1.1. The 1940s and 1950s

During the 1940s and 1950s, researchers in later language development published innovative studies that generated new information. They also paved the way for future studies on topics such as proverb understanding, word knowledge, and the word learning process.

Proverbs are figurative expressions that can serve a variety of pragmatic functions. Examples include offering encouragement (e.g., Every cloud has a silver lining), warning a person of danger (e.g., Look before you leap), or commenting on a situation (e.g., When the cat’s away, the mice will play). To fully understand a proverb, an individual must be able to relate the expression to events in the broader context, a process of metaphorical mapping that requires analogical reasoning. For example, to interpret “Every cloud has a silver lining,” uttered by a parent to a disappointed child, it is essential to know that in this case, the “cloud” refers to a cancelled birthday party and that the “silver lining” refers to a joyful replacement activity.

Watts (Reference Watts1944) conducted the first known study in this area. Children who were ages 11 through 14 years old completed a written multiple-choice task that examined their comprehension of proverbs. Each proverb (e.g., Empty vessels make the most noise) was accompanied by four interpretations, only one of which was correct (e.g., The people who talk the most often know the least). Each child read the proverb and the answer choices and selected the best interpretation. Although accuracy improved in relation to age, even the oldest children struggled with the task, perhaps because of the absence of supportive contextual information.

Gorham (Reference Gorham1956) later conducted a large-scale study to examine the understanding of proverbs using a written multiple-choice task that consisted of 40 questions. The participants in his study were children, adolescents, and young adults (n = 651) who were ages 10 through 21+ years old. Although the youngest group had difficulty with the task (mean accuracy = 33%), age-related improvements occurred during adolescence and into adulthood (mean accuracy = 81%).

Similar findings were reported by Richardson and Church (Reference Richardson and Church1959) who asked a group of 9- to 12-year-old children (n = 47) and a group of adolescents and young adults (n = 30) to explain in writing the meanings of seven proverbs (e.g., You cannot teach an old dog new tricks). Although the youngest participants often interpreted the proverbs literally (e.g., “It means it’s hard for the dog because he’s so old”), gradual age-related gains occurred in the participants’ ability to interpret the proverbs figuratively (e.g., “It means it’s futile to try to change old habits”). Together, these three studies provide evidence of quantitative and qualitative advancements in the understanding of proverbs between late childhood and early adulthood.

Regarding the development of word knowledge, Feifel and Lorge (Reference Feifel and Lorge1950) conducted a large-scale investigation in children who were ages 9 through 14 years old (n = 600). Each child was asked to explain the meanings of 45 words. The study revealed age-related improvements in the number of words that were accurately defined (“Skill means you can do something well”), a quantitative change, and in the ability to define words using their superordinate category term (e.g., “An orange is a fruit”), a qualitative change that reflects higher-order reasoning.

In a study that examined development of the word learning process, Werner and Kaplan (Reference Werner and Kaplan1952) tested the ability of children ages 8 through 13 years old (n = 125) to infer the meanings of unfamiliar words that occurred in sentences, using a multiple-choice task. Those researchers found that during this age range, children’s understanding of the words gradually improved as they learned to use context clues to infer meaning, a strategy called contextual abstraction. When language is analyzed in this way to discern meaning, this reflects metalinguistic competence.

Metalinguistic competence, the ability to step back, reflect, and examine language as an entity itself (Gombert, Reference Gombert1992), is a driving force in all aspects of later language development. That is, as children grow older, their expanding analytical skills enable them to make sense of the smaller components of language – including its phonology, morphology, syntax, semantics, and pragmatics – leading to greater insight into the structure, function, and meaning of language.

1.2. The 1960s

Studies of lexical development continued into the 1960s with researchers examining the understanding of different types of words. Asch and Nerlove (Reference Asch, Nerlove, Kaplan and Wapner1960), for example, examined children’s understanding of double function terms. These are polysemous words that have both a concrete physical meaning and a related abstract psychological meaning (e.g., sweet, cold, bright). In their study, children ages 3 through 12 years old (n = 50) were interviewed individually and asked to explain the meanings of some common double function terms. The results showed that the physical meanings were well understood even by the youngest children but that an understanding of the more difficult psychological meanings developed more slowly. Moreover, only the oldest group of children could explain the relationship between the physical and psychological meanings of the terms (e.g., “Bright stars and bright people both stand out”).

Other types of words that were studied during the 1960s included different types of connectives. Robertson (Reference Robertson1968), for example, examined the development of coordinating (e.g., but, and, for) and subordinating (e.g., because, if, when, although) conjunctions drawn from textbooks written for upper elementary grade students. An experimenter-designed task was administered to students from grades 4, 5, and 6 (ages 9–12 years old; n = 402). The task consisted of 150 multiple-choice items where an incomplete sentence that contained a conjunction was presented (e.g., The man held the rod and…) and the student was asked to select from four options the best way to complete the sentence (e.g., the horse jumped over it). Results indicated a pattern of gradually increasing accuracy on the task, with mean scores of 57%, 66%, and 75% earned by the fourth-, fifth-, and sixth-grade groups, respectively. Hence, not even the oldest students showed mastery of the conjunctions. The study also showed that accuracy on the task was associated with students’ scores on measures of reading, writing, and listening. This led the author to conclude that mastering conjunctions is important for literacy attainment.

Related to the use of coordinating and subordinating conjunctions is the development of syntax, which focuses on growth that occurs in the length and complexity of sentences. Early studies of syntactic development beyond the preschool years were conducted during the 1960s by researchers such as Hunt (Reference Hunt1965), Loban (Reference Loban1963), and O’Donnell et al. (Reference O’Donnell, Griffin and Norris1967). Collectively, those researchers documented the existence of gradual age-related increases in the length and complexity of sentences produced in spoken and written language during the school-age and adolescent years. Hunt, for example, studied the development of syntax by examining essays written by students from grades 4, 8, and 12 (ages 9–18 years; n = 54). In the course of his research, he designed a metric called the terminable unit (T-unit) which consists of an independent clause and any subordinate clauses that are attached to it. A writer’s mean length of T-unit (MLTU) is therefore calculated by dividing the total number of words produced in an essay by the total number of T-units it contains. A related metric, the communication unit (or C-unit), was later designed by Loban (Reference Loban1976) to examine syntactic development in spoken language produced during a conversation or other genre (e.g., explanation, narration). Unlike the T-unit, the C-unit may include utterances that are less than an independent clause; otherwise, it is identical to the T-unit. Both MLTU and mean length of C-unit (MLCU) continue to be widely used by researchers and clinicians to chart age-related increases that occur in the use of complex syntax in the written or spoken language, respectively, of school-age children, adolescents, and adults. They are also used to measure the level of syntactic competence of individual students.

During the 1960s, researchers also began to study the development of persuasive discourse (e.g., Flavell et al., Reference Flavell, Botkin, Fry, Wright and Jarvis1968; Wood et al., Reference Wood, Weinstein and Parker1967). Flavell et al., for example, examined the persuasive strategies used by children and adolescents ages 8, 13, and 16 years (n = 20 per group) to convince another person to perform some action (to purchase an item or to give them a gift), using hypothetical role-playing tasks. Results showed that the 13- and 16-year-olds generated a greater number of different arguments to convince the listener than the 8-year-olds, a quantitative change, and that the older participants presented their arguments in a more organized fashion, a qualitative change. Another qualitative change that occurred was an age-related increase in the use of arguments that appealed to the listener’s perceived needs, interests, and values, reflecting growth in social perspective-taking, the ability to infer others’ viewpoints.

1.3. The 1970s

Studies of spoken and written persuasion continued during the 1970s (e.g., Clark & Delia, Reference Clark and Delia1976; Crowhurst & Piche, Reference Crowhurst and Piche1979; Delia et al., Reference Delia, Kline and Burleson1979; Finley & Humphreys, Reference Finley and Humphreys1974; Piche et al., Reference Piche, Rubin and Michlin1978; Rubin & Piche, Reference Rubin and Piche1979), with researchers documenting age-related advancements in various persuasive skills in school-age children and adolescents, including social perspective-taking. Researchers also began to study another genre, narrative discourse (i.e., storytelling), and the concept of story grammar was introduced (Mandler & Johnson, Reference Mandler and Johnson1977; Stein & Glenn, Reference Stein, Glenn and Freedle1979). A story grammar is an organizational framework that a storyteller can use when generating or retelling a narrative coherently. Typically, it contains seven essential elements: the setting, characters, episodes, initiating events, attempts, outcomes, and internal responses.

Using that framework, Stein and Glenn (Reference Stein, Glenn and Freedle1979) examined the development of narrative speaking in children who were ages 6 and 10 years old (n = 48). Each child listened to and retold an unfamiliar folktale that contained the basic story grammar elements. Results indicated that the 10-year-olds included more information in their retells and were more likely to mention the characters’ internal responses than the 6-year-olds. However, both groups mentioned all other story grammar elements in their retells and retold the events of the story in the proper sequence.

During the 1970s, researchers also began to study the development of linguistic ambiguity, the ability to detect the double meanings of words, phrases, and sentences. In an early study, Kessel (Reference Kessel1970) examined how well children ages 6 through 11 years old (n = 50) understood sentences that represented different types of ambiguity, including lexical (e.g., The boy picked up the bat) and syntactic (e.g., The chicken is ready to eat). With a lexical ambiguity, the double meaning stems from the presence of a word that has two meanings (a baseball bat or an animal bat). With a syntactic ambiguity, the subject of the sentence (chicken) is either an agent (the chicken will eat its dinner) or an object (the children will eat the roasted chicken). Kessel designed a task that consisted of four pictures on a page, two of which illustrated the correct meanings of an ambiguous sentence and two of which were foils. The examiner read each sentence aloud and asked the child to point to the picture(s) that matched the sentence. Following this, the examiner asked the child to explain why each picture was chosen. Results showed that most of the 6- and 7-year-olds understood both meanings of the lexical ambiguities but that the syntactic ambiguities were more difficult and not well understood until age 10 or 11 years.

Following Kessel’s (Reference Kessel1970) study, Shultz and Horibe (Reference Shultz and Horibe1974) examined the ability of children ages 6 through 12 years old (n = 120) to explain what was funny about jokes that were based on different types of linguistic ambiguity: phonological, lexical, and syntactic. With a phonological ambiguity, the humor stems from different ways in which words can be pronounced (e.g., Diner: “What’s this?” Waiter: “It’s bean soup.” Diner: “I’m not asking what it’s been, I want to know what it is!”). Gradual age-related improvements occurred, with jokes based on phonological ambiguity easier to understand than those based on lexical or syntactic ambiguity.

During the 1970s, adolescents’ use of slang terms was also investigated (Leona, Reference Leona1978; Nelsen & Rosenbaum, Reference Nelsen and Rosenbaum1972). Nelsen and Rosenbaum, for example, examined the ability of small groups of adolescents ages 12–18 years old, working together, to generate as many slang terms as possible when given a particular topic (e.g., cars, smoking, boys, girls). Results demonstrated substantial age-related increases in the number of different terms they could generate. In another study, Leona, who observed students in a large urban high school, reported that adolescents tended to form various “cliques” (e.g., motorheads, fleabags, preppies) and that each clique made up its own slang terms that helped to distinguish their group from the outsiders.

The 1970s also saw the publication of numerous studies of figurative language development in children, adolescents, and adults. Collectively, those studies documented gradual age-related advancements in the understanding of metaphors, similes, idioms, and proverbs (e.g., Billow, Reference Billow1975; Boswell, Reference Boswell1979; Douglas & Peel, Reference Douglas and Peel1979; Holden, Reference Holden1978; Honeck et al., Reference Honeck, Sowry and Voegtle1978; Lodge & Leach, Reference Lodge and Leach1975; Malgady, Reference Malgady1977; Ortony et al., Reference Ortony, Schallert, Reynolds and Antos1978; Pollio & Pollio, Reference Pollio and Pollio1979; Strand & Fraser, Reference Strand and Fraser1979).

Lodge and Leach (Reference Lodge and Leach1975), for example, examined idiom understanding in students who were ages 6, 9, 12, and 21 years old (n = 80). Those researchers designed a picture-pointing task that contained 10 idiomatic sentences (e.g., He spilled the beans). With four pictures presented on a page, one illustrated the literal interpretation of the idiom (e.g., a boy spilling beans on the floor), one illustrated the figurative meaning (e.g., a boy giving away a secret), and the remaining two were foils. After the examiner read each sentence aloud, the student was asked to select the two pictures that correctly illustrated its meaning. The results showed that all four age groups understood the literal meanings of the idioms, that the figurative meanings were more challenging, and that only the 21-year-olds demonstrated a full understanding of both meanings.

Douglas and Peel (Reference Douglas and Peel1979), however, questioned those results. They constructed a task in which six idiomatic sentences (e.g., Billy jumped the gun) were presented, each in the context of a supportive story (a boy who was excited to race his toy car started too soon) rather than in isolation. Students who were ages 7, 9, 11, and 13 years old (n = 120) listened to each story and explained the figurative meaning of the idiomatic sentence. Results showed that even the 7-year-olds understood some of the idioms, that the 9- and 11-year-olds understood more of them, and that the 13-year-olds understood most of the idioms. Hence, the provision of relevant contextual information assisted the students to correctly interpret the figurative meanings of the idioms.

Most studies of later language development have been cross-sectional in design where researchers compared groups of participants of different ages on one or more measures rather than tracking the development of the same participants over time, as in a longitudinal design. A notable exception was a 13-year longitudinal study conducted by Loban (Reference Loban1976). Loban examined the development of syntax in a diverse group of students (n = 211), starting when they were in kindergarten (age 5) and continuing every year through Grade 12 (age 18). Based on a random subgroup (n = 35) that represented high, average, and low levels of language proficiency, Loban reported that sentence length gradually increased during these years. For example, at Grades 1, 6, and 12, respectively, the average MLCU for spoken language was 6.88, 9.82, and 11.70 words. He also reported that as sentence length increased, so did the use of all types of subordinate clauses, including relative, adverbial, and nominal. Loban suggested that education was a significant factor driving these changes such that, as students’ knowledge of a wide range of complex topics increased, so did their need to use complex language to explain what they knew.

1.4. The 1980s

Studies of syntactic development continued during the 1980s, with researchers documenting age-related advancements at the clausal, sentential, and discourse levels (e.g., Crowhurst, Reference Crowhurst1980; Karmiloff-Smith, Reference Karmiloff-Smith, Fletcher and Garman1986; Klecan-Aker & Hedrick, Reference Klecan-Aker and Hedrick1985; Morris & Crump, Reference Morris and Crump1982; Perera, Reference Perera, Fletcher and Garman1986; Scott, Reference Scott and Nippold1988). Scott, for example, provided detailed descriptions of the subtle and nuanced developments that occurred in the use of low-frequency syntactic structures during the school-age and adolescent years. These included noun phrase post modification through the use of prepositional phrases (e.g., The visitors rang the bell at the top of the stairs), nonfinite clauses (e.g., The visitors sat on a bench, waiting for their host), and appositives (e.g., Their host, a gourmet chef, was often late), structures that enhanced the clarity and precision of sentences.

Throughout the 1980s, studies of figurative language development also continued, with researchers seeking to identify factors that made those expressions easier or more difficult for students of different ages to understand. Regarding metaphors, similes, and proverbs, studies found that when the expressions conveyed physical or perceptual concepts, they were easier to understand than those that conveyed mental or psychological meanings; that knowledge of the relevant semantic features of the keywords contained in an expression was essential to students’ understanding; that comprehension improved when the expressions were presented in supportive story contexts rather than in isolation, a finding that was consistent with research on idioms; and that performance was higher on receptive than on expressive language tasks (e.g., Baldwin et al., Reference Baldwin, Luce and Readence1982; Douglas & Peel, Reference Douglas and Peel1979; Evans & Gamble, Reference Evans and Gamble1988; Nippold et al., Reference Nippold, Leonard and Kail1984, Reference Nippold, Martin and Erskine1988; Resnick, Reference Resnick1982; Reynolds & Ortony, Reference Reynolds and Ortony1980; Siltanen, Reference Siltanen1989; Waggoner & Palermo, Reference Waggoner and Palermo1989; Winner et al., Reference Winner, Engel and Gardner1980).

Regarding idioms, Gibbs (Reference Gibbs1987) made an important distinction between transparent and opaque expressions. With transparent idioms (e.g., go by the book, skating on thin ice), the figurative meaning (i.e., to follow the rules carefully, to place oneself in a dangerous situation) is a metaphorical extension of the literal meaning; however, with opaque idioms (e.g., beat around the bush, paint the town red), the two meanings have little overlap (i.e., to avoid discussing a topic; to go out and celebrate wildly). In a study of children ages 5–9 years (n = 80), Gibbs found that transparent idioms were easier to understand than opaque, and that both types were easier to understand when presented in supportive story contexts rather than in isolation.

Relatedly, researchers during the 1980s continued to examine the development of contextual abstraction (e.g., Carnine et al., Reference Carnine, Kame’enui and Coyle1984; Nagy & Herman, Reference Nagy, Herman, McKeown and Curtis1987; Sternberg, Reference Sternberg, McKeown and Curtis1987; Wysocki & Jenkins, Reference Wysocki and Jenkins1987), a word learning strategy that had been introduced decades earlier (Werner & Kaplan, Reference Werner and Kaplan1952). Once students have learned to read fluently, they are able to gain exposure to challenging new words that occur more often in print than in daily conversations. This affords them the opportunity to learn thousands of new words each year, especially when they frequently read complex academic material. The learning process begins when the reader attends closely to linguistic clues surrounding an unfamiliar word. In the following sentence, for example, the medical term vertigo is explained by the appositive construction that immediately follows it: The lecturer experienced vertigo, a sudden sensation of dizziness, and had to sit down. Contextual abstraction is an efficient strategy for expanding one’s knowledge of words.

During the 1980s, researchers also initiated studies of the development of metalinguistic and metacognitive verbs (e.g., Astington & Olson, Reference Astington and Olson1987). Whereas metalinguistic verbs refer to acts of speaking (e.g., argue, assert, predict), metacognitive verbs refer to acts of thinking (e.g., analyze, infer, remember). Also called meta-verbs, they reflect an awareness that verbal and mental events of the self and others can be analyzed. As students advance through middle school, high school, and college, they encounter these words with increasing frequency, particularly when reading books and other materials that focus on academically rigorous topics (e.g., science, history, literature). Research has shown that comprehension of meta-verbs improves during these years but that some verbs (e.g., extrapolate, renounce) may remain challenging well into adulthood.

During the 1980s, researchers also examined the development of derivational morphology (e.g., Templeton & Scarborough-Franks, Reference Templeton and Scarborough-Franks1985; Tyler & Nagy, Reference Tyler and Nagy1989; Wysocki & Jenkins, Reference Wysocki and Jenkins1987), the understanding of sarcasm (e.g., Ackerman, Reference Ackerman1982, Reference Ackerman1986; Demorest et al., Reference Demorest, Silberstein, Gardner and Winner1983; Winner, Reference Winner1988), and the interpretation of ambiguity found in advertisements (Nippold et al., Reference Nippold, Cuyler and Braunbeck-Price1988). Each of these areas also requires metalinguistic competence to discern the intended meaning of words, phrases, and sentences.

1.5. The 1990s

Developmental studies of derivational morphology in relation to metalinguistic competence continued during the 1990s, with researchers learning more about the substantial contribution it makes to word learning, spelling, decoding, and reading comprehension during the school-age and adolescent years (e.g., Carlisle & Nomanbhoy, Reference Carlisle and Nomanbhoy1993; N. Jones, Reference Jones1991; Nagy et al., Reference Nagy, Diakidoy and Anderson1993; Windsor, Reference Windsor1994). Indeed, the contribution of metalinguistic competence to later language development became a prominent theme during the 1990s. As mentioned earlier, metalinguistic competence is the ability of an individual to step back and reflect upon or analyze various aspects of language. Different types of metalinguistic competence include meta-phonology, meta-morphology, meta-semantics, meta-syntax, and meta-pragmatics (Gombert, Reference Gombert, Van Lier and Corson1997), activities that focus on the use and understanding of speech sounds, word parts, meaningful units, sentential structures, and social skills, respectively. Age-related improvements in the ability to engage in these activities promote all aspects of later language development.

Illustrating this phenomenon, Anglin (Reference Anglin1993) examined children’s ability to learn the meanings of words through morphological analysis. Children who were ages 6–10 years old (n = 96) were individually interviewed and asked to explain the meanings of some unfamiliar words. Performance improved during this age range in the ability to determine the meanings of compound words (e.g., firesafe) and derived adjectives (e.g., workable) by attending to the meanings of the lexical (e.g., fire, safe, work) and derivational (e.g., −able) morphemes contained within the words and then putting them together to determine the larger meaning.

Progress also occurred in clarifying the linguistic nature of reading and writing, debunking the once-prominent view that these activities were primarily a function of visual, auditory, kinesthetic, and tactile perceptual processes (Fernald, Reference Fernald1943). Research conducted in the 1990s showed that reading and writing were language-based skills, and in particular, that a child’s level of phonological and morphological awareness contributed substantially to the acquisition of early literacy skills (Moats, Reference Moats1994; Vellutino, Reference Vellutino1991). Accordingly, researchers advocated for teaching these skills to young children who were just learning to read and to older children who had not yet mastered them.

Reading and writing are critical to later language development. As children progress through school, their ability to read and write promotes their acquisition of more sophisticated words, syntactic structures, and figurative expressions. This, in turn, enhances their comprehension and production of complex materials they may read or write and allows them to engage in more sophisticated forms of discourse (e.g., expository). As the acquisition of advanced language skills feeds back into their expanding literacy skills, a symbiotic relationship develops, which supports continued growth in both areas into adulthood.

Thus, it is not surprising that studies published during the 1990s documented age-related advancements in the ability of school-age children, adolescents, and adults to define abstract nouns such as freedom, humility, and compassion (McGhee-Bidlack, Reference McGhee-Bidlack1991; Nippold et al., Reference Nippold, Hegel, Sohlberg and Schwarz1999); to use and understand adverbial conjuncts such as consequently, furthermore, and nevertheless (Nippold et al., Reference Nippold, Schwarz and Undlin1992); to interpret sarcastic remarks (e.g., Capelli et al., Reference Capelli, Nakagawa and Madden1990; Dews et al., Reference Dews, Winner, Kaplan, Rosenblatt, Hunt, Lim and Smarsh1996; Keenan & Quigley, Reference Keenan and Quigley1999); and to comprehend idioms and proverbs (e.g., Nippold et al., Reference Nippold, Taylor and Baker1996, Reference Nippold, Uhden and Schwarz1997; Nippold & Haq, Reference Nippold and Haq1996; Nippold & Rudzinski, Reference Nippold and Rudzinski1993; Nippold & Taylor, Reference Nippold and Taylor1995; Spector, Reference Spector1996).

Studies of syntactic development also continued, with researchers examining the influence of genre. Leadholm and Miller (Reference Leadholm and Miller1992), for example, elicited conversational and narrative language samples from children who were ages 3–13 years (n = 266). Although age-related increases occurred in mean length of utterance (MLU) in both genres, syntactic complexity was greater in narrative than in conversational speaking. Similarly, Gutierrez-Clellen and Hofstetter (Reference Gutierrez-Clellen and Hofstetterl994) examined the development of narrative speaking in children ages 5–8 years old (n = 77). Their participants who were native Spanish speakers viewed a silent movie and retold the story. Age-related increases occurred in MLU and in the use of relative clauses.

1.6. 2000–2009

Studies of syntactic development and the influence of genre continued during the early 2000s. In addition to conversational and narrative discourse, researchers examined expository and persuasive discourse. Expository discourse is the ability to explain complex phenomena (e.g., the steps, stages, and materials needed to build a plastics recovery system), whereas persuasive discourse is the ability to convince other people to perform some action (e.g., to volunteer at a plastics recovery program at a local beach). Both genres tend to elicit greater syntactic complexity than conversational discourse by requiring more precise and detailed topic knowledge and verbal expression that is accurate, clear, and efficient. Thus, it has been argued that with syntactic development, “Complex talk reflects complex thought” (Nippold, Reference Nippold2014).

Studies involving different languages (e.g., Dutch, English, French, Hebrew, Spanish) have shown that syntax continues to develop during the school-age years, adolescence, and into adulthood, with gains most apparent in narrative, expository, and persuasive discourse (e.g., Berman & Verhoeven, Reference Berman and Verhoeven2002; Nippold, Hesketh, et al., Reference Nippold, Hesketh, Duthie and Mansfield2005; Nippold, Ward-Lonergan, & Fanning, Reference Nippold, Ward-Lonergan and Fanning2005; Ravid & Berman, Reference Ravid and Berman2006; Scott & Windsor, Reference Scott and Windsor2000; Verhoeven et al., Reference Verhoeven, Aparici, Cahana-Amitay, van Hell, Kriz and Viguié-Simon2002). During these years, utterances become longer and more complex as the number of clauses (main and subordinate) increases, a phenomenon known as clause packaging. Moreover, as the level of syntactic complexity expands, speakers and writers tend to use a greater number and variety of difficult words in their sentences. These may include, for example, low-frequency meta-verbs (postulate, conceptualize), derivational nouns (inaccessibility, conservancy), derivational adjectives (inspirational, pervasive), and morphologically complex adverbs (considerably, substantially) – words that a student is likely to encounter when reading high-level academic materials such as textbooks in history classes. In this way, lexical and syntactic development work together to enhance each other, reflecting the lexicon–syntax interface (Ravid, Reference Ravid and Berman2004).

During the early 2000s, researchers also studied the development of derivational morphology, revealing its close association to the development of literacy skills. These include, for example, the ability to decode, spell, and learn the meanings of words, and to comprehend written material (e.g., Carlisle, Reference Carlisle2000; Carlisle & Katz, Reference Carlisle and Katz2006; Deacon & Bryant, Reference Deacon and Bryant2006; Larsen & Nippold, Reference Larsen and Nippold2007; Mahony et al., Reference Mahony, Singson and Mann2000; Singson et al., Reference Singson, Mahony and Mann2000).

Other topics studied during this decade included the development of word definition, with studies confirming earlier findings that the ability to explain the meanings of words gradually improved during the school-age years, adolescence, and into adulthood; that nouns were increasingly defined in terms of their superordinate category terms (e.g., A car is a type of vehicle); and that abstract and low-frequency words were especially difficult to define. Studies also showed that verbs and adjectives were defined differently from nouns (e.g., Marinellie & Chan, Reference Marinellie and Chan2006; Marinellie & Johnson, Reference Marinellie and Johnson2003, Reference Marinellie and Johnson2004). Whereas verbs were defined by stating the key activity involved (e.g., To run means to go forward by moving your legs quickly), adjectives were defined by offering a synonym of the target word (e.g., Beautiful means gorgeous).

Additional studies of figurative language were also conducted during the early 2000s, showing that, as the understanding of idioms and proverbs improves during the school-age years, adolescence, and into adulthood, so does the ability to describe relevant mental images of the expressions (Duthie et al., Reference Duthie, Nippold, Billow and Mansfield2008; Nippold & Duthie, Reference Nippold and Duthie2003). For example, Duthie et al. reported that for the proverb “One bad apple spoils the whole barrel,” an 11-year-old child described an image of seeing a large barrel of apples covered with worms. In contrast, a 21-year-old adult described an image of an unpleasant person disrupting a group of peers who were enjoying a conversation. The authors suggested that having a clear mental image of a proverb or idiom is associated with having a deeper understanding of its figurative meaning.

1.7. 2010–2019

Numerous studies published between 2010 and 2019 focused on the development of discourse, including conversational, narrative, and expository. Often, the goal was to determine the levels of performance that could be expected of children, adolescents, or adults of different ages (e.g., Frizelle et al., Reference Frizelle, Thompson, McDonald and Bishop2018; Heilmann et al., Reference Heilmann, Miller, Nockerts and Dunaway2010; Heilmann & Malone, Reference Heilmann and Malone2014; Nippold et al., Reference Nippold, Frantz-Kaspar, Cramond, Kirk, Hayward-Mayhew and MacKinnon2015; Nippold, Cramond, & Hayward-Mayhew, Reference Nippold, Cramond and Hayward-Mayhew2014; Nippold, Frantz-Kaspar, et al., Reference Nippold, Frantz-Kaspar, Cramond, Kirk, Hayward-Mayhew and MacKinnon2014; Nippold & Sun, Reference Nippold and Sun2010; Nippold, Vigeland, et al., Reference Nippold, Vigeland, Frantz-Kaspar and Ward-Lonergan2017; Sun & Nippold, Reference Sun and Nippold2012; Westerveld & Moran, Reference Westerveld and Moran2011, Reference Westerveld and Moran2013; Westerveld & Vidler, Reference Westerveld and Vidler2016).

For example, Nippold, Frantz-Kaspar, et al. (Reference Nippold, Frantz-Kaspar, Cramond, Kirk, Hayward-Mayhew and MacKinnon2014) compared the level of syntactic complexity used in conversational versus narrative speaking in a group of 14-year-old adolescents (n = 40). To begin, the examiner engaged each participant in a conversation about topics of general interest (e.g., friends, school, sports). Then, to elicit narrative discourse, the participant was asked to listen to two fables from Aesop (The Mice in Council, The Monkey and the Dolphin) and then to retell each one. Results indicated that the narrative task elicited greater syntactic complexity than the conversational task as measured by MLCU and CD.

Subsequently, Nippold, Frantz-Kaspar, and Vigeland (Reference Nippold, Frantz-Kaspar and Vigeland2017) administered the same conversational and narrative tasks to a group of 22-year-old adults (n = 40). Once again, the narrative task elicited greater syntactic complexity than the conversational task on both measures (MLCU, CD). In this study, the researchers also compared the performance of the adults to that of the adolescents who had participated in the previous study (Nippold, Frantz-Kaspar, et al., Reference Nippold, Frantz-Kaspar, Cramond, Kirk, Hayward-Mayhew and MacKinnon2014). Results showed that on the conversational task, the adults outperformed the adolescents on both measures of syntactic complexity (MLCU, CD). On the narrative task, however, the groups did not differ with both groups using high levels of syntactic complexity.

Regarding expository discourse, Westerveld and Moran (Reference Westerveld and Moran2011) elicited spoken language samples from two groups of children, one younger (ages 6 and 7 years; n = 66) and one older (age 11 years; n = 20), using the Favorite Game or Sport (FGS) task. This task required the participant to explain the basic rules and strategies associated with a FGS (e.g., chess, checkers, football, soccer). The results showed that the FGS task was effective in eliciting expository discourse even from the younger group; that the older group outperformed the younger group on verbal productivity (total T-units produced), verbal fluency (fewer mazes), and grammatical accuracy; but that the two groups did not differ on syntactic complexity (MLTU).

Heilmann and Malone (Reference Heilmann and Malone2014) also used the FGS task to elicit expository language samples. The participants in their study were children and adolescents who were ages 10 through 15 years old (n = 235). The results indicated age-related increases in syntactic complexity (MLCU) and in lexical diversity (number of different words produced) during this age range.

Researchers also examined other topics during this decade, documenting gradual age-related improvements in a variety of areas. These included, for example, the development of word definition, humor, sarcasm, proverb and idiom understanding, reading comprehension, written expression, metalinguistic competence, and pragmatics (e.g., Apel et al., Reference Apel, Wilson-Fowler, Brimo and Perrin2012; Baines & Howe, Reference Baines and Howe2010; Berman & Ravid, Reference Berman and Ravid2010; Cameron et al., Reference Cameron, Fox, Anderson and Cameron2010; Garcia & Cain, Reference Garcia and Cain2014; Glenwright et al., Reference Glenwright, Parackel, Cheung and Nilsen2014; Glenwright & Pexman, Reference Glenwright and Pexman2010; Hung & Nippold, Reference Hung and Nippold2014; Matloubi et al., Reference Matloubi, Zarifian, Shirazi and Bakhshi2015; Nation, Reference Nation2019; Wagner et al., Reference Wagner, Puranik, Foorman, Foster, Wilson, Tschinkel and Kantor2011; Yoon et al., Reference Yoon, Schwarz and Nippold2016).

1.8. 2020–2025

Since 2020, research in later language development has continued unabated. Areas of investigation have included persuasive and narrative discourse, past tense counterfactual (PTCF) sentences, and critical thinking (e.g., Heilmann et al., Reference Heilmann, Malone and Westerfeld2020; Nippold, LaFavre, & Shinham, Reference Nippold, LaFavre and Shinham2020; Nippold, Nehls-Lowe, & Lee, Reference Nippold, Nehls-Lowe and Lee2020; Nippold, Shinham, & LaFavre, Reference Nippold, Shinham and LaFavre2021; Wallis et al., Reference Wallis, Westerveld, Waters and Snow2021; Wallis & Westerveld, Reference Wallis and Westerveld2024).

Heilmann et al. (Reference Heilmann, Malone and Westerfeld2020), for example, examined the development of persuasive speaking in adolescents who were enrolled in Grades 8, 9, 10, 11, and 12 (ages 13–18 years; n = 179). Each participant was asked to try to convince an adult authority figure to change a specific policy or rule. A planning sheet with topic headings (e.g., What do you believe? What are your reasons?) was provided to assist the participant to think about the topic and organize a response before speaking. Although no age-related differences were found, even the youngest adolescents produced high levels of syntactic complexity and persuasive effectiveness on this task.

To examine different types of narrative speaking, Wallis and Westerveld (Reference Wallis and Westerveld2024) elicited language samples from adolescents who were ages 12–18 years old (n = 44). Each participant was asked to listen to and retell a fable (The Mice in Council); to generate a fictional story from a wordless picture book (One Frog Too Many); and to tell a personal story about an emotional experience (e.g., encountering a bully, feeling excited, having a problem). Results indicated that the fable retell elicited the highest level of syntactic complexity (MLCU = 13.45 words) and that the personal story elicited the highest verbal productivity (mean number of C-units = 85.57). However, no age-related differences were found on any of the narrative speaking measures.

A PTCF sentence (e.g., If the coach had supervised the workout, the athletes would have completed the drills) consists of a subordinate clause that contains the past perfect verb form (had supervised) and a main clause that contains the present perfect verb form (would have completed). A type of low-frequency complex sentence, the PTCF expresses conditions that, in reality, did not happen (i.e., the coach did not supervise the workout; the athletes did not complete the drills), hence the term counterfactual. These sentences are a late linguistic attainment. Although accuracy in producing and comprehending PTCF sentences gradually improves during the school-age and adolescent years, they remain challenging even for many young adults (Nippold, LaFavre, & Shinham, Reference Nippold, LaFavre and Shinham2020; Nippold, Nehls-Lowe, & Lee, Reference Nippold, Nehls-Lowe and Lee2020).

Critical thinking is the ability to analyze an argument or claim and then decide for oneself what to believe or do, based on having clear reasons and solid evidence. This is a higher-order cognitive skill that requires advanced levels of language ability in all areas (syntax, morphology, semantics, pragmatics) and a strong foundation in literacy, including speaking, listening, reading, and writing. In the current era of widespread disinformation and misinformation, often espoused through social media, critical thinking is an essential skill for people of all ages to cultivate.

Nippold, LaFavre, and Shinham (Reference Nippold, LaFavre and Shinham2020) examined the development of critical thinking in adolescents who were ages 13 and 16 years old (n = 40 per group). Participants completed a written language task that involved four fables by Aesop (the Fox and the Grapes, the Crow and the Pitcher, the Ant and the Grasshopper, the Rooster and the Fox), each of which ended in a moral message (It is easy to despise what you cannot get, Necessity is the mother of invention, It is best to prepare for the days of necessity, Beware of the sudden offers of friendship, respectively). After reading a fable, participants were asked to indicate if they agreed or disagreed with the moral message or were unsure, and to explain their answer in writing. A 4-point scoring system was used to evaluate the quality of critical thinking that was expressed. The explanations were also evaluated for linguistic sophistication. The results indicated that the 16-year-olds outperformed the 13-year-olds in providing explanations that reflected independent thinking and application to real-life situations, showing that critical thinking improved during adolescence. However, not even the older group showed mastery of the task. Linguistic analyses also revealed age-related advancements in verbal productivity and metacognitive verb use, but not in syntactic complexity, with both groups already using high levels of syntactic complexity.