3.1.1. Childhood stage

Humans, compared to other primates and mammals, exhibit a uniquely extended period of vulnerability and dependency during childhood, along with delayed maturation and independence from their parents (Kaplan et al., Reference Kaplan, Hill, Lancaster and Hurtado2000). This extended vulnerable life stage necessitates reliance on adults for both material resources of food and shelter and social learning essential for development (Nelson, Reference Nelson2020). Because kin represents the primary source of these critical resources, alloparental care substantially enhances children’s survival, maintenance, and growth (Jaeggi & Gurven, Reference Jaeggi and Gurven2013; Kramer, Reference Kramer2010). Consequently, environments with high genetic relatedness are likely perceived as offering opportunities for reliable alloparental investment (Crittenden & Marlowe, Reference Crittenden and Marlowe2008; Kenkel et al., Reference Kenkel, Perkeybile and Carter2017; Martin et al., Reference Martin, Ringen, Duda and Jaeggi2020), whereas environments lacking kin presence are likely perceived as constraining access to consistent resources and protection.

How do children navigate the ecological affordances of genetic relatedness? We hypothesize that children—mostly unconsciously—leverage evolved mechanisms to optimize care and support in the environment they interact with by (a) detecting kin and showing preferences toward them, (b) calibrating sensitivity and behaviors for monitoring and engaging caregivers, and (c) adjusting attachment strategies.

First, children possess evolved mechanisms for kinship detection that emerge early in development. Newborns demonstrate the ability to recognize their mother’s face within hours after birth (Bushnell, Reference Bushnell2001; Walton & Bower, Reference Walton and Bower1993), and children show sensitivity to reliable kinship cues—including patterns of parental caregiving and shared developmental history (Lieberman et al., Reference Lieberman, Tooby and Cosmides2007; Sznycer et al., Reference Sznycer, De Smet, Billingsley and Lieberman2016). These abilities facilitate identification of potential alloparents, leading children to prefer individuals who resemble their parents or those who have received substantial parental investment, such as their siblings and cousins. Such abilities are likely more important in an environment with low genetic relatedness, where accurately identifying reliable caregivers is essential for meeting developmental goals.

Second, children actively monitor caregiver support and act if care becomes insufficient. Even infants seek attention by establishing eye contact, tracking gazes (Hrdy, Reference Hrdy2009, Reference Hrdy2007), vocalizing (e.g., babbling; Hrdy, Reference Hrdy2007), and displaying emotions (e.g., expressing fear; Grossmann, Reference Grossmann2022). The intensity of this monitoring and engagement is likely calibrated to kin prevalence in the environment. In an environment with high genetic relatedness, where reliable alloparents are abundant, children may not need to constantly seek attention; in an environment with low genetic relatedness, children may intensify their monitoring and engagement to secure adequate care; and in an environment with extremely low genetic relatedness, such as orphanages, children may conserve energy by reducing their efforts to solicit attention, given the low likelihood of success.

Third, children form attachment bonds with multiple individuals while maintaining a primary bond, typically with the mother (Cassidy, Reference Cassidy, Cassidy and Shaver1999). The depth and breadth of these attachments likely calibrate to the kin availability in their environment. Because kin attachments (even if shallow) can buffer against negative life events, such as losing a primary attachment figure (Perry, Reference Perry2021), children in an environment with high genetic relatedness may form multiple attachments with several relatives without heavily relying on any particular kin relationship or non-kin relationships. Conversely, children in an environment with low genetic relatedness may focus on forming stronger and deeper attachments with the limited relatives available or even with non-kin (e.g., godfather, parent’s friends) to secure adequate care and overall investment.

3.1.2. Mating stage

Although perceived as advantageous by children, an environment of high genetic relatedness will likely be perceived as affording both threats and opportunities for individuals in the mating stage. Because young adults face developmental trade-offs between immediate reproduction and long-term reproductive success, their perceived ecological affordances likely depend on their specific reproductive strategies. On the one hand, an environment of high genetic relatedness makes finding genetically diverse mates more challenging, thereby increasing the risk of inbreeding depression. On the other hand, a strong kin network within an environment of high genetic relatedness can provide resources and skills essential for future mating and parenting.

The management strategies to mitigate the threats and exploit the opportunities are likely shaped by individuals’ prioritized mating goals and reproductive strategies. For example, those prioritizing immediate or short-term reproduction may address the challenges of high genetic relatedness by (a) heightening their sensitivity to detect and avoid inbreeding risks, (b) altering their surroundings within the given environment to increase contact with non-kin, and (c) relocating to an environment with a lower kin prevalence to expand mating opportunities.

First, although incest aversion shows a universal pattern—being more disgusted and avoidant for close versus distant relatives due to reproductive fitness costs (Antfolk et al., Reference Antfolk, Lieberman, Harju, Albrecht, Mokros and Santtila2018; Kresanov et al., Reference Kresanov, Kotler, Seto, Lieberman, Santtila and Antfolk2018)—its intensity appears calibrated to environmental threat. Living in environments of high genetic relatedness poses greater risks of inbreeding, requiring heightened sensitivity to detect and avoid such risks. Supporting this, young adults surrounded by many genetic relatives exhibit stronger moral condemnation of incest compared to those in low-relatedness environments (Sng et al., Reference Sng, Choi and Ackerman2024). This calibration of incest aversion is especially relevant during the mating life stage when it has significant implications for reproductive success.

Second, because interactions with non-kin became important for young adults seeking mates, they likely alter their surroundings to reduce kin interactions. Young adults begin spending less time with their parents and more time with friends and peers (Larson & Verma, Reference Larson and Verma1999; Roberts & Dunbar, Reference Roberts and Dunbar2011). Moreover, during the high fertility phase of their ovulatory cycle, women selectively avoid interactions with their fathers (Lieberman et al., Reference Lieberman, Pillsworth and Haselton2011). These behaviors are likely more pronounced in environments with high genetic relatedness.

Third, young adults are likely to relocate to an environment with lower genetic relatedness to expand their mating opportunities. In small hunter-gatherer societies, where susceptibility to low genetic diversity can be high, members of the more prevalent sex often disperse from their natal group to find mates (Kramer et al., Reference Kramer, Schacht and Bell2017). Similarly, in the U.S., young adults are more likely than other age groups to move to large urban areas (Johnson et al., Reference Johnson, Winkler and Rogers2013), where genetic diversity is greater, possibly in search of better mating opportunities, even if it means losing support from kin.

Conversely, those prioritizing future or long-term reproductive potential are likely to maximize their opportunities by delaying both courtship and dispersal from proximity to their local kinship networks. In small-scale hunter-gatherer societies, young adults often delay independence until gaining the necessary skills, physical strength, and social alliances to thrive independently (Kramer, Reference Kramer2011; Natterson-Horowitz & Bowers, Reference Natterson-Horowitz and Bowers2020). Similarly, in modern industrialized societies, many individuals undergo a phase known as ‘emerging adulthood’ (Arnett, Reference Arnett2000, Reference Arnett2007), during which they explore education, relationships, and careers while remaining dependent on their parents. These delaying strategies are likely to be more effective in an environment with high genetic relatedness, particularly where mating competition is intense.

3.1.3. Parenting stage

For parents, especially those with young children, access to alloparents is critical for both parental and offspring success. Maternal commitment—the strongest predictor of infant survival—largely depends on allomaternal support from the community (Hrdy, Reference Hrdy2007, Reference Hrdy2009). Such support also enables mothers to shorten birth intervals (Hrdy, Reference Hrdy2007; Kaplan et al., Reference Kaplan, Hill, Lancaster and Hurtado2000; Richerson & Boyd, Reference Richerson and Boyd2020). Because genetic relatives often serve as alloparents, an environment with high genetic relatedness is likely perceived to afford opportunities for parents to enhance the quality and quantity of their offspring.

How do parents with young children manage the opportunities afforded by environments of high genetic relatedness and the challenges afforded by those of low genetic relatedness? We hypothesize that parents (a) alter their immediate surroundings to facilitate alloparenting by kin or, if necessary, non-kin, depending on the prevalence of kin within their environment, and (b) relocate their family to access kin support.

To maximize the alloparenting opportunities, parents likely alter their immediate social environment by pooling their “energy budget” with available family members (Kramer & Ellison, Reference Kramer and Ellison2010; Reiches et al., Reference Reiches, Ellison, Lipson, Sharrock, Gardiner and Duncan2009). Even after forming new family units through marriage, individuals typically maintain lifelong connections with their birth families (Rodseth et al., Reference Rodseth, Wrangham, Harrigan, Smuts, Dare, Fox, King, Lee, Foley, Muller, Otterbein, Strier, Turke and Wolpoff1991) to preserve access to this shared energy budget. When biological kin are scarce in an environment, people form “fictive kin” networks—family-like relationships with individuals unrelated by blood or marriage. For instance, immigrants who live at great distance from their biological families often create social capital with others from similar backgrounds to simulate and recreate the benefits typically afforded by environments of high genetic relatedness (Ebaugh & Curry, Reference Ebaugh and Curry2000; Regnerus & Burdette, Reference Regnerus and Burdette2006; Taylor et al., Reference Taylor, Chatters, Woodward and Brown2013). Religious groups similarly foster a subjective sense of fictive kin by promoting “brotherhood” and “sisterhood” among members and establishing ritual kin, such as godparenthood (E. B. Hubbard et al., Reference Hubbard, Shannon and Pisor2023), thereby enhancing offspring fitness through shared resources and group support (Moon et al., Reference Moon, Krems, Cohen and Kenrick2019; Weeden & Kurzban, Reference Weeden and Kurzban2013).

As a strategy to maximize the alloparenting opportunities, parents likely relocate to be near genetic relatives, particularly their own parents—children’s grandparents. For instance, foragers exhibit flexible and facultative residence patterns across their life course based on environmental resources, social needs, and kin availability (Marlowe, Reference Marlowe2010). Similar patterns persist in contemporary societies, where parents commonly relocate closer to their own parents for childcare support, or grandparents migrate to assist in childrearing (Hank, Reference Hank2007). This strategy is especially beneficial for resource-constrained families; for example, individuals with lower education and income tend to live closer to their parents than those with higher socioeconomic status (Costa & Kahn, Reference Costa and Kahn1990). The ubiquity of three-generational households reflects, in part, how people alter local genetic relatedness to maximize alloparenting benefits (Hooper et al., Reference Hooper, Gurven, Winking and Kaplan2015).

3.1.4. Summary

Individuals likely perceive ecological affordances of genetic relatedness differently across life stages. In childhood, they may perceive high genetic relatedness as affording opportunities to receive support for survival and development. During the mating stage, they may perceive it as affording both threat and opportunity—limiting immediate reproduction while enhancing future reproductive potential. In parenting, they may perceive it as affording opportunities for alloparenting to improve offspring success. Individuals thus likely calibrate their strategies to optimize these stage-specific affordances of genetic relatedness.

3.2.1. Childhood stage

An environment with a high prevalence of physical violence is likely perceived as affording threats to children by (a) increasing the risk of family violence, (b) limiting caregiving when parents are harmed, and (c) exposing children to community violence. First, child victimization is often linked to several risk factors that are more prevalent in environments with high physical violence, such as social and economic instability (Daly & Wilson, Reference Daly and Wilson1988; Frankenhuis & Amir, Reference Frankenhuis and Amir2022; Pinheiro, Reference Pinheiro2006), parental mental health issues (Kohl et al., Reference Kohl, Jonson-Reid and Drake2011), and domestic violence (Jouriles et al., Reference Jouriles, McDonald, Smith Slep, Heyman and Garrido2008). These factors elevate caregiving stress, leading parents to become perpetrators or victims, ultimately compromising their caregiving abilities (Margolin & Gordis, Reference Margolin and Gordis2004). Second, parental victimization also deprives children of consistent care and access to essential resources, heightening children’s vulnerability and hindering their development (Belsky, Reference Belsky1984; Shonkoff et al., Reference Shonkoff, Garner, Siegel, Dobbins, Earls, Garner, McGuinn, Pascoe and Wood2012). Third, children in such environments often become targets of community violence—whether as direct victims, collateral damage, or through forced recruitment into gangs—perpetuating cycles of harm (Howell & Griffiths, Reference Howell and Griffiths2018).

How do children mitigate the threats of living in an environment with high physical violence? We hypothesize that children (a) calibrate their attention and sensitivity to identifying potential perpetrators and (b) adjust their behavioral strategies according to their own physical formidability.

Children in violent environments are likely to exhibit heightened attention and sensitivity to cues of potential harm. Historically, children have faced significant mortality risks from outgroup members (Hill & Hurtado, Reference Hill and Hurtado2018). As an evolved protective mechanism, infants exhibit greater fear toward male strangers once they begin to crawl. This stranger anxiety is observed across cultures regardless of the primary caregiver’s gender, challenging the idea that this fear stems solely from familiarity with females (Hahn-Holbrook et al., Reference Hahn-Holbrook, Holbrook, Bering, Holbrook and Bering2010). Additionally, children exposed to physical abuse develop greater accuracy, response bias, and sensitivity in detecting anger—an emotion strongly associated with imminent physical harm. Notably, these children do not show the same level of attunement to other negative emotions, such as disgust, fear, or sadness, indicating a selective focus on cues most relevant to their safety (Pollak et al., Reference Pollak, Cicchetti, Hornung and Reed2000, Reference Pollak, Klorman, Thatcher and Cicchetti2001). Children in violent environments are likely to develop increased fear of outgroup males and enhanced sensitivity to cues relevant to harm, thereby enhancing their ability to identify potential perpetrators and mitigate risks.

In addition, children in violent environments are likely to calibrate their behavioral strategies based on their own physical formidability. For physically larger and stronger children, aggression functions as an adaptive strategy to secure resources and deter threats (Dodge & Frame, Reference Dodge and Frame1982; Ellis et al., Reference Ellis, Bianchi, Griskevicius and Frankenhuis2017; Hubbard et al., Reference Hubbard, Dodge, Cillessen, Coie and Schwartz2001; Light & Dishion, Reference Light and Dishion2007). This strategy manifests through (a) bullying—targeted aggression toward more vulnerable peers (Kolbert & Crothers, Reference Kolbert and Crothers2003; Olthof & Goossens, Reference Olthof and Goossens2007; Veenstra et al., Reference Veenstra, Lindenberg, Munniksma and Dijkstra2010; Volk et al., Reference Volk, Camilleri, Dane and Marini2012a, Reference Volk, Camilleri, Dane and Marini2012b), (b) engaging in antisocial behaviors that signal strength and deter potential rivals (Ellis et al., Reference Ellis, Del Giudice, Dishion, Figueredo, Gray, Griskevicius, Hawley, Jacobs, James, Volk and Wilson2012), and (c) affiliating with peer groups characterized by volatility or risk-taking behaviors, which enhances perceived dominance and social capital (e.g., Schwartz & Proctor, Reference Schwartz and Proctor2000). In contrast, children who perceive themselves as physically vulnerable are likely to adopt different strategies, such as (a) forming friendships with older, stronger, or socially influential peers to gain protection and bolster their security or (b) forming alliances with peers who share similar vulnerabilities, thereby fostering mutual support that enhances resilience in a threatening environment.

3.2.2. Mating stage

After puberty, young males became more effective at employing violence due to their increased strength, muscle mass, and aggressive inclinations. Because males can increase reproductive success by mating with multiple partners, young males face intense intrasexual competition for mates, leading them to pursue high-risk aggressive strategies (Tooby & Cosmides, Reference Tooby and Cosmides1988; Wang et al., Reference Wang, Kruger and Wilke2009). Successful warriors and hunters historically dominated material, social, and reproductive resources (Chagnon, Reference Chagnon1988; Wilson, Reference Wilson1989). As a result, much of male aggression is directed toward other males, manifesting as intergroup conflict, public assaults, and peer aggression, especially during the mating stage (McDonald et al., Reference McDonald, Navarrete and Van Vugt2012). Homicide rates, for instance, peak between the ages of 15 and 29, with over 80% resulting from male-on-male violence (United Nations Office on Drugs and Crime, 2019a; Hirschi & Gottfredson, Reference Hirschi and Gottfredson1983; Moffitt, Reference Moffitt1993).

Young men also often engage in physical violence toward females—particularly those in their reproductive prime—aimed at controlling female sexuality for their reproductive advantage (Smuts, Reference Smuts1992). In the U.S., 90% of rape victims are female, one in four females experience some form of physical and sexual violence by an intimate partner, and half of female murder victims are killed by intimate partners (Ertl et al., Reference Ertl, Sheats, Petrosky, Betz, Yuan and Fowler2019; Smith et al., Reference Smith, Zhang, Basile, Merrick, Wang, Kresnow and Chen2018; Snyder, Reference Snyder2000). Young females in their mating stage are more vulnerable to sexual violence than young men or older women (National Coalition against Domestic Violence, 2020; Smith et al., Reference Smith, Zhang, Basile, Merrick, Wang, Kresnow and Chen2018) as they are perceived as more desirable mates (Buss, Reference Buss1989; Conroy-Beam & Buss, Reference Conroy-Beam and Buss2018; Kenrick & Keefe, Reference Kenrick and Keefe1992).

An environment with a high prevalence of physical violence is likely perceived as affording threats to young men, elevating the risks of mortality and morbidity mainly through male-male aggression. However, for a few dominant, physically formidable, and aggressive men, this environment can be perceived as affording an opportunity to gain access to material, social, and sexual resources (Daly & Wilson, Reference Daly and Wilson1988; Wilson, Reference Wilson1989). In such environments, the immediate benefits for young men able to dominate through violence can outweigh the long-term risks of injury or death. In contrast, for young women, such environments are likely perceived as affording a threat of sexual victimization. War zones illustrate this pattern, where young women are frequently targeted for sexual violence and trafficking (United Nations Office on Drugs and Crime, 2019b; Vikman, Reference Vikman2005).

How do young men and women navigate the ecological affordances of an environment of high physical violence? Given sex differences in vulnerability and the varying costs and benefits of aggression, we hypothesize that people develop sex-specific strategies by (a) adjusting cognitive biases to identify those most capable of causing harm, (b) calibrating attention and sensitivity to detect the types of aggression they are most vulnerable to, and (c) forming strategic alliances and relationships.

First, people in violent environments—both males and females across life stages—are likely to develop heightened fear responses specifically to the anger of young men, who typically possess the greatest ability and inclination to inflict harm. Because those most capable of causing harm are especially likely to signal this via displays of anger (Sell et al., Reference Sell, Tooby and Cosmides2009), missing cues of anger carries high potential costs (Haselton & Buss, Reference Haselton and Buss2000; Nesse, Reference Nesse2005). Thus, people tend to overperceive anger in the faces of young men, even when their expressions are neutral (Maner et al., Reference Maner, Kenrick, Becker, Robertson, Hofer, Neuberg, Delton, Butner and Schaller2005; Neel et al., Reference Neel, Becker, Neuberg and Kenrick2012). Anger displayed by young men is particularly potent in triggering persistent fear conditioning, unlike anger from women (Mazurski et al., Reference Mazurski, Bond, Siddle and Lovibond1996). In an environment of high physical violence, people are likely to develop heightened vigilance and cognitive biases towards aggression cues from young men.

Second, young men in violent environments are likely to develop a heightened sensitivity to detect aggressive intentions from other men, whereas young women are likely to become more attuned to recognizing cues of sexual aggression. Indeed, male biases against outgroup males are frequently driven by concerns over aggression, status competition, and dominance, whereas female biases are often rooted in fear of sexual coercion (Navarrete et al., Reference Navarrete, McDonald, Molina and Sidanius2010). In a violent environment, young men and women—each target of violence, but in different ways—are expected to exhibit pronounced sensitivities and biases toward the threats to which they are most vulnerable.

Third, young men and women in violent environments are likely to adopt distinct social strategies tailored to their vulnerabilities. To minimize the threats and maximize the opportunities, physically formidable, aggressive, and dominant young men are likely to prioritize forming strong same-sex peer groups, such as gangs, to consolidate power, reduce vulnerability, and facilitate resource acquisition in the face of intergroup threats (Balliet et al., Reference Balliet, Li, Macfarlan and Van Vugt2011; Van Vugt et al., Reference Van Vugt, De Cremer and Janssen2007). Conversely, young men vulnerable to aggression are likely to form alliances with stronger or socially influential males for protection or seek support from other males who share similar vulnerabilities, fostering mutual resilience.

In contrast, young women in violent environments are likely to prioritize partnerships with men who can offer protection. Women in violent environments prefer partners who are physically formidable and socially dominant, as these traits provide security (Ryder et al., Reference Ryder, Maltby, Rai, Jones and Flowe2016; Snyder et al., Reference Snyder, Fessler, Tiokhin, Frederick, Woo and David2011). Higher national homicide rates are linked to greater female preferences for masculine features that signal dominance (Brooks et al., Reference Brooks, Scott, Maklakov, Kasumovic, Clark and Penton-Voak2011). However, women face a complex trade-off: Although physically formidable partners may provide external security, they may also pose internal risks of intimate partner violence. Navigating this balance between the protective benefits and potential costs of such partnerships is likely a critical challenge for women in violent environments.

3.2.3. Parenting stage

An environment of high physical violence is likely perceived as affording threats to parents by (a) endangering their own safety and survival, limiting their reproductive potential and reducing overall fitness, (b) impairing their ability to invest in existing offspring due to injury or death, indirectly lowering fitness (Hurtado & Hill, Reference Hurtado and Hill1992; Scelza, Reference Scelza2010), and (c) directly threatening the safety and survival of their offspring, further compromising parental fitness.

How do parents mitigate these threats of living in an environment with a high prevalence of physical violence? We hypothesize that parents (a) exhibit heightened vigilance toward potential threats to their children, (b) calibrate their risk-taking behaviors to balance personal safety with the need to protect their offspring, (c) adopt stricter moral judgments toward transgressors to enforce social order and reduce conflict, and (d) actively seek to modify both physical and social environments to enhance security and communal support.

First, parents are likely to calibrate their vigilance and thresholds for detecting potential threats to their children. Parental precaution systems are particularly attuned to threats to offspring (Hahn-Holbrook et al., Reference Hahn-Holbrook, Holbrook and Haselton2011), functioning similarly to evolved fear responses to predators or heights (Boyer & Bergstrom, Reference Boyer and Bergstrom2011). Throughout human evolutionary history, violence from unrelated males posed significant threats to children (Hahn-Holbrook et al., Reference Hahn-Holbrook, Holbrook, Bering, Holbrook and Bering2010; Hill & Hurtado, Reference Hill and Hurtado2018), perhaps explaining why parents still express heightened concern about harm or abduction by strangers despite the contemporary statistical rarity of such events (Hahn-Holbrook et al., Reference Hahn-Holbrook, Holbrook and Haselton2011). This evolved sensitivity manifests when parental status is activated: parents tend to overestimate both the danger posed by formidable men (Fessler et al., Reference Fessler, Holbrook, Pollack and Hahn-Holbrook2014) and risks in ambiguous situations (Eibach et al., Reference Eibach, Libby and Gilovich2003; Eibach & Mock, Reference Eibach and Mock2011) while becoming more sensitive to cues of outgroup danger (Gilead & Liberman, Reference Gilead and Liberman2014)—all as a part of a conservative error management strategy (Haselton & Buss, Reference Haselton and Buss2000; Nesse, Reference Nesse2005). Such parental precautionary patterns are likely even more pronounced for parents navigating violent environments.

Second, parents are likely to calibrate their risk-taking behaviors to balance personal safety with offspring protection. Parenthood typically reduces risk-taking, particularly when decisions affect their children’s safety and well-being (Görlitz & Tamm, Reference Görlitz and Tamm2020; Wang et al., Reference Wang, Kruger and Wilke2009). Even parents with criminal backgrounds tend to engage in less criminal and violent behavior when they have strong, satisfying relationships with their children, particularly if the child was wanted (Ganem & Agnew, Reference Ganem and Agnew2007; Giordano et al., Reference Giordano, Seffrin, Manning and Longmore2011). However, when offspring face threats, parents’ protective instincts often override their usual risk aversion, leading to increased risk-taking behaviors prioritizing child protection over personal safety (Boyer & Bergstrom, Reference Boyer and Bergstrom2011; Trivers, Reference Trivers1972). Neural evidence supports this flexibility: fathers-to-be show increased activation in motor areas of the brain when imagining threats to their own infants, as compared to unknown infants, suggesting a neural readiness to act in defense of their offspring (Rilling & Mascaro, Reference Rilling and Mascaro2017; van ‘t Veer et al., Reference van ‘t Veer, Thijssen, Witteman, Van IJzendoorn and Bakermans-Kranenburg2019). These risk-taking behaviors prioritizing child protection likely become more pronounced in violent environments where threats to offspring are more frequent and severe.

Third, parents are likely to calibrate their moral judgments to protect their offspring from social threats, with this tendency becoming particularly pronounced in violent environments. Research shows that parenting motivation predicts greater social conservatism and harsher moral evaluations (Kerry & Murray, Reference Kerry and Murray2018). Parents express stronger condemnation of adult transgressions that threaten social order while showing greater leniency toward children’s misbehavior that carries lower social costs (Hofer et al., Reference Hofer, Buckels, White, Beall and Schaller2018). Because maintaining social order and minimizing conflict is crucial for family survival and offspring well-being, parents in violent environments likely develop even more stringent moral judgments toward adult transgressors who pose potential threats to social stability.

Last, parents are likely to alter both their physical and social environments to protect their children. At the physical level, parents likely enhance security through direct modifications such as installing stronger locks or fencing. When possible, relocating to a safer, more stable environment may be likely (Sharkey & Sampson, Reference Sharkey and Sampson2010). At the social level, parents likely create protective buffers for their children by engaging in community advocacy or political action to reduce local violence. Additionally, parents may cultivate strategic networks or fictive kin relationships (e.g., godparents) to strengthen communal bonds that offer mutual support. These networks can offer social buffers, providing emotional coping, practical support (e.g., shared childcare), and a greater sense of community, which helps families navigate the challenges of living in violent neighborhoods (Ceballo & McLoyd, Reference Ceballo and McLoyd2002; Sampson et al., Reference Sampson, Raudenbush and Earls1997). By leveraging both physical and social strategies, parents can better mitigate the threats posed by violent environments and create a more secure space for their families.

3.2.4. Summary

Consideration of the ecological dimension of physical violence reveals that perceived ecological affordances can differ for males and females, even within the same life stage. In childhood, both boys and girls may perceive such an environment as affording threat via family violence, compromised caregiving, and exposure to community violence. During the mating stage, perceptions become sex-specific: whereas most young men may perceive threats from male-male aggression, physically formidable men may see opportunities for status and resource gain, and women may perceive threat from sexual victimization. In the parenting stage, adults may perceive environments of high physical violence as affording threats to offspring survival and parenting capacity. These life-stage- and sex-specific perceptions likely guide how individuals navigate violent environments.

3.3.1. Childhood stage

Especially in early childhood, the sex ratio of peers has limited relationship affordance implications, given that children’s primary social interactions occur with caregivers rather than peers. Although children begin forming sex-segregated playgroups around age three or four as friendship goals emerge (Maccoby, Reference Maccoby1988; Serbin et al., Reference Serbin, Moller, Gulko, Powlishta and Colburne1994), opposite-sex peers do not yet afford specific opportunities or threats.

In contrast, the sex ratio of genetically related juveniles can affect alloparenting opportunities for young children (Kramer, Reference Kramer2011, Reference Kramer2019). In traditional societies, older siblings are the most common alloparents for infants (Kramer & Veile, Reference Kramer and Veile2018), contributing through various subsistence activities (Bird & Bird, Reference Bird and Bird2002; Crittenden et al., Reference Crittenden, Conklin-Brittain, Zes, Schoeninger and Marlowe2013; Kramer, Reference Kramer2005). The affordance implications of juvenile sex ratio, however, likely depend on ecological factors, local food targets, social organization, and opportunity costs (Bock, Reference Bock2002). For example, in an environment requiring great physical strength for subsistence, male-biased juvenile sex ratios likely afford greater opportunities for younger kin.

Adult sex ratios present complex implications for child development. In environments with female-biased ratios among genetically related adults, children likely benefit from enhanced alloparenting opportunities from their older female relatives. Research consistently shows that maternal grandmothers and older sisters contribute more significantly to child survival than maternal grandfathers and older brothers (Chapman et al., Reference Chapman, Pettay, Lummaa and Lahdenperä2018; Mace & Sear, Reference Mace and Sear2005; Sear & Mace, Reference Sear and Mace2008), reflecting evolved sex differences in nurturant behavior (Geary, Reference Geary2000). Conversely, male-biased sex ratios among unrelated adults likely afford greater paternal investment for children through increased longer-term relationship commitment from fathers (e.g., Uggla & Mace, Reference Uggla and Mace2017). Cooperative biparental investment has played a critical role in enhanceing offspring fitness (Geary, Reference Geary2000, Reference Geary2005; Geary & Flinn, Reference Geary and Flinn2001) in both small-scale (Hurtado & Hill, Reference Hurtado and Hill1992; Kaplan et al., Reference Kaplan, Lancaster, Hurtado, Booth and Crouter1998; Winking et al., Reference Winking, Gurven and Kaplan2011) and modern societies (B. L. Barber & Demo, Reference Barber and Demo2006; N. Barber, Reference Barber2000b).

How do children manage the ecological affordances posed by sex ratios across different life stage groups? We hypothesize children indirectly manage them through preferences for certain caregivers or social settings, potentially influencing caregiving allocation in extended family or communal living situations. From early development, infants show greater proficiency in processing female faces (de Haan et al., Reference de Haan, Johnson, Maurer and Perrett2001; Ramsey et al., Reference Ramsey, Langlois and Marti2005; Walton & Bower, Reference Walton and Bower1993) and display increased sociability with female strangers (Lamb et al., Reference Lamb, Hwang, Frodi and Frodi1982), suggesting an evolved expectation and preference for female caregiving engagement.

Although children can readily distinguish adults by sex (Fagot & Leinbach, Reference Fagot and Leinbach1993; Leinbach & Fagot, Reference Leinbach and Fagot1993), their ability to detect relative sex ratios remains unclear, as this requires complex cognitive mechanisms they may lack. Nevertheless, ecological sex ratios likely have important affordance implications for children through their influence on parental and alloparental investment patterns.

3.3.2. Mating stage

For young adults, the sex ratios of non-reproductively viable groups—such as children or the elderly—present minimal affordances since these groups neither facilitate nor hinder young adults’ primary developmental goals at this life stage.

The operational sex ratio—the ratio of reproductively viable adults—however, creates significant affordances that shape reproductive strategies. These ratios influence both the available pool of potential partners and the intensity of mate competition. In heterosexual populations, members of the less prevalent sex benefit from enhanced mating opportunities and greater leverage in partner selection, enabling them to maintain high selectivity. Conversely, members of the more prevalent sex face intensified intrasexual competition, necessitating strategic adjustments to better align with opposite-sex preferences for successful mate acquisition (Emlen & Oring, Reference Emlen and Oring1977; Maner & Ackerman, Reference Maner and Ackerman2020; Secord, Reference Secord1983).

How do young adults maximize the opportunities and minimize the threats of living in an environment with a biased operational sex ratio? We hypothesize that they (a) increase vigilance toward the operational sex ratio and track changes in it, (b) calibrate their reproductive strategies to align with mating competition, (c) alter their immediate environments to enhance mating opportunities, and (d) relocate to new environments.

First, young adults are likely attuned to the operational sex ratio within mating-relevant subgroups, actively tracking changes in these demographic patterns. Humans can rapidly perceive and process sex ratios, even from minimal cues such as facial photographs (Brandner et al., Reference Brandner, Dillon and Brase2020) and voice patterns (Neuhoff, Reference Neuhoff2017; Neuhoff & Sikich, Reference Neuhoff and Sikich2018). This tracking is likely refined based on both preferred partner characteristics and one’s own mate value. For instance, given that people typically form long-term relationships within their own racial or ethnic group, they may mentally construct operational sex ratios specific to their group rather than to the broader population. Similarly, age-based mating preferences lead younger individuals, such as teens and adolescents, to focus on the ratio within their age cohort to predict future mating prospects (e.g., Barber, Reference Barber2000a, Reference Barber2003). Given that one’s own mate value determines bargaining power in the mating market, individuals with lower mate value may perceive sex ratio imbalances as relatively more unfavorable and adopt cautious strategies in response (e.g., Millar et al., Reference Millar, Westfall and Walsh2019; Uggla & Mace, Reference Uggla and Mace2017).

Second, young adults of the more prevalent sex—who face greater mating competition and fewer opportunities—are likely to calibrate their mating strategies toward those preferred by the less prevalent sex. When reproductive-aged males are abundant, they tend to shift toward female-preferred strategies that favor long-term, committed, monogamous relationships. This facultative calibration often results in higher marriage rates, earlier marriages, and lower out-of-wedlock or teenage birth rates (Barber, Reference Barber2001; South & Trent, Reference South and Trent1988; Stone, Reference Stone2019). In contrast, when reproductive-aged females are abundant, they tend to shift toward male-preferred strategies that favor short-term, uncommitted, casual-sex relationships, resulting in lower marriage rates and higher teen pregnancy rates.

Third, young adults of the more prevalent, unfavored sex are likely to reconstruct their environments to improve their mating prospects, both by selectively occupying specific spaces within their environments and by expanding the environment. They may spend more time in spaces that increase their chances of success (e.g., Matz & Harari, Reference Matz and Harari2020), such as spaces where others are also actively seeking partners (e.g., clubs or bars on Friday nights) or where the sex ratio is more favorable—like coffee shops if seeking women or gyms if seeking men. However, when local options remain limited, young adults may expand their search. For instance, males in male-biased sex ratios show greater interest in and higher engagement with sex tourism (Kock, Reference Kock2021) and in seeking or purchasing brides from abroad to offset local mate shortages (Kaur, Reference Kaur2004; Kawaguchi & Lee, Reference Kawaguchi and Lee2017).

Last, young adults of the more prevalent, unfavored sex may be more likely to relocate to a new environment. In hunter-gatherer societies, which often have locally constrained mating options, a biased operational sex ratio motivates dispersal outside of the group (Kramer et al., Reference Kramer, Schacht and Bell2017). Similarly, modern young adults often migrate to urban areas, ostensibly for career or educational opportunities that enhance their reproductive potential (Johnson et al., Reference Johnson, Winkler and Rogers2013). Such relocations are likely to be even more pronounced among individuals of the unfavored sex facing heightened local competition.

3.3.3. Parenting stage

An environment with a biased operational sex ratio is likely perceived as affording both opportunities and threats for parental goals depending on the parent’s sex and their offspring’s sex and life stage. A favorable operational sex ratio for parents enables (a) securing partner investment for offspring care or (b) pursuing additional mating opportunities, whereas a favorable operational sex ratio for offspring in the mating stage enhances their offspring’s reproductive success.

First, the ecological affordances of a biased operational sex ratio likely diverge between male and female parents due to the inherent differences in obligatory reproductive investment, whereby females invest more through gestation and early care. Although greater availability of alternative mates is linked to declines in marital stability in both traditional and modern societies (Jones et al., Reference Jones, Marlowe, Hawkes and O’Connell2000; Uggla & Andersson, Reference Uggla and Andersson2018), the opportunities and threats of a male- versus female-biased operational sex ratio vary for each sex. A male-biased operational sex ratio is likely to be perceived as affording opportunities for female parents to enhance their bargaining power, increasing their chances of securing committed partners who can provide resources and care for their offspring. Such environments, however, are likely perceived as threatening by male parents, especially for those with lower mate value, as their female partners would have opportunities to leave the relationship for higher mate value males who can offer better support and investment.

Conversely, a female-biased operational sex ratio is likely perceived as affording increased mating opportunities for male parents. Although human males generally invest in their offspring, such a sex ratio provides an opportunity for male parents to engage in extra-pair mating, enabling them to father additional offspring with relatively lower investment. This strategy is particularly advantageous in environments where external threats reduce the likelihood that heavy investment in a single offspring will guarantee its survival or future success (Kokko & Jennions, Reference Kokko and Jennions2008). Such environments are likely perceived as posing threats to female parents in securing partnerships and paternal investment (Uggla & Mace, Reference Uggla and Mace2017).

Second, the ecological affordances of a biased operational sex ratio may also have implications for parents with offspring in the mating stage. The reproductive success of offspring is crucial to the parent’s long-term fitness; if offspring fail to secure mates and reproduce, parental investment loses value as the flow of genes to future generations is interrupted. These affordances should thus vary based on the sex of the offspring, not the sex of the parent. For example, a male-biased sex ratio is likely to be perceived as affording threat by parents with sons but as affording opportunity for parents with daughters when their children are in the mating stage.

We hypothesize that parents navigate the ecological affordance of a biased operational sex ratio in two ways. First, they are likely to adjust their reproductive strategies by calibrating their own parenting and mating efforts while closely monitoring their partner’s. In a female-biased operational sex ratio, male parents are likely to increase their (extra-pair) mating efforts for additional reproductive opportunities. In response, female parents likely become more vigilant and exhibit heightened jealousy toward their partner’s interactions with other women to protect their relational and paternal investment. Conversely, in a male-biased operational sex ratio, female parents are likely to demand more parenting effort from the father and may pursue additional mates if unsatisfied, prompting father vigilance and jealousy to protect relational and maternal investment.

Second, parents likely calibrate their financial or material investment in offspring based on expected reproductive success in the environment. In China, for example, parents with sons in male-biased regions are more likely to increase their savings (Wei & Zhang, Reference Wei and Zhang2011). They may recognize that boosting their son’s financial resources improves his relative mate value in a competitive marriage market, thereby increasing their own reproductive success.

3.3.4. Summary

Considering the sex-age ratio reveals that perceived ecological affordances vary by the life stages and sexes of both the actor and the target. For children, the sex ratio of their same-age peers is largely perceived as irrelevant, whereas the sex ratio of the adults in their ecology is likely perceived as posing significant affordances for their development. For young adults in the mating stage, the sex ratio of reproductively viable targets is likely perceived as affording significant mating opportunities and threats. For parents, the sex ratio of reproductively viable targets is likely perceived as having significant affordance implications for both their own mate retention prospects and for their offspring’s mate-seeking prospects once they reach the mating stage. This nuanced pattern of ecological affordances of sex-age ratios necessitates equally nuanced psychological and behavioral management strategies.