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    Lancy, David F. 2016. Evolutionary Perspectives on Child Development and Education. p. 33.

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Abstract

The human species is more reliant on cultural adaptation than any other species, but it is unclear how observational learning can give rise to the faithful transmission of cultural adaptations. One possibility is that teaching facilitates accurate social transmission by narrowing the range of inferences that learners make. However, there is wide disagreement about how to define teaching, and how to interpret the empirical evidence for teaching across cultures and species. In this article I argue that disputes about the nature and prevalence of teaching across human societies and nonhuman animals are based on a number of deep-rooted theoretical differences between fields, as well as on important differences in how teaching is defined. To reconcile these disparate bodies of research, I review the three major approaches to the study of teaching – mentalistic, culture-based, and functionalist – and outline the research questions about teaching that each addresses. I then argue for a new, integrated framework that differentiates between teaching types according to the specific adaptive problems that each type solves, and apply this framework to restructure current empirical evidence on teaching in humans and nonhuman animals. This integrative framework generates novel insights, with broad implications for the study of the evolution of teaching, including the roles of cognitive constraints and cooperative dilemmas in how and when teaching evolves. Finally, I propose an explanation for why some types of teaching are uniquely human, and discuss new directions for research motivated by this framework.

1. The adaptive value of teaching

The human species is more reliant on cultural adaptation than is any other species (Boyd & Richerson 1985; Boyd et al. 2011; Dean et al. 2012, Hill et al. 2009; Whiten & Erdal 2012). Much of the knowledge and behavior that allows humans to adapt to a uniquely broad range of ecologies is accumulated over multiple generations, leading to adaptations more complex than any one individual could produce in a lifetime (Boyd & Richerson 1996; Tennie et al. 2009). For example, Oceania was only settled through the combination of sophisticated navigational knowledge and complex double-hulled canoes (see Kirch 2002), and the Arctic could not have been settled without new technologies for clothing and shelter, as well as food-gathering techniques (Boyd et al. 2011).

In culture as in biology, the accumulation of adaptive changes across many generations occurs only when transmission is sufficiently faithful. A variety of mechanisms work to make genetic replication incredibly accurate, but it is less clear what makes cultural transmission faithful, or how much fidelity is necessary (see Dawkins 1982; Henrich & Boyd 2002; Henrich et al. 2008). The problem is that – unlike genes – cultural variants do not physically replicate. Instead, they replicate through the inferential process of social learning, by which social learners use – among other inputs – others' behavior to make and support inferences about the world. This often entails acquiring the same behaviors or mental representations held by others. In such cases the range of inferences that can be made based on a single behavior may be quite broad, and making an accurate inference can depend on background knowledge (see Boyer 1998; Sperber & Wilson 1995). One possible way that faithful transmission may be maintained under such circumstances is that variation in learner inferences is decreased by attractors, that is, innate or learned psychological dispositions that restrict the range of inferences made by learners (Boyer 1998; Sperber 2000). One problem with attractors as a solution to this inferential frame problem is that attractors can only change over historical time, at the pace of the factors of the environment, physical world, or psychology in which they are rooted. Culture can change faster than biology (Perreault 2012), such that genetically evolved cognitive attractors may not be able to keep pace with culturally evolving mental representations. This may also apply to attractors shaped by environmental change. As a result, attractors may help to explain faithful transmission for domains of social learning in which content varies only superficially over time and space. However, attractors are difficult to reconcile as a mechanism for explaining the diversity of complex, locally adaptive beliefs and technologies used by humans. For example, our information-rich psychology might include attractors that influence the human creation and use of sharp tools to cut materials. Still, it is a long way from “things that cut” to the sophisticated production process required to produce a Damascus steel blade, a product which modern techniques and present-day scientists have yet to recreate (see Reibold et al. 2006).

In addition to an inherited information-rich psychology, humans may also rely on help from knowledgeable models to narrow the range of a learner's inferences, making social learning accurate beyond the proper domains of existing attractors. Overt teaching can function this way, and psychologists studying social learning have pointed out that models can provide subtle cues that greatly enhance the accuracy of social learning. For example, models may use gaze to establish joint attention (Tomasello et al. 2005), or use ostensive cues to mark some knowledge as generalizable (Csibra & Gergely 2009; 2011).

In most forms of social learning, the mental capacities and strategies for copying others evolve in the learners. Such learner-driven mechanisms can be adaptive, provided they are mixed with asocial learning strategies that produce new, locally adaptive information (Boyd & Richerson 1985; 1995; Galef 1995; Giraldeau et al. 2002; Rogers 1988). In contrast, teaching also requires the evolution of mental capacities and strategies for when to teach on the part of models who perform teaching behaviors. As a result, teaching may only be adaptive in a subset of the situations in which social learning more generally evolves.

Mathematical modeling suggests that teaching – defined as behavior evolved to facilitate learning in others – is adaptive when information is difficult for learners to acquire independently or through observation, but not when it is so rare that relatives are unlikely to possess it (Castro & Toro, 2014; Fogarty et al. 2011). Assuming that these conditions are met by cumulative cultural knowledge in humans and not by other forms of culture, this could explain why humans teach more than other animals. There is some experimental evidence suggesting that humans do facilitate cumulative cultural learning through teaching (Dean et al. 2012; cf. Caldwell & Millen 2009), and psychologists often characterize teaching as ubiquitous across human societies and as unique to humans (see Csibra & Gergely 2009; 2011; Kruger & Tomasello 1996; Premack & Premack 2004; Strauss et al. 2002). However, cultural anthropologists studying childhood and learning in diverse societies claim that teaching is unique to Western cultures and absent elsewhere (Gaskins & Paradise 2010; Lancy & Grove 2010; Paradise & Rogoff 2009; see also review by Hewlett et al. 2011). At the same time, biologists have documented teaching in nonhuman (and some non-cultural) animals including ants, meerkats, pied babblers, and several additional species (see reviews: Caro & Hauser 1992; Hoppitt et al. 2008; Thornton & Raihani 2008). Precisely what qualifies as “teaching” is still hotly debated (see Csibra 2007; reply by Thornton 2007; see also Byrne & Rapaport 2011; reply by Thornton & McAuliffe 2012).

I argue here that discrepancies in empirical claims across disciplines follow from deep-rooted theoretical differences among them, due to disciplinary differences in focal research questions. To better address the evolutionary origins and adaptive design of teaching behavior, I propose a framework that synthesizes existing comparative and anthropological research on teaching behavior. In a review of the three major approaches to the definition and study of teaching – mentalistic, culture-based, and functionalist – I outline the research questions that drive the discipline-specific approaches, and address the ways in which each approach might benefit from this synthesis (Section 2.4). I then propose a framework based in the functionalist approach that structures the study of teaching according to the adaptive problems each type solves (Section 3), and demonstrate its utility by restructuring existing comparative and anthropological data on teaching (Section 4) and by building on this preliminary empirical review to generate novel research questions unique to the integrative approach (Sections 5, 6, and 7). Specifically, I discuss the implications for the study of the psychology of teaching, and clarify how problems of cooperation may constrain the evolution of teaching differently for the roles of learner and teacher. Finally, I synthesize these insights to illuminate why humans are such prolific and intensive teachers, relative to other animals.

2. What is teaching, in theory?

2.1. Mentalistic definitions of teaching

Mentalistic approaches define teaching as behavior with the intent to facilitate learning in another (Pearson 1989, p. 63). This approach seeks to explain the design of mental mechanisms that make teaching possible, often in order to explain individual-level and species-level variation in teaching abilities. For example, Tomasello et al. (1993) argue that the establishment of joint attention and holding representations about the mind of the other are necessary prerequisites to teaching. It follows that teachers need theory of mind (ToM) to identify the need for teaching, to figure out what it is that they ought to teach, and to tailor the difficulty of the task to match the skill level of the pupil (Kruger & Tomasello 1996). Elsewhere, researchers from this school of thought apply the same line of reasoning to social learning capacities, including imitation (for review, see Caldwell & Whiten 2002). In this view, the absence of theory of mind in nonhuman animals explains why only humans engage in “powerful forms of cultural learning” such as teaching and imitation (Tomasello 1999).

Similarly, the mentalistic approach uses variation in theory of mind capacities to explain variation in human teaching capacities, at the individual level. For example, Ziv and Frye (2004) argue that teaching is only possible when teachers and pupils both consciously recognize (a) intentionality and (b) knowledge differences between individuals. They cite a range of studies showing that children who have better success on theory of mind (ToM) tests also teach more or use more effective methods for teaching (Davis-Unger & Carlson 2008; Strauss et al. 2002; Ziv & Frye 2004), and perform better as pupils (Wellman & Lagattuta 2004). Olson and Bruner (1996) argue that without theory of mind there can be no ascription of ignorance, and therefore there will be no attempt to teach. According to Strauss et al. (2002), “in order to teach, one needs to know when knowledge, beliefs, skills, etc. are missing, incomplete, or distorted, as well as how people learn” (p. 1476). This goes beyond simply having the intent to teach, to having the intent to teach based upon a range of mental representations of others' mental states. This approach builds on Vygotsky's (1978) concept of the “zone of proximal development,” the narrow range of learning just outside a child's developing abilities, within which a child can learn, building on his or her present competencies through scaffolding by knowledgeable others (Guberman & Greenfield 1991; for review, see Pelissier 1991). In this view, only a teacher who identifies this proximal zone of development (through ToM) can understand that teaching is needed, and can then intend to teach.

2.2. Culture-based definitions of teaching

Culture-based definitions of teaching focus on teaching as it happens in formal classrooms in Western societies, in contrast to informal social learning in more “traditional” societies. This approach is most commonly used in sociocultural anthropology and cross-cultural psychology. The research goals include describing cross-cultural variation in the prevalence of teaching, with some work focused on direct comparison of formal schooling versus everyday learning in childhood (e.g., Maynard 2004; Paradise & Rogoff 2009). These definitions underlie nearly all of the cross-cultural data on teaching, which are crucial to our understanding of human variation in teaching behavior.

Researchers taking this approach contrast teaching with other forms of social and individual learning outside of the Western cultural context (e.g., Lancy 2010). Rather than using an explicit definition, researchers in this approach identify teaching from a shifting set of ostensive features, including: (1) the teacher intends to teach, (2) knowledge transmission is unidirectional (teacher to pupil), (3) pupils are passive recipients of knowledge who do not collaborate interactively with the teacher, (4) knowledge is communicated explicitly, often by verbal instruction, and (5) the activity is marked in some way and recognized as “teaching” by its participants. These criteria are not applied as a strict checklist, but as a set of characteristics arrived at inductively via contrasts between teaching as the ethnographer understands it, and other types of social learning. Note that in this approach, “social learning” is used in the colloquial sense to refer to learning that happens in a social context or interaction; therefore, it is not limited to the functionalist learning mechanisms that have evolved to facilitate the transfer of social information.

Types of social learning that are not considered “teaching” are described as natural, simple, informal, observational, practical learning, or guided instruction (Paradise & Rogoff 2009), in contrast with “formal” learning via teaching. In informal social learning, (a) learning takes place within an activity, the focus is on completing a task rather than on learning or teaching; (b) learners are often expected to observe rather than participate; and (c) the responsibility for attending, learning, and ending a learning period lies with the learner rather than the model (Gaskins & Paradise 2010). In other words, learning is common through intent participation, defined as “listening-in” and “keen observation” (Rogoff et al. 2003), or through legitimate peripheral participation, in which learning a given activity is also inextricably linked to the learner's building a sense of shared identity with other practitioners (Lave & Wenger 1991). Learners must, therefore, identify with their models prior to learning about an activity.

In contrast, teaching is a “marked” event, such that a behavior is only teaching when participants label the activity as “teaching” instead of something like “line-fishing” or “weaving.” In this framework, “teaching” that is embedded in another activity is informal learning, rather than formal learning or teaching. It is so fully integrated into the learner's everyday experiences that they appear to learn through “a kind of osmosis” (Gaskins & Paradise 2010, p. 87). This type of learning is thought to allow for automatic, collaborative, highly effective learning through experience, “with little dependence on coercion and explicit teaching” (Paradise & Rogoff 2009, p. 124). This fashion of learning works, researchers argue, because informal learners are always interested, such that learning happens without any need for teaching, and learning happens without fail (Paradise & Rogoff 2009; Spindler & Spindler 1989).

From this perspective, teaching is not a generally useful mechanism to make difficult learning easier. Instead, it is a means of forcing passive or uninterested pupils to learn. A number of researchers suggest this is unique to Western societies. Mead (1970, p. 12), for example, argues that in the shift toward Western ways of learning, “the emphasis has shifted from learning to teaching, from the doing to the one who causes it to be done, from the spontaneity to coercion, from freedom to power,” and that the shift toward teaching means “the shift from the need for an individual to learn something which everyone agrees he would wish to know, to the will of some individual to teach something that it is not agreed that anyone has any desire to know” (p. 3). Rogoff et al. (2003) contrast informal learning with the “factory-efficiency” model of teaching, where information transfer is unilateral and divorced from culturally valued activities. Rather than being collaborative social partners, in this factory-efficiency model, “[t]eachers were cast as technical workers who were supposed to insert information into the children, who were seen as receptacles of knowledge or skill” (p. 181).

Lancy and Grove (2010) equate teaching with explicit or abstract verbal instruction, which they claim is rare in non-Western societies. They discuss “the near total absence of children being taught (in the explanatory, didactic sense) by adults” (p. 145), and explain in an endnote that only three examples of this kind of teaching exist in the ethnographic literature on non-Western societies. Lancy and Grove also present several examples of guided learning that they do not consider “teaching,” but which would be considered teaching by the mentalistic definition mentioned above. For example, teaching is absent in canoe-building among the Warao, where “there is not much verbal instruction… but the father does correct the hand of his son and does teach him how to overcome the pain in his wrist from working with the adze” (Wilbert 1976; quoted in Lancy & Grove 2010, p. 161). Weaving apprenticeships in which “actual instruction begins in earnest when the master sits beside the boy at the loom and begins to demonstrate some simple patterns, which the novice copies” are not classed as “teaching,” because prior to instruction, pupils offset the cost of teaching through menial labor, and again there is little explicit verbal explanation (Lancy & Grove 2010, p. 160).

2.3. Functionalist definitions of teaching

Functionalist definitions of teaching are grounded in the observable behavioral causes and outcomes of teaching behavior, instead of the teacher's motivational state or on local context. This approach aims to explain the evolutionary roots and adaptive design of teaching behavior, in part by demonstrating that “teaching” is not a uniquely human behavior. Functionalist approaches have similarly been applied to social learning more broadly, opening up the spectrum of social learning mechanisms beyond pure imitation, to include a range of mechanisms by which nonhuman (and human) animals may gather information through social transmission (for reviews, see Heyes & Galef 1996; Fragaszy & Perry 2003). The functionalist approach to teaching differs from the mentalistic one in that there is less focus given to mental mechanisms – a practical innovation for studying nonverbal species (see Caro & Hauser 1992). To this end, functionalist definitions build on a basic theoretical definition of teaching as behavior evolved to facilitate learning in others. To enable the empirically rigorous study of teaching in nonhuman animals, Caro and Hauser (1992) established three operational criteria for identifying behavior that functions as teaching: (1) behavior is contingent on the presence of a naïve learner, (2) it provides no immediate benefit (or even generates a cost) for the teacher, and (3) it can be shown to facilitate learning in others.

Subsequent functionalist approaches to teaching have sought to update Caro and Hauser's (1992) operational criteria, but not their conceptual definition. Hoppitt et al. (2008) argue that the cost criterion does not reliably distinguish between behavior that evolved for teaching versus alternative functions, so it may lead to false positives. For example, food provisioning may result in offspring learning about parental food preferences, but probably evolved because it increases offspring survival rates (Hoppitt et al. 2008). However, if food provisioning leads to learning by offspring at an immediate cost to parents, the Caro and Hauser (1992) definition would categorize this as teaching. Without rejecting the usefulness of the Caro and Hauser operational definition, Hoppitt et al. (2008) offer a revision of the broader theoretical definition of teaching that separates the process of learning from the behavior of teaching, such that teaching can be viewed as an accessory to other well-established social learning processes, and teaching does not have a single behavioral profile. For example, learning may happen through teaching by tolerance of close observation – where the adaptation of tolerance is on the part of the teacher – as compared with close observation, where the adaptation is on the part of the learner, who persists in observing. In this way, teaching can evolve alongside existing social learning processes, increasing learning accuracy through small modifications of a model-turned-teacher's behavior. The typology laid out by Hoppitt and colleagues distinguishes among teaching via local enhancement, observational conditioning, imitation, opportunity provisioning, and coaching. Teaching behavior is conceptually distinguishable from other social learning behavior because non-teaching is inadvertent, such that the adaptation for teaching is not on the part of the demonstrator.

Thornton and Raihani (2008) point out that the Caro and Hauser (1992) operational definition can also lead to false negatives in the study of teaching in nonhuman animals (see also Thornton & McAuliffe 2012; cf. Byrne & Rapaport 2011). Thornton and Raihani suggest that teaching be defined instead by “key characteristics” (p. 1825): “(1) it is a form of cooperative behavior with response-dependent fitness payoffs; (2) its function is to facilitate learning in others; and (3) it involves the coordinated interaction of a donor and a receiver of information.” The key characteristics do not stipulate behavioral guidelines for identifying these key characteristics in the field, but do predict that teaching will be observed where its utility in increasing the pupil's learning efficiency is the highest (e.g., it most improves on other learning mechanisms). A major function of these key characteristics is to distinguish teaching from other forms of social learning, communication, and social interactions such as “punishment,” which do not evolve because of the learning benefits they create for the learner.

Thornton and Riahani (2008) and Hoppitt et al (2008) do not include human-specific forms of teaching within their frameworks. However, the novel functionalist definition of Csibra and Gergely (2009; 2011) focuses on teaching as a uniquely human trait. Although like the mentalistic approaches, this definition describes a number of mental characteristics of teaching, it differs from mentalistic approaches (e.g., Kruger & Tomasello 1996), in that its primary research question is on the functional or adaptive role that teaching serves, and on how teaching evolved (in humans). Csibra and Gergely (2009; 2011) argue that the adaptive value of teaching is the driving causal force in the evolution of those mental capacities, rather than the capacities acting as constraints on the evolution of teaching. Since they argue that the adaptive function of teaching is to facilitate transfer of abstract, generalizable knowledge, their definition of teaching is narrower than other functionalist definitions. In Csibra and Gergely's approach to teaching, which they term natural pedagogy, the adaptation that makes knowledge transmission possible is a motivational system. This system evolves to facilitate the sharing of generalizable knowledge with others, and its adaptive function is to speed up the rate at which naïve (human) learners gain “reliable, new, and relevant information” about the world (Gergely et al. 2007, p. 140).

According to Csibra and Gergely (2006) this function does not necessarily require theory of mind capacities or a conscious intentionality to achieve, but rather any evolved psychology that would function to facilitate learning in others, for this particular informational domain. Thus, the teaching adaptation includes communicative capacities: behavioral markers, or ostensive cues, that highlight and mark the act of teaching for teacher and pupil. By this definition, teaching behavior requires: “(1) explicit manifestation of generalizable knowledge by an individual (the ‘teacher’), and (2) interpretation of this manifestation in terms of knowledge content by another individual (‘the learner’)” (Csibra & Gergely 2006, p. 5). It does not require that teachers or pupils are consciously aware of these processes, unlike the mentalistic or culture-based definitions. By this definition, the teaching adaptation is both a special learning mechanism (because it requires an adaptation by both parties) and a special type of communication (because it evolved specifically to convey generalizable, abstract content such as “tigers are always dangerous,” as opposed to fleeting, concrete content about the current state of the world, such as “there is a tiger behind you”). Thus, it characterizes teaching as an adaptation to facilitate learning in others, for a particular type of content (generalizable knowledge) and kind of learning (knowledge transfer) in a single species (humans).

2.4. On the benefits of integrating existing approaches

The mentalistic, culture-based, and functionalist approaches are each tailor-made research programs designed to address questions about specific aspects of teaching. Mentalistic approaches investigate the psychological prerequisites for teaching, and use these factors to explain individual- and species-level variation in teaching capacities. Culture-based definitions focus on the features of Western-style classroom teaching, and highlight qualitative cross-cultural differences in the means by which children learn. Finally, functionalist definitions of teaching focus on the evolutionary costs and benefits of teaching behavior to better understand the evolutionary origins and functional design features of teaching behavior. The problem with the current division of labor is that each approach could make better use of the findings in the other two fields to address their own primary research questions. Recent attempts at dialogue across these approaches have, unfortunately, been stymied by definitional debates about what constitutes teaching (see, e.g., Csibra 2007; Thornton et al. 2007). The systematic framework I propose subsequently aims to integrate core research questions from each approach, and synthesize extant data on teaching behaviors, as identified by these approaches.

Mentalistic definitions would benefit from integration with the functionalist and culture-based approach in several ways. First, integrating mentalistic questions about the psychological prerequisites for teaching with functionalist studies of teaching-like behavior in nonhuman animals would reshape mentalistic endeavors to ask why intentional and theory-of-mind based teaching might have evolved in humans alone. Some nonhuman animals regularly facilitate learning in conspecifics without human-like theory of mind capacities, so the answer cannot be that only humans can facilitate others' learning. Mentally representing the intent-to-teach has no adaptive value by itself; only the teaching behavior has adaptive value. It follows that as long as pupils act as if they expect to receive relevant information, and learn it effectively, an interaction can fulfill the adaptive function of teaching. What, then, is unique about intentional teaching? Mentalistic approaches integrated with a functionalist framework could make better use of cross-species comparative data by relating psychological capacities more closely to behavioral adaptations, and to the social learning problems these behaviors address. One possibility is that theory of mind helps to address only a subset of adaptive problems in which one individual facilitates learning in another, and that there are alternative psychological mechanisms that might also suffice (see sect. 5 for possibilities). This can generate new and interesting questions for the mentalistic approach to teaching. For example, are there socio-ecological conditions that are specific to human evolution, under which conscious intent-to-teach might provide adaptive benefits? Under what conditions is our species' apparently unique teaching psychology better-adapted than the simpler alternatives?

Integrating the mentalistic approach with the culture-based approach's emphasis on documenting cross-cultural variation in learning capacities could provide a broader range of human behaviors against which to test hypotheses about the psychology of intentional teaching. When humans facilitate others' learning, do they always use theory of mind, or are there other mental mechanisms that promote functionally equivalent behavior? Descriptively, what sorts of social learning problems does intentional teaching solve for humans, and how does this vary cross-culturally? Explaining the full range of variation in human teaching behavior may shed light on teaching's role in making cumulative cultural adaptation possible.

Culture-based approaches would benefit from integration with the mentalistic and functionalist approaches. While useful in generating rich descriptions of cross-cultural differences in how children learn, culture-based approaches could benefit from the integrative study of teaching in order to better explain how such cross-cultural variation emerges, why it persists, and why this type of variation is characteristic across human populations but atypical in nonhuman animals. By establishing a baseline of what is universal behavior across human societies, researchers could more precisely focus on aspects of teaching behavior that may be culture-specific, in contrast. Both the mentalistic and functionalist approaches assume that teaching behavior is driven by a species-typical psychology of teaching. Researchers adopting the culture-based approach, especially those conducting long-term ethnographic fieldwork, are in the unique position to evaluate the validity of these claims; however, this requires a common framework across the major approaches. This framework would provide a yardstick by which to compare cross-cultural variation quantitatively, strengthening the original research aims of the culture-based approach to the study of teaching.

Functionalist definitions of teaching are focused primarily on the evolutionary origins and adaptive consequences of teaching. Functionalist approaches are to some degree already the basis for productive comparative work. For example, one typology links kinds of teaching behavior to the forms of learner-driven social learning behavior from which teaching may be derived (Hoppitt et al. 2008), and the earliest functionalist definition was an explicit attempt to enable cross-species comparisons on the basis of behavior rather than a teacher's mental state (Caro & Hauser 1992). Similar efforts have proven fruitful in the social learning literature more broadly (e.g., see Whiten [2011] on chimpanzee learning). However, the functionalist approach could benefit from integration with the culture-based and mentalistic approaches in several ways. Despite acknowledging exceptional levels of teaching in humans, existing functionalist frameworks do not explain the apparently unique qualities of human teaching within the same framework they use to study teaching in other species. This is akin to studying kin selection according to a framework that does not explain the behavior of eusocial insects. One exception is Csibra and Gergely's (2009; 2011) explanation of the evolution of teaching as an adaptation; however, their approach defines teaching as a uniquely and universally human adaptation, while not explaining behaviors that function to facilitate learning in others for nonhuman animals.

If humans are abnormally prolific and sophisticated teachers in comparison to other animals, it is all the more important that any framework for understanding the evolution of teaching should be tested against the human case. This is not to say that functionalists should abandon their study species of choice, only that the frameworks used would benefit by integrating research on human teaching. The best data available on the range of human variation in teaching behavior come from ethnographers in the culture-based approach, and this evidence suggests that Western-style teaching is cross-culturally rare. Even by the broader functionalist definition, there is a lack of systematic empirical work documenting that teaching happens across all human societies. Only a few papers have recently begun to address the question at all, and none include direct cross-cultural comparisons, in part because there has so far been no comprehensive framework by which to collect comparative data (Hewlett et al. 2011; Kline et al. 2013). Despite this, functionalist researchers state that “[t]eaching is ubiquitous in human societies” (Thornton & McAuliffe 2006, p. 227), and that “[i]ndeed, all human children must be taught” (Thornton & McAuliffe 2012, p. e8). This disconnect is, again, most likely due to a difference in definitions: surely, humans in all societies facilitate learning in other humans – but only an integrative framework can allow us to evaluate human uniqueness in theoretically interesting ways, and with quantitative data.

Without a common framework across which human and nonhuman behavioral data can be compared, functionalist frameworks will continue to only hint at the explanation for uniquely human teaching behavior, and as a result may only be providing part of the evolutionary explanation for why teaching evolves in any species. It may be that functionalist researchers prefer to eschew ethnographic data because of its qualitative, often subjective nature. However, the range of variation in human teaching behavior documented by ethnographic descriptions can inform the development and evaluation of integrative functionalist frameworks (see sect. 4.2) and the richness of anthropological descriptions of human behavior can shape operational criteria, for example, in the development of an ethogram of human teaching behavior for cross-cultural application (Kline 2013). A synthesis along these lines might also help to make sense of the theoretical and empirical rift between traditional functionalist approaches as applied to nonhuman animals, and Csibra and Gergely's (2006; 2009; 2011) functionalist approach to natural pedagogy as a human-specific adaptation for teaching. The typology proposed below (see sect. 3) will suggest that these two approaches are discussing two different teaching types, evolved to solve two different adaptive problems in the domain of social learning.

The functionalist approach would also benefit from further integration with the mentalistic approach, and such a synthesis need not detract from the ease with which functionalists can study teaching behavior in nonhuman animals, as it has in the past (see Caro & Hauser 1992). Integrating questions from the mentalistic approach with functionalist methodologies may generate and address novel research questions. For example, functionalist frameworks excel at outlining adaptive problems and the behavioral adaptations that might solve them. Mentalistic approaches work to enumerate the psychological mechanisms that produce behavioral consequences. Translating empirical research between approaches via a cohesive framework would, therefore, shed light on the socio-ecological circumstances under which a given psychology might be selected for. These psychological adaptations need not be established as criteria for identifying teaching, but rather could be used to generate novel hypotheses about the evolution of teaching psychology and the psychological underpinnings of cumulative cultural evolution. Investigating such hypotheses can shed light on the role of psychological constraints on the evolution of teaching and other social learning capacities, across species. Given the potentially important function of teaching as a means of faithful cultural transmission, this question is relevant to broader problems in the comparative study of the evolution of culture, and cumulative cultural adaptation.

3. A taxonomy of teaching adaptations

The taxonomy that I propose is based on a functionalist approach, but endeavors to provide a framework that unites functionalist, mentalistic, and culture-based inquiries into the study of teaching behavior in human and nonhuman animals. As a conceptual definition, the framework defines teaching as behavior that evolved to facilitate learning in others. This framework focuses on a number of distinct teaching types, many of which have been proposed elsewhere (e.g., Caro & Hauser 1992; Hoppitt et al. 2008), but differs from previous functionalist frameworks in that the teaching types as defined here are directly tied to specific adaptive problems that are inherent in social learning. In contrast, Hoppitt et al. (2008) base their categories on documented forms of social learning from which teaching may be derived; Caro and Hauser (1992) originally proposed a simpler taxonomy based on descriptions of teacher behavior (coaching vs. opportunity teaching).

The taxonomy I propose aims to cover the full range of possible adaptive problems that teaching could evolve to address, and incorporates all known teaching mechanisms in humans and other animals into a cohesive theoretical framework. By focusing on the specific adaptive function of particular teaching types, the framework is a tool tailored for integrating research on the contexts in which teaching is likely to have evolved, and for creating an ethogram of what each teaching type may look like “in the wild.” In addition, the framework can be used to integrate existing data from the mentalistic, functionalist, and culture-based approaches for comparative analysis across species (sect. 4.3) and across human populations (sect. 4.2). Distinguishing between multiple adaptive problems and the teaching mechanisms that address them can generate novel research questions with respect to the psychological adaptations required for each teaching mechanism (sect. 5); the costs, benefits, and cooperative dilemmas each type poses from the perspective of the pupil (sect. 6.2) and the teacher (sect. 6.1); the relative prevalence of the variety of teaching mechanisms cross culturally (see further on in this section); and for questions about teaching and human uniqueness (sect. 7).

The illustrative examples that follow below are based on long-term fieldwork in Fiji, and should not be regarded as defining the teaching types. (For ethnographic background that includes an evaluation of the importance of teaching versus other learning mechanisms, see Kline et al. [2013].) As more than one teaching type can solve a given learning problem, the taxonomy is meant to be modifiable and expandable, should additional teaching types be observed in the laboratory or field. Social learning problems may overlap in real life, so that more than one teaching type might occur on a given occasion.

For all the teaching types discussed below, it will be important in practice to distinguish between actors' baseline behaviors versus their behavior during an episode of teaching. Likewise, it will be important to distinguish between teaching and other forms of social learning. Since other forms of social learning do not require any modification of the model's behavior, demonstrating such differences will be sufficient to rule out alternative forms of social learning. Ruling out direct benefits to the teacher may be sufficient to distinguish teaching from other forms of social influence. However, precisely how these behavioral changes are quantified will vary by activity and by species (and in humans, perhaps by culture), so the possibilities are not laid out in detail. To facilitate comparative work, it may be necessary to use an ethogram of behavioral features of teaching tailored for a species or population (e.g., see ethogram for human teaching by Kline [2013]).

In some cases, demonstrating differences from baseline behavior may not be enough to rule out alternate explanations. Functionalists have typically followed Caro and Hauser (1992) to distinguish teaching from alternate explanations by demonstrating that the benefits to the teacher are indirect and derived through improved pupil learning. Documenting that there are direct benefits to the teacher would suggest that an alternative explanation is possible, though it would not necessarily rule out teaching, since behaviors can evolve due to more than one type of benefit. This heuristic can be used as an operational definition of teaching, but it is only one of many possible criteria that can be used as evidence of adaptive design. For additional possibilities, see Kline (2013), Hoppitt et al. (2008), and Thornton and Raihani (2008).

3.1. Teaching types

Teaching potentially solves a number of different adaptive problems that arise in social learning, and that cannot be addressed by learner behavior alone. In order to learn, pupils must (a) attend to, and (b) have access to, relevant information, defined as novel information which is useful and/or in some way connected to the pupil's prior knowledge (Sperber & Wilson 1995). This includes information that is accessed through experience, observation, or direct communication. When attention or relevant information (or both) would otherwise be lacking, teaching may make learning possible, more efficient, or more accurate. Teaching can therefore be thought of as a set of adaptive behaviors, perhaps derived from existing social learning behaviors (see Hoppitt et al. 2008).

3.1.1. Teaching by social tolerance

Adaptive problem: A pupil attends to relevant stimuli, but does not have the knowledge or skill to undertake some task because it requires observing a conspecific's behavior. This means that purely asocial learning mechanisms would not be sufficient, and that social learning by any means is only possible with modification of the model's behavior. The learning problem could be solved through teaching by social tolerance – defined as teaching in which the teacher does not stop the pupil's close and intrusive observation. For example, in Fiji, women who are cooking often tolerate close observation by children, even to the degree of pausing their own work, as children stick their hands into bowls or grab and manipulate mixing tools that are crucial to the women's food preparation tasks. This kind of teaching by social tolerance can be distinguished from the social tolerance that evolved for other functions, in that the degree of tolerance toward pupils is greater than species-typical tolerance toward other conspecifics. It is worth noting here that many species are highly tolerant of young conspecifics, in general (presumably because they impose little threat or cost to adults). Where this is the case, evidence for teaching by social tolerance might include heightened social tolerance in situations, or during activities, when the potential gains in learning for the young are especially high. For example, in a quantitative observational study in Fiji – a hierarchical society where corporal punishment of children is permissible – I found that 100% of children's physical intrusions into others' activities were tolerated (Kline 2013).

3.1.2. Teaching by opportunity provisioning

Adaptive problem: A pupil attends to relevant stimuli, but lacks the opportunity to undertake some task because it is too difficult or dangerous to explore independently. When this is true, teaching by opportunity provisioning may solve the learning problem. Teaching by opportunity provisioning occurs when a teacher creates opportunities for the pupil to practice – opportunities for asocial learning – that would otherwise not exist (cf. Caro & Hauser 1992; Hoppitt et al. 2008). For example, adults in Fiji sometimes make day-to-day tasks easier so that children can participate, as when one 4-year-old boy who apparently knew (in theory) how to fetch water from a well using a bucket and rope could only do so with the help of his uncle's physical strength. (The task was far slower and much more water was spilled than if the uncle had completed the task alone). Other instances of teaching by opportunity provisioning may or may not include a scaffolding component, in which the teacher scales up the difficulty of the learning opportunities with the maturity or skill of the pupil.

3.1.3. Teaching by stimulus or local enhancement

Adaptive problem: A learner may not attend to a relevant stimulus, though it is accessible. This category may include cases where the pupil is afraid of, or otherwise repelled by, a learning-relevant stimulus. Since asocial and social learning both depend upon the attention of pupil, learning is unlikely to happen in this context without intervention by a teacher. Once attention is stimulated by a teacher, other learning mechanisms might suffice for producing adaptive behavior on the part of the pupil. Therefore, teaching by stimulus or local enhancement might evolve to facilitate initial bouts of individual learning. Teaching by stimulus or local enhancement is defined as a teaching type in which the teacher stimulates the pupil's interest in a stimulus or location, potentially leading to a discovery or skill development following the pupil's individual learning. This may occur with varying levels of effort by the teacher and resistance by the pupil. As in many other societies (Bryant & Barrett 2007), Fijian adults often make use of both pointing and motherese (infant-directed speech) to manipulate a child's attention toward relevant stimuli. The target of attention can include anything from a relative walking across the village, to a breaching dolphin, or even an otherwise unremarkable object such as a palm leaf or a hinged house door.

3.1.4. Teaching by evaluative feedback

Adaptive problem: Pupils may not attend to existing feedback or possible consequences regarding their behavior, although they have access to that information. That is, the pupil either over- or under-uses a behavior, and does not have cognitive access to information about the resulting outcome. As in the previous category, learning by other mechanisms is unlikely without intervention by a teacher. Teaching by evaluative feedback can solve this adaptive problem, when a teacher provides positive or negative reinforcement conditioned on the pupil's behavior, and in a format to which the learner is receptive. This may be particularly common for opaque social rules or costly outcomes. For example, Fijian adults and even older children teach young children that touching another person's head is tabu (“taboo,” or forbidden), by scolding children whenever they happen to do so. There are many opportunities for children to make this “mistake,” because a toddling child is often at head-height with adults who are sitting cross-legged on the floor, as is typical in Fiji. Elsewhere, teaching by evaluative feedback is called coaching, training, or encouragement/ discouragement (e.g., Caro & Hauser 1992). This teaching type may include the extremes – removing a behavior from the pupil's repertoire altogether, or making it omnipresent – as well as reinforcement that depends on the particulars of the situation.

3.1.5. Direct active teaching

Adaptive problem: A pupil lacks both attention and access to a relevant stimulus or information. As a result, the pupil cannot gain this information through individual learning mechanisms, and has no way to solve the “frame problem” by observing others' behavior. This may arise either in an evolutionarily novel situation, when no alternative social or asocial learning mechanisms exist to interpret the fitness-relevant components of that situation, or when a pupil simply lacks background knowledge. Hence, the frame problem may shift with pupils' changing knowledge levels. The frame problem may be solved by direct active teaching. This is similar to what Gergely and Csibra (2009; 2011) define as teaching, but is not necessarily limited to humans, and does not require ostensive cues (at least by definition). Direct active teaching is characterized by (1) manifestation of relevant information by the teacher to the pupil and (2) interpretation of this manifestation in terms of knowledge content by the pupil. It differs from other teaching adaptations in that it requires some shared background knowledge as well as a means of direct communication, so that the teacher can identify and communicate the relevant information to the pupil. In Fiji, for example, direct active teaching might include a father's verbal explanation to his 8-year-old daughter, complete with pointing and illustrative hand movements, of how to extract a turtle intestine from the rest of the carcass. It could also include a non-verbal demonstration, punctuated with exaggerated movements, by an expert weaver to a novice weaver. Alternatively, it might include an exclusively verbal description of where turtles lay their eggs, what butterflies eat, and where dolphins sleep, as when one mother replied to her 4-year-old son's questions about wild animals.

Note that while direct active teaching may include all “teaching” as categorized by the culture-based definitions, it is a much broader definition. Direct active teaching does not use the same criteria for identifying teaching as the culture-based definitions. In practical applications it would subsume the culture-based definition, but dispenses with the requirements of teacher intentionality, unidirectional transmission, pupil passivity, and labeling of the activity as “teaching” by the teacher and pupil. The culture-based definition and this definition of direct active teaching share in common the requirement for “direct” or “explicit” communication of information. However, they differ slightly even here, in that direct communication is not exclusively verbal under the definition of direct active teaching.

3.2. How this framework integrates across existing approaches

Much of the present debate over the form and distribution of teaching behaviors results from different schools of researchers focusing on different teaching adaptations as if these represent the entire category of “teaching.” The more fine-grained approach outlined here has the potential to restructure these debates by integrating the full range of documented variation in teaching adaptations across human and nonhuman animals. Frameworks with similar goals have proven fruitful for asocial and social learning mechanisms other than teaching (e.g., Heyes 1994), especially concerning the evolutionary origins of imitation and its purported uniqueness in humans (for reviews, see Caldwell & Whiten 2002; Tennie et al. 2009; Tomasello et al. 1993), leading to a more sophisticated understanding of why humans alone have cumulative culture (e.g., Enquist & Ghirlanda 2007; Tennie et al. 2009). Because this framework covers all of the informational contexts in which teaching might evolve, it provides a comprehensive structure by which to categorize known (and future) teaching behaviors – a necessity for future comparative and anthropological studies of teaching.

This framework categorizes teaching types according to theoretically significant learning problems, rather than by behavioral profile, or based on known mechanisms of social learning. The latter is important, because the adaptive problems solved by teaching may not be a one-to-one match to those solved by already-catalogued social or asocial learning mechanisms. These learning problems are not content-specific, but instead describe informational contexts in which teaching could facilitate learning. While some mechanisms may have non-teaching analogues (e.g., Hoppitt et al. 2008), this approach allows for straightforward comparative empirical applications, since learning problems can be analyzed in terms of these contexts rather than according to domains of content to be learned. This approach does not prevent researchers motivated by the research questions of mentalistic or culture-based approaches from continuing to focus such questions. Instead, it clarifies the interpretation of their research as it is relevant to the other approaches, and facilitates comparative and anthropological work. Subsequently, I demonstrate the utility of this framework by using it to reinterpret the existing data on the prevalence of teaching in humans and other animals, and to re-examine what evolutionary theory can tell us about why humans are such prolific and intensive teachers.

4. Reconsidering the prevalence of teaching

4.1. Broad claims and limited evidence

The prevalence of teaching in humans and other animals is a contentious issue. Some researchers make sweeping statements about the omnipresence of teaching cross-culturally, contrasting animal societies with “human societies, where teaching is common” (Thornton & Raihani 2010, p. 297) or claiming that “it is (almost) incontrovertible that teaching is ubiquitous in human beings, which means that, with few exceptions, every person in every society has taught and has been taught by others” (Strauss et al. 2002, p. 1476). In contrast, others claim teaching is uniquely Western (Gaskins & Paradise 2010; Lancy & Grove 2010; Paradise & Rogoff 2009; for review, see Hewlett et al. 2011). In reality, the story may be more complicated. For example, my own field work in Fiji suggests that not only is teaching common there, contrary to previous claims (Hocart 1929; Lancy & Grove 2010; Ritchie & Ritchie 1979), but also, that the prevalence of teaching varies adaptively across domains and life stage (see Kline et al. 2013). This suggests that a study including only some age groups or domains might seem to document an absence of teaching, especially if teaching is defined as Western-style formal verbal instruction, as it has been in the mentalistic approach.

Based on mentalistic or culture-based definitions of teaching, it seems obvious that animals lacking in the ability to consciously represent others' mental states cannot teach. On the other hand, animal behaviorists identify a number of species that display what they call teaching, based on observable behavior. Unlike much of the anthropological field research on teaching in non-Western human populations, these studies typically adhere to strict criteria enumerated in operational definitions (e.g., Caro & Hauser 1992), which may cause false negatives in the study of teaching (Thornton & Raihani 2010). Even so, Csibra (2007, p. 96) critiques established examples of teaching in nonhuman animals on the grounds that they “do not seem to be particularly good examples of the activity that, as humans, we would call ‘teaching’” and suggests further that these behaviors might instead be labeled “scaffolding,” or “charitable information donation.” The problem with these and other claims about teaching-in-general is that each is based on a definition of teaching that is aimed at a focused research question within particular disciplines, and disagreements often result when researchers have not settled on the same approach. The present taxonomy illuminates the distribution of different teaching behaviors across species and societies, and thereby facilitates comparative studies of the psychological adaptations they may entail, their evolutionary function and origin, and their cross-cultural prevalence. In the following two sections, I use qualitative and descriptive observations of teaching behavior in humans and nonhuman animals to illustrate how the framework might be applied. The examples given below are best viewed as candidate instances of teaching behavior, rather than as clear evidence of teaching behavior, in both humans and animals.

4.2. Teaching in humans

Here, I provide examples for each type of teaching distinguished in the present framework. This is not a comprehensive review of teaching in humans, but rather a collection of illustrative examples drawn from throughout the ethnographic literature on teaching in humans. In many cases, these same ethnographic examples have been used as examples of “non-teaching” under culture-based definitions. Ethnographers often describe these behaviors in terms of teachers' mental states, including teachers' expected outcomes. These mental states are sometimes included in the descriptions below. However, in this framework, mental states do not function to distinguish between teaching types, so these details should be regarded only as contextual information, not as diagnostic features of teaching behavior.

4.2.1. Teaching by social tolerance

In humans, it is common for children to learn by observation and participation (Lancy & Grove 2010; Lave & Wenger 1991; Paradise & Rogoff 2009; Rogoff et al. 2003). Where knowledgeable actors tolerate interference or some other cost in order to permit learners to observe or participate, learning-by-observation can be classified as a low-effort form of teaching. This form of learning is common in Fijian villages where children's poking and prodding laundry to be washed, or fish to be cleaned, is not stopped by the adult even though it slows down the adults' work (personal observation). During early stages of a human apprenticeship, targeted tolerance of observation may be the predominant method of teaching (see Coy 1989). Hogbin (1970, p. 143) explains that in Wogeo, New Guinea: “Children are also encouraged to work side by side with their parents even when their efforts are likely to be a hindrance,” and children's interference is tolerated during canoe making, with the expectation that they will learn and be able to contribute in the future.

4.2.2. Teaching by opportunity provisioning

In humans, it is common for adults to assign “chores” to children in many different cultural contexts (Lancy 2008). These chores are often assigned according to the child's physical capabilities (Bird & Bliege-Bird 2002; Bliege-Bird & Bird 2002; Kline et al. 2013), and are sometimes explained solely in terms of their role in the economy of the household, which is one possible explanation for their existence (Lancy 2008). However, chores can also function to provide learning opportunities for a large variety of tasks that adult humans must learn to perform, so may sometimes constitute teaching by opportunity provisioning, especially where they are fused with play (see Bock [2002] for a life history treatment that analyzes these alternative explanations as tradeoffs). Minimally, opportunity provisioning may simply involve providing access to a stimulus that is otherwise unavailable to children. In a description of music lessons in Bali, McPhee (1969) explains that a music teacher simply plays music for his students without modification, and expects that the students will learn as a result of their unique access to his playing. This is opportunity provisioning (rather than teaching by enhancement or social tolerance) because the music is produced only for the students, creating an otherwise nonexistent opportunity for observation. Some storytelling can provide opportunities for naïve individuals to learn from second-hand experience; for example, inexperienced hunters may learn a great deal from the details of stories (MacDonald 2007). When these stories are preferentially provided to naïve learners, they can be considered teaching. In a study of Inuit childhood, Briggs (1998) reports that adults may create dramatic situations or social dilemmas to challenge children, causing them to learn important lessons about social conduct and emotional regulation through these adult-manufactured experiences. Quizzing or question-and-answer sessions may also provide an opportunity for learning to make high-risk decisions without actual danger. Among the Fort Norman Slave, fathers may verbally quiz their sons about travel paths according to hypothetical ice conditions – if boys answer incorrectly, they are “mildly chastised and urged to reconsider” (Basso 1972, p. 40).

4.2.3. Teaching by stimulus or local enhancement

Human adults point, vocalize, gaze at, or touch objects in the presence of infants, sometimes using stereotyped speech (see Brand et al. 2002) and/or gesture (see Fernald & Mazzie 1991). These behaviors often persist until the infant engages with the stimulus. There is some evidence that the degree to which different modalities are used to manipulate children's attention varies cross-culturally (Akhtar & Gernsbacher 2008) and that the overall frequency of stimulus enhancement also differs across populations, along with socio-cultural models of proper childcare (Lancy 2007). Nevertheless, prosodic features of infant-directed speech may be a human universal (Bryant & Barrett 2007). Among the Warao of South America, canoe makers may require boys to be present when they plan to build boats, with the assumption that they will learn through exposure without any instruction (Wilbert 1976). This differs from opportunity provisioning in that the canoe-makers must make the boats whether or not the boys observe them. In Native North American societies, it is common for adults to expect children to learn by seeing (Cazden & John 1971), and adults may specifically direct their attention by pointing or otherwise focusing children's attention, and sometimes naming stimuli (Lee 1967).

4.2.4. Teaching by evaluative feedback

Human adults often encourage or discourage a broad range of behaviors in children (Gaskins & Paradise 2010; Lancy & Grove 2010; Paradise & Rogoff 2009; Rogoff et al. 2003). Adults might slap a child's hand as the child tries to touch a fire, poisonous animal, or other dangerous object. In cultures where “early” walking is undesirable, adults may also physically punish a child who attempts to walk (e.g., among the Beng; Gottlieb 2004). In Nepal, adults give explicit verbal feedback, or may shame children who are considered old enough to know better, in a variety of domains (Levy 1996). Shaming through verbal teasing is common across cultures (see Lancy 2010, p. 87; Lancy & Grove 2010, p. 157; Rogoff et al. 2003). In Javanese society, adults tell stories about children who failed to learn and grew up to be useless to society, to motivate children to learn their chores (Geertz 1961).

Direct active teaching may be common in humans in a range of behavioral domains including food selection and preparation, hygiene, mating behaviors, religious ritual, abstract concepts (like mathematics), and word learning. Further, many populations of humans have formal or semi-formal institutions to promote direct active instruction, including Western-style schools, bush-schools, apprenticeships, or tutor–pupil privileged relationships. Among the Wogeo of New Guinea, orphans are considered to be disadvantaged because they must learn despite a lack of the “deliberate instruction” that most children can expect from their parents (Hogbin 1970, p. 143). Yurok-Karok adults provide “special instruction” for children who seem interested in weaving (Pettitt 1946, p. 46). Pettitt suggests that this is because learning by observation alone would not be sufficient. Social norms and kinship terms of address are often explicitly taught through verbal instruction and labeling of relatives (see Lancy & Grove 2010, p. 150). In New Guinea, Wogeo men verbally critique children's hand-made toy boats, offering theories and solutions for the boats' poor performance, which children then implement and retest (Hogbin 1970).

Direct active teaching is not always verbal. Among the Warao of South America, a father might physically reposition his son's wrist as the son learns to make a canoe, in order to prevent wrist pain (Wilbert 1976). Similarly, a Wogeo man in New Guinea might reposition his son's hands on a digging stick, while also explaining that it prevents back pain (Hogbin 1970). Instruction may also consist of demonstration without extensive verbal explanation, as when Dioula master weavers sit side by side with their apprentices in order to show them simple patterns, after years of more menial and basic tasks (Tanon 1994). Demonstration can also be combined with hands-on practice, as when a Tlingit mother shows her daughter how to weave, then takes turns with her daughter, weaving alternate rows on the same basket (Laguna 1965). This method is also common in Fiji, where one woman reported that this is how she learned to weave from her mother, and how she taught her adopted daughters. Her own mother agreed this is how she taught all of her daughters, and said that her mother taught her the same way – covering a total of four generations (personal observation). As Navajo girls learn to weave, they may also be shown rather than told (Reichard 1934), and any verbal or nonverbal feedback is likely to be immediately relevant rather than abstract.

4.3. Teaching in other animals

As with the review of teaching in humans, the review of teaching in nonhuman animals is not meant to be exhaustive; therefore, some possible examples of teaching may not be included here. In addition, it should be noted that the examples offered below include quantitative studies, some of which adhere to Caro and Hauser's (1992) criteria for documenting teaching, as well as qualitative observations and anecdotal evidence. I include the latter in order to be consistent across discussions of humans and nonhuman animals, and because anecdotal cases presented here may inspire excellent future quantitative studies.

Teaching by social tolerance can also be described as tolerance of information scrounging. High-tolerance behavior such as this has been documented in several nonhuman primate species, though it has not always been characterized as having evolved to facilitate learning in others. In vervets, yellow baboons, mantled howlers, and capuchins (see Rapaport & Ruiz-Miranda 2002), adults are highly tolerant of close observation and even intrusive touching by immatures. Unlike food scrounging among adults, social tolerance of interference by immatures cannot be explained by the potential cost of an ensuing fight (Blurton-Jones 1967), because of the relative weakness and small size of immatures as compared to the adults they are observing – such that the costs of exclusion would predict the opposite pattern (exclude immatures, tolerate more formidable adults) from that predicted by teaching by social tolerance.

Teaching by opportunity provisioning has also been observed in the wild, especially among predators. Wild meerkats hunt and disable prey (Ewer 1969), then present it to meerkat pups. Careful quantitative work shows that adult meerkats disable the prey to different degrees depending on the developmental stage of the pups, as gauged by the pups' food calls. The pups then finish processing the prey and so develop the skills to manage the prey without being exposed to the risk of injury, for instance from scorpion stings (Thornton & McAuliffe 2006). Domestic cat mothers also capture and recapture prey, allowing their offspring to interact with it (observations by Ewer 1969; followed by experimental work by Caro 1980). This has also been observed in river otters (Liers 1951), cheetahs (Kruuk & Turner 1967), and tigers (Schaller 1967). Observational studies suggest that Orcas that are beached along with their offspring sometimes flip live prey toward their offspring, and may do the same in the ocean (Lopez & Lopez 1985).

Teaching by stimulus or local enhancement has been described in animal skill-acquisition, and especially foraging, across species. In river otters, some mothers have been seen to drag their offspring into the water repeatedly until they learn to swim (Liers 1951), and a similar behavior has been observed in the California sea lion (see Caro & Hauser 1992). Quantitative observational data show that long-tailed macaque mothers in one population intensify and exaggerate tool use (in this case, using human hair as dental floss) in the presence of their infants (Masataka et al. 2009). Golden lion tamarin adults have occasionally been observed to locate hidden prey and then emit a food-transfer call (typically used for transferring already-obtained food items), leaving the young to extract the prey on their own before eating it (Rapaport & Ruiz-Miranda 2002). Experimental work shows that domestic hens increase pecking intensity towards palatable food, when their chicks appear to be eating unpalatable food (Nicol & Pope 1996). Experiments also show that tandem-running ants that have located a food source will lead naïve ants to that food source and are responsive to feedback from the naïve ants (Franks & Richardson 2006; Richardson et al. 2007). Quantitative observational data show that Atlantic spotted dolphins chase prey longer and make more referential body-orienting movements in the direction of prey when calves are present (Bender et al. 2009). Observational work suggests that Orcas accompany and may lead offspring toward hunting grounds, and charge alongside their offspring in beaching attempts. The adult does not actually beach in these cases, but the offspring do (Lopez & Lopez 1985). In some of these foraging examples, it would most likely be more efficient for adults to kill the prey and provision offspring directly – which suggests that these behaviors may function to facilitate learning in offspring.

Teaching by evaluative feedback has been observed across a number of nonhuman animal taxa. Female river otters have been seen to nip at the noses of their young if they run ahead, rather than following behind (Liers 1951). In chimpanzees, mothers or older siblings may take unfamiliar food away from an infant or juvenile, restricting its diet, and captive macaques may prevent offspring from exploring novel stimuli, or stimuli known to be dangerous (see Caro & Hauser 1992). However, it is not clear how widespread these food-limiting behaviors are among nonhuman primates (Fairbanks 1975; see also Hikami et al. 1990). Primate mothers of many species encourage their offspring to walk by setting them down and then looking/calling for them to join (e.g., chimps, gorillas, rhesus macaques, free-living yellow baboons, spider monkeys: see Caro & Hauser 1992; see also Maestripieri 1995; 1996). Experimental work shows that adult pied babblers emit a purr-call while provisioning nestlings, such that nestlings learn to associate the call with food (Raihani & Ridley 2008); further work shows that the purr-call encourages fledglings to approach adults in both food-related and non-food-related circumstances (Raihani & Ridley 2007). An alternative explanation for such examples is that giving feedback evolved to alter behavior in real time – for instance, to lower the costs of parental care by eliciting independent locomotion – rather than to facilitate learning. For this teaching type, it may often be necessary to distinguish between direct and indirect benefits to the potential teacher, in order to distinguish between teaching and other explanations for behavior. For example, while feedback from female cowbirds shapes male cowbird song development and content, there is no evidence that female fitness is dependent upon males modifying their song (Smith et al. 2000). As a result, this kind of feedback resulting in behavioral modification is more likely to have evolved via intersexual selection than through selection on teaching behavior.

4.3.1. Direct active teaching

There are no observed examples of direct active teaching in nonhuman animals, with the exception of Boesch's (1991) anecdotal account of two potential instances of “demonstration” of nut-cracking techniques by chimpanzee mothers. While much of the evidence given above is similarly anecdotal, the lack of similar observations of direct active teaching in chimpanzees or in any other nonhuman species suggests that these observations should be cautiously interpreted.

5. Psychologies of teaching

The present taxonomy of learning problems paired with specific adaptations for teaching generates novel hypotheses about the landscape of possible teaching psychologies. Specifically, this framework suggests that a suite of psychological mechanisms could underlie each of the different teaching types, and that for at least some teaching behaviors, more than one evolved psychological mechanism is possible. In addition, it suggests that pupil and teacher psychologies should be considered separate adaptations that may have coevolved under selection pressures for particular teaching types. For some teaching types, pupil psychologies will match those of social learners, as argued by Hoppitt et al (2008), while teacher psychologies will be adapted for each teaching type.

An overarching problem for teachers and pupils alike is to limit teaching to information that is useful for the pupil's learning process. In other forms of social learning, this is a problem for the pupil, but, by definition, not for the model, who simply serves as a stimulus. There are three categories of information to consider in this context (and in communication more broadly): prior information which the recipient already knows, novel information which has no utility, and novel information which is useful and/or in some way connected to the pupil's prior knowledge (Sperber & Wilson 1995). The latter type is useful in the learning process, and considered relevant information (Sperber & Wilson 1995), because it is novel and can be interpreted by the recipient, who may use it to generate new inferences. By definition, only relevant information will promote pupil learning.

5.1. Pupil psychology

For teaching to have any adaptive consequences, the pupil's psychology must be sensitive to relevant information in the context of teaching. For teaching by social tolerance and by opportunity provisioning, pupils are already motivated to learn about relevant stimuli, and sensitive to relevant information (in both cases, the learning problem is access rather than interest). For these teaching types, teacher behaviors evolved to take advantage of pre-existing behaviors or motivations in pupils, and pupil behaviors and motivations would need little to no modification for teaching to be effective, at a minimum.

In teaching by local enhancement and by evaluative feedback, pupils must be sensitive to teacher cues and feedback that indicate relevant information. This is not a teaching-specific psychological adaptation in pupils, because the same kinds of cues are germane to other forms of social behavior. For example, social learners may already attend to others' demonstrated interest and to positive or negative social reinforcement. In all of these teaching types, the adaptation is on the part of the teacher, who displays these behaviors preferentially toward naïve learners, thereby facilitating learning.

In contrast, direct active teaching is characterized by manifestation of relevant information by the teacher to the pupil, as well as the pupil's interpretation of that information as generalizable. Because direct active teaching is a solution to the frame problem, it is expected to evolve only when pupils have very limited or very costly alternative individual or social learning mechanisms available. As a result, in direct active teaching, the pupil's only indication that information is relevant comes from the teacher.

There are a number of hypothetical pupil psychologies that would make this possible. First, in mentalistic approaches to teaching, pupils must theorize about the teacher's motivations and recognize that a teacher intends to teach. In this model of the pupil's psychology, a pupil who recognizes teaching may be receptive, or may reject a teacher's influence (see sect. 6.2 on skeptical pupils). This model also predicts that direct active teaching will be limited to species with theory of mind capacities, since a pupil's receptiveness to direct active teaching depends upon recognizing the teacher's intent.

Alternatively, teachers and pupils may have evolved a system of communication to reliably indicate the exchange of relevant, generalizable information. This suggests that direct active teaching is a result of teacher behavior creating new selective pressures on pupils. Such a process of coevolution may have created a novel teaching type in humans, derived from ancestral ones shared with nonhuman animals. Csibra and Gergely (2006) argue that such a special communication system is a defining feature of (human) teaching: When teachers mark their behavior using ostensive cues (e.g., eye gaze, use of pupil's name, soliciting joint attention, etc.), pupils act as if they expect to receive relevant information that should be learned. This does not necessarily require that teachers or pupils consciously theorize about the contents of the other's mind. It only requires that ostensive cues reliably result in learning receptivity on the part of the pupil. This is not a novel style of reasoning that applies only to teaching. For example, animals may respond to behavioral solicitations of play or grooming, without theorizing about the mind of the conspecific making the request. This suggests that direct active teaching need not be restricted to species with theory of mind capabilities, at least not from the perspective of the evolved psychology of the pupil.

5.2. Teacher psychology

For most teaching types, teachers can build upon existing pupil behaviors and motivations to shape pupil learning. However, teachers must still have psychological adaptations that make teaching more likely in situations in which it will be adaptive. This means that teachers must be selective in their efforts, teaching only those pupils from whose learning the teacher will also benefit (see sect. 6). All else equal, teachers must provide teaching when it is beneficial for the pupil – and that means providing information when it is relevant.

Again, theory of mind is one solution to the problem of relevance. For all teaching types, teachers could use theory of mind to assess what pupils know or do not know, and could subsequently provide information that is useful given the pupil's mental state. Theory of mind capacities may be necessary when the teacher's explicit proximate goal is to produce conceptual change in a pupil – shaping the pupil's understanding to match a predetermined concept as known by the teacher (e.g., Carey 2000). In other cases, the need for theory of mind capacities is less clear. For example, theorizing about the pupil's mind may only be necessary when there is expected to be great heterogeneity in pupils' knowledge states, and when their knowledge status is not tied to any cues that would be reliably present (over an evolutionary time span).

In cases when pupils' learning needs are reliably tied to cues of age or maturity, teachers do not need to assess a particular pupil's mental state in order to provide relevant information. Instead, teachers may simply provide information that is typically relevant to pupils at a particular life stage or in a given situation. For instance, human adults in many societies interact with pre-verbal infants through modified infant-directed language (see review by Snow 1995) and gesture (Iverson et al. 1999). The proximate mechanism for this is not necessarily that adults theorize about the contents of infants' minds and strategically expose them to intensified emotional expression and to age-relevant vocabulary. Instead, it may be that the adult's own emotional reaction to young children, or their desire to see babies react, directly motivates behaviors such as motherese or “baby talk,” and other infant-directed behaviors. At the ultimate or evolutionary level, the motivational system matters very little, so long as children are exposed to relevant stimuli. The same sort of mechanism is evidently at work in meerkats. Adult meerkats provide pups with dangerous prey (scorpions), and depend primarily on vocal cues of immaturity to determine how much to disable the prey before provisioning (Thornton & McAuliffe 2006).

Alternatively, teachers may provide relevant knowledge based on pupil-specific cues that depend on a metacognitive understanding of a behavior or task. Meta-representation for the purposes of teaching means that the teacher holds representations of her own knowledge, which is itself in the form of representations (Sperber 2000). In common parlance, this is “knowing about” or having conscious knowledge of a task. Csibra and Gergely (2006) argue that teachers must meta-represent their own knowledge, analyze it, and selectively provide information to pupils that is relevant to the task at hand and adjusted to the learner's performance. As long as teachers meta-represent their own knowledge, they could use cues from pupil behavior or from the context to deduce which bits of their knowledge are relevant, without employing theory of mind (Csibra & Gergely 2009; 2011). When relevance is based on behavioral (as opposed to mental) assessment, the teacher does not distinguish between what the pupil knows and what the pupil does, and only provides feedback on aspects of the behavioral performance of a task.

There is some evidence for a role for metacognition in teaching. Conscious access to one's own knowledge may play a minor role in language-learning (Karmiloff-Smith 1985) and a more important role in what appears to be metacognition of metacognitive instruction – instruction on learning how to think about one's own metacognition in order to learn more effectively (Adams et al. 1998; Gourgey 1998). There is also evidence that metacognitive awareness of teaching may make children more sensitive and sophisticated teachers (Davis-Unger & Carlson 2008).

Intuitively, it may seem far easier to theorize about another's mind than to evaluate that person's task performance through metacognition and behavior-matching. However, it is important to realize that while theory of mind may be an “easy” task for humans, it is not necessarily the case for other animals. Both theory of mind and metacognition are hypothetically feasible solutions for the problem of providing relevant information from the perspective of the teacher. Alternatively, teaching behaviors that are produced reliably in response to a cue (of pupil age or maturity, for example) may not require either theory of mind or metacognition. This suggests that predictions about the role and prevalence of teaching across species that are based only on species differences in theory of mind or metacognition may not be robust. It may require more than considering cognitive constraints to explain why some teaching types are more common than others, some species are more avid teachers than others, or why direct active teaching seems to be so rare. Another factor to consider is the cooperative nature of teaching.

6. Teaching as cooperation

Teaching is a cooperative behavior, as it benefits both the pupil and teacher through the pupil's improved social learning outcomes. This distinguishes teaching from other forms of social influence or reinforcement, which are better explained by direct benefits to the actor, or an actor's self-interested preferences. As a result, teachers may only evolve when assortment between pupils and teachers is possible – by kinship, reciprocity, or spatial assortment, for example. Formal models suggest that teaching (in general) should be exceedingly rare, and limited only to information that pupils cannot learn by other social or individual learning mechanisms, and to information that can be learned from relatives (Castro & Toro 2014; Fogarty et al. 2011). This section goes beyond those analyses to examine the costs and benefits of specific teaching types, and to consider two cooperative problems that may act as barriers to the evolution of teaching. First, from the perspective of the teacher, why facilitate another's learning? Second, from the learner's perspective, why believe what another teaches you? Thus far, research on teaching as a cooperative problem has not considered the problem from the perspective of the pupil.

6.1. Teacher as donor

From the perspective of a teacher, teaching may be costly in terms of (a) the effort required for teaching, and (b) future competition with skilled or knowledgeable pupils. In addition, teachers can (sometimes) be thought of as information donors (see Thornton & Raihani [2008] for review), especially if that information is otherwise costly to acquire. If there are costs of teaching, then there must be some benefit to the teacher, in order for teaching to evolve. In teaching, these benefits are indirect and due to the pupils' gains in learning – in contrast to other forms of social reinforcement. Because the costs and benefits vary across different teaching types, the ways and degree to which benefits accrue to teachers should also be quite variable. For example, teachers may be related to their pupils, or pupils may be future allies or mates.

In addition, the relative costs and benefits of teaching will matter: teaching behaviors that require little teaching effort should be the most common, both across and within species. This may have greater predictive power than considering the cognitive capacities of a species, since more than one possible psychology could enable each teaching type (see sect. 5). A priori, lower-effort forms of teaching behavior include: teaching via social tolerance, teaching via evaluative feedback, and teaching via local or stimuli enhancement. Higher-effort forms of teaching include opportunity provisioning – because the teacher must modify the environment and transform a learning situation – and direct, active teaching, because teachers may completely cease their ongoing activities in order to verbally and/or gesturally communicate abstract, generalizable knowledge to pupils. Of course, these a priori assertions ought to be tested in terms of measurable costs, such as time, opportunity, or energetic costs to teachers. The brief review of teaching behavior given above does seem to support this prediction – all else equal, the less costly forms of teaching are apparently more common across species, and the more costly forms more rare. Observational data from Fiji show that relatedness predicts teaching between child–other dyads, and that high-cost teaching types are less common (Kline 2013). Further, Fijians are more likely to report teaching as a means of learning for domains transmitted from parents to offspring, and for domains that are highly important to success in village life (Kline et al. 2013).

If teachers are donors, they may recoup costs through indirect fitness benefits, by selectively teaching biological kin. Based on Hamilton's (1964) rule, teaching should evolve only when the cost to the pupil for independent learning, discounted by the teacher/pupil relatedness, is less than the cost to the teacher (Dewar 2002; Hoppitt et al. 2008). This suggests that teaching should be common among closely related individuals, and in species that breed cooperatively (Burkart & van Schaik 2010; van Schaik & Burkart 2010). Alternatively, it may be common where background relatedness within groups is high, and teaching happens within groups while resource competition happens between them (Taylor 1992). Where resource competition is primarily within the groups, background relatedness would have no such effect (Taylor 1992). This is consistent with the literature review above: Relatively costly teaching via opportunity provisioning occurs in meerkats, which breed cooperatively (Thornton & McAuliffe 2006); other forms of teaching happen in pied babblers and tandem-running ants (see sect. 4). High-cost forms of teaching, including direct active teaching, seem to be most common in humans, who have also been described as cooperative breeders (Hrdy 2008). In addition, humans often live in highly interdependent communities, which may shape caretaking, sharing, and cooperative behavior in the same ways as cooperative breeding does (Nettle et al. 2011; Roberts 2005; Tomasello et al. 2012). Such high-effort teaching is rare or absent in closely related chimpanzees (cf. Boesch 1991), which are not cooperative breeders.

6.2. Pupil as skeptic

Teaching is, in one form or another, modification of one individual's behavior by another's influence. In this sense, the pupil is vulnerable to the same pitfalls that any social learner experiences. However, pupils may be especially at risk of manipulation by “cheating” teachers who deceive pupils for their own benefit, because the benefits to the pupil depend upon some measure of credulity. This necessary trade-off suggests that pupils should practice epistemic vigilance to guard against inaccurate or deceptive teaching that harms their fitness (Sperber et al. 2010). Even when teachers are the pupil's parents, pupil and teacher interests may differ – for example, parents often prefer to distribute resources across multiple offspring, while each individual offspring may prefer not to share with their siblings. This possibility should lead to counter-strategies used by skeptical pupils to resist manipulative teachers, while maintaining receptivity to credible teachers. Some of these strategies overlap with strategies for choosing good models for social learning more generally (for a review see Henrich & McElreath 2003), while others are unique or especially applicable to teaching behavior. Pupil skepticism may be conscious and explicit, or it may be captured in learning heuristics. Heuristics may include biases based on social context of transmission, such as cues about the model or teacher's abilities or success (Boyd & Richerson 1985; Henrich & McElreath 2003). Alternatively, they may be based on the content of what is being learned, based on its plausibility or compatibility with what the pupil already knows (Boyd & Richerson 1985; Henrich & McElreath 2003; Sperber 1996).

The teaching types outlined above differ in their potential for manipulation of pupils by teachers, which will shape the type of skepticism expected to evolve in pupils. Teaching types that work by eliciting, directing, or limiting the learner's individual learning are generally least vulnerable to teacher abuse. This includes teaching via social tolerance, teaching by stimulus/local enhancement, and teaching by opportunity provisioning. In teaching by social tolerance, pupils are driven by internal motivations for social learning to seek access to another's behavior, and the “teaching” takes the form of tolerating the learner's interference. In order to achieve deception, a teacher would need to anticipate the learner's attention, and then perform the behavior in a way that would mislead the pupil to the advantage of the teacher. This type of deception would be cognitively demanding with limited effectiveness, especially if pupils have mental mechanisms for interpreting others' actions and goals, or for checking socially learned information against personal experience or observation. Similarly, teaching via stimulus or local enhancement is not prone to manipulation by selfish teachers, because teachers do not have direct influence on what pupils learn, but only facilitate pupils' individual learning by facilitating or directing their interest toward certain stimuli or sources of information. Teaching by opportunity provisioning functions similarly, as it depends on existing motives and nascent skill sets in the pupil. For example, when social predators such as meerkats provide their offspring with maimed prey, they have little effect on the pupil's motivations or the content of pupils' learning, but allow the learner to gain experience that they could not achieve safely on their own. Pure opportunity provisioning will always have these properties. Because humans often mix opportunity provisioning with other types of teaching (e.g., teaching via evaluative feedback, direct active teaching) in childhood chore assignments, opportunity provisioning contexts may be more prone to deception in humans – but this question is separate from the behavior of teaching via opportunity provisioning. For this category of teaching types, there is little benefit to pupils in being skeptical at all, as the risk of deception is very low, perhaps no greater than during any act of individual learning.

Teaching by evaluative feedback is more prone to abuse by selfish teachers, because its efficacy depends upon the teacher's modification of a pupil's behavior, in that effective teaching by evaluative feedback serves to change the likelihood of repeating a behavior that already exists in the pupil's repertoire. Detecting deception in teaching via evaluative feedback could be achieved if pupils are selectively receptive to encouragement or discouragement that is consistent with their intrinsic motivations, but only magnifies it (a content bias). Alternatively, pupils might be receptive to this type of teaching only from trustworthy teachers – for instance, close relatives who have little conflict of interest (a context bias). Teaching via evaluative feedback is unique because it is low-cost for teachers, but high-risk for pupils. As such, this teaching type should be relatively common in its use by teachers, but would be often accompanied by resistance and skepticism by pupils. Resistance may be less pronounced where content or context biases support the teacher's influence.

Direct active teaching is exceptionally vulnerable to exploitation by selfish teachers because it is the only teaching behavior that may include the direct communication of abstract knowledge, entirely independent of related behavioral observation. (Though note that gestural communication, or direct active teaching through demonstration, are both possible.) Since direct active teaching especially facilitates learning where pupils have little background knowledge to solve the frame problem, content biases will often be unhelpful. Instead, pupils may rely heavily on context biases dependent on characteristics of the teacher (Henrich & McElreath 2003).

As mentioned, learners may be more likely to trust related teachers, because inclusive fitness benefits mean they are less likely to deceive pupils. Pupils can also assume that kin have especially relevant knowledge, because they share genes and are likely to experience similar environments (Boyd & Richerson 1985; 1988). Some experimental evidence supports this conjecture: In the absence of information about accuracy, American 4–5-year-old children prefer their mothers' verbal testimony about object names and functions over a stranger's (Corriveau et al. 2009) – though when faced with accurate testimony from a stranger and inaccurate testimony from their mothers, only some children persist in favoring mother's testimony.

Without direct evidence, pupils may also choose to learn from teachers who are known to be accurate and reliable more broadly. Verifying teachers' claims against reality means the teacher is both knowledgeable and – at least in the past – truthful. This matters because learning from an honest-but-wrong teacher can be equally deleterious to a pupil's fitness as learning from a manipulative teacher. Studies with children in the United States suggest that 4-year-olds are capable of distinguishing teacher quality by tracking model accuracies, as well as preferring previously accurate models (Koenig et al. 2004) and knowledgeable models (Koenig & Harris 2005; Sabbagh & Baldwin 2001), and generally preferring accurate models when learning about novel object functions (Birch et al. 2008). Children also prefer accurate models over familiar ones when the models disagree (Corriveau & Harris 2009), and will accept testimony from a presently well-informed model who was previously inaccurate due to being misinformed in the past – but they will not “trust” information from a model who was both well-informed and inaccurate in the past (Nurmsoo & Robinson 2009). Children also seem to be capable of learning according to statistical evidence drawn from multiple observations of similar events, and can integrate this with information about the model (Buchsbaum et al. 2011). (For an extensive review of the developmental psychology of children's learning in pedagogical contexts, see Skerry et al. 2013). These mechanisms only apply when the pupil can evaluate teacher accuracy or gauge a teacher's reputation for being trustworthy. Outside the laboratory, accuracy and knowledge levels may be difficult for naïve learners to assess, especially when pupils depend upon teachers for learning about dangerous or causally opaque domains.

When direct verification of teacher accuracy is not possible, pupils may use alternative strategies. Some of these strategies may have evolved for social learning more broadly (see Boyd & Richerson 1985), but would also apply to cases of teaching. Simple content-based heuristics such as conformist bias (Henrich & Boyd 1998) or prestige bias (Henrich & Gil-White 2001) could be particularly important for direct active teaching when the content of what is being taught is divorced from observable outcomes. For example, conformist bias explains the believability of social gossip, where information heard from more sources is rated as more credible (Hess and Hagen 2006). Similarly, children are more likely to believe testimony endorsed by two informants in competition with a single one (Corriveau et al. 2009). Learners may also be credulous of teachers who are respected by third parties (Boyd & Richerson 1985). Prestige bias is thought to allow naïve learners to assess the quality of teachers based on the number of followers and extent of their deference (Henrich & Gil-White 2001). Consistent with this argument, 3- to 4-year-old Canadian children prefer to imitate models who were observed by others rather than models who were not watched (Chudek et al. 2011).

Finally, in some cases of direct active teaching, neither direct verification of content to be learned, nor properties of the learning context may be available to pupils. In these conditions, when teachers profess beliefs in the abstract, a pupil may avoid manipulation by learning only from teachers who seem to act in accord with their own stated beliefs (Mascaro & Sperber 2009; Sperber et al. 2010). For example, knowledge of particularly rich fishing grounds was a closely kept secret among highly competitive fishermen in pre-contact Hawaii – so much so, that if a deep-sea fisherman discovered a new productive location, he might share this information only with his children, even at the threat of physical assault (Kamakau 1976). In this situation, pupils should be skeptical of any information received; fishermen might misdirect others in order to protect their own preferred fishing grounds from becoming publicly known. Pupils should only believe fishermen who behave in ways that would be costly if their actual beliefs were in conflict with their stated beliefs – for instance, those who travel long distances to exploit particular fishing grounds. The same reasoning works for other behaviors, like not eating tasty but supposedly poisonous fruit. If such behaviors do not already exist, credibility-enhancing displays (CREDs) may culturally evolve along with beliefs, because they facilitate the spread of otherwise untestable beliefs (Henrich 2009). For direct active teaching, comparing a teacher's observed behavior against his or her professed belief may serve as a proxy for testing the validity of their beliefs. For example, imagine a number of teachers who claim that pleasurable behaviors like sex or alcohol consumption are spiritually harmful. They could be making these claims in earnest, or could be making false claims to manipulate others' behavior. If a teacher abstains from sex and alcohol, this can serve as evidence of a teacher's credibility, since it is only worth giving up pleasurable activities if one believes the negative consequences are real. (This is not evidence that a belief is true – only that the teacher believes it). On the other hand, if a teacher engages in sex and alcohol consumption, learners can infer that the teacher does not sincerely believe that these are harmful and against his or her best interest. The evolution of credibility-enhancing displays to highlight this kind of distinction may be especially prevalent for the teaching of beliefs that are difficult or dangerous to verify directly, including beliefs about disease transmission, poisonous foods, or the supernatural.

7. Why humans teach more than other animals

The constraints of cognition and cooperation do not seem sufficient to explain why direct active teaching appears to be limited to humans, or why humans seem to be more prolific teachers in general in comparison with other species. How, then, can this be explained?

Based on the existing data and in light of the new framework proposed above, there are several possibilities. First, it may be that this is not a real difference. It could be that other species do use direct active teaching, but that because of a bias favoring the study of teaching in species with theory of mind and general cognitive capacities, researchers have been focusing on teaching in the wrong set of species. Similarly, it may be that there are simply more studies of human teaching – and a lower bar for “establishing” teaching in humans – than for any other species, so that a better sample might document teaching in nonhuman species. A related explanation is that existing operational criteria are too stringently focused on cost/benefit analyses, so that some cases of direct active teaching may go unreported. This explanation seems plausible after considering the last 20 years of studies on social learning and culture in nonhuman animals. This has changed our understanding of the varied ways in which chimpanzees – our nearest ape relative and perhaps best-studied comparison species – learn socially (see Whiten 2011). The framework presented here could be used across species and across human populations, for structured quantitative comparisons of teaching behavior (see, e.g., Kline 2013).

A second explanation of the empirical record on teaching is consistent with existing mentalistic definitions: It may be that direct active teaching is uniquely human, and that this is due to our species' derived cognitive capacities. The above-mentioned framework suggests that direct active teaching could depend upon either theory of mind or metacognition and behavior-matching capacities (or both). This suggests that researchers should not limit the study of teaching to species with forms of mind-reading or theory of mind, because it is not an a priori necessity for behavior that functions as teaching to evolve. Theory of mind and degrees of mind-reading capacities are notoriously difficult to identify even in species that are closely related to humans (see, e.g., Call & Tomasello 2008; Heyes 1998; Penn & Povinelli 2007), which makes a theory of mind–centered approach impractical, as Caro and Hauser (1992) point out. This is in part because of a focus on false belief tasks as the test for theory of mind, a practice that is itself of debatable value (Bloom & German 2000).

Rather than focus on psychological prerequisites for teaching, comparative researchers could gain more ground with a focus on the socio-environmental niches in which particular teaching types are likely to evolve. The framework proposed restructures the study of teaching to better link psychological mechanisms to particular teaching types, each of which is based on an informational context that poses an adaptive challenge. To study a particular psychological mechanism, researchers could focus on species where that type of adaptive challenge is common in the environment, and where the social organization suggests it is likely to evolve.

As a general direction, this might include species with cooperative breeding, interdependence, or heavy parental investment in offspring. It might also include species that depend on socially learned information for adapting to a spatiotemporally variable environment. It may be more profitable to compare humans with species that depend on complicated, socially learned behavior, for example, cetaceans (Rendell & Whitehead 2001; see also Schusterman et al. 2013) or social carnivores (Smith et al. 2012), rather than nonhuman primates. This shift in focus may lead to a change in the data researchers collect on direct active teaching, as well as other teaching types.

Finally, the most theoretically valuable explanation for why direct active teaching is observed only in humans is that ours is the only species in which it is adaptive; that is, ours is the only species in which the frame problem is fitness-relevant, and that is sufficiently cooperative for this form of teaching to evolve. I argue that this is because humans (and only humans) evolved in the “cultural niche,” such that our species depends to a great degree on cumulative cultural adaptations too complex for any one individual to create on his or her own, which coevolved with species-specific cultural capacities (Boyd et al. 2011). If this is the case, then direct active teaching may be a derived form of teaching that coevolved with culture, for the purpose of transmitting hard-to-learn cultural adaptations. Testing this explanation requires an integrated, unifying framework of the study of teaching across species and societies like the one proposed in this article.

8. Conclusion

To date, the study of teaching has been conducted from three different perspectives, each of which focuses on a particular research question and tailored definition of teaching, often at the expense of integrating each study with the broader range of variation in teaching behavior across cultures and taxa. This diversity of research questions is a strength, especially when definitional disagreements are put aside in favor of an integrative framework for comparative and anthropological work, such as the one proposed here. This article refocuses the study of teaching on categorizing and studying an array of teaching types, based on a framework of adaptive problems that teaching solves for social learners. It can be used to integrate the empirical literature on teaching in human and nonhuman animals, and highlights two major areas of inquiry in the evolution of teaching: psychological underpinnings, and cooperative dilemmas.

In light of this new framework, it makes little sense to focus research on establishing that a species or specific human population “has teaching” or “does not have teaching,” because teaching – like social learning – is an array of adaptive behaviors that function to solve a set of problems for learners. Instead, researchers should identify the adaptive problem that teaching solves in a particular context or environment of evolutionary adaptiveness, and pursue research on the mental, functional, and cultural features of teaching based in this shared framework. Teaching should be distinguished from other forms of social influence on a case-by-case basis, with operational rules tied to the theoretical definitions at work, situated in the particular species and behavioral domain of study. Developing these practical methods requires differentiating among the social learning contexts in which different teaching types may evolve, because the form of teaching behavior will depend upon its specific adaptive function. This framework can illuminate the adaptive design and evolutionary origins of teaching behavior, by providing a unifying approach through which to study the form, function, and prevalence of different teaching behaviors across human and nonhuman populations.

ACKNOWLEDGMENTS

This research was supported by a National Institutes of Health (NIH) Challenge Grant, National Science Foundation Doctoral Dissertation Improvement Grant, a Leakey Foundation General Research Grant, and an International Society for Human Ethology Owen Aldis Grant. Rob Boyd, Clark Barrett, Susan Perry, Joan Silk, Joe Henrich, and Matthew Gervais all provided helpful commentary, as did members of the UCLA Center for Behavior, Evolution, and Culture, the UCLA Experimental Behavioral Anthropology Lab group, and the Arts and Humanities Research Council Culture and the Mind Project. This article has also benefited from the comments of Alex Thornton and a number of anonymous reviewers, who were generous with their time and thoughts.

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