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On a warm June evening, nestled among tightly packed houses on the outskirts of Chiang Mai, Northern Thailand’s largest city, a mother explained that her 7-year-old was neither a boy nor a girl. Rather, the child was tung ting, a Thai term that our interpreter translated as “girlish.” As dusk settled in, she went on about how her child was born male but did not tend to display interest in the kinds of rough-and-tumble play that are characteristic of boys. Instead, her child preferred feminine toys, liked to imitate female characters from television and movies, and enjoyed dance. Right on cue, the tung ting child cut in to perform a dance for us, stepping into the soft glow spilling from the window as if it were a small spotlight. As we cheered and clapped, the mother remarked, “When he (sic) grows up, he can be kathoey [a Thai term referring to transfeminine individuals assigned male at birth who are typically attracted to men] and support the family. He can do his dancing to make money and be a success.”
Looking past this tender moment of family life and a mother’s quiet hopes, we can glimpse a broader story about the place of sexual and gender diversity within the ongoing history of our species—a story grounded in the principles of kin selection. From an evolutionary perspective, a trait that leads to reduced rates of reproduction, as would be the case with same-sex sexuality, should gradually diminish in frequency unless it offers some compensatory advantage elsewhere in the evolutionary ledger. Kin selection provides a solution to this conundrum. It proposes that a trait not only persists when it enhances an individual’s own reproductive success but also when it benefits the reproductive success of close relatives who share the same genes—a process known as indirect fitness. In the context of same-sex sexuality, the kin selection hypothesis suggests that any genes associated with the expression of same-sex sexuality can be passed down to future generations, not because same-sex-attracted individuals reproduce themselves, but because they contribute to the well-being of their genetic relatives. In turn, these relatives are better able to survive and reproduce, allowing shared genes—including those linked to same-sex sexuality—to be passed to future generations. Thus, contrary to viewing same-sex sexuality as an evolutionary dead end, this perspective suggests that it is maintained across generations through shared genes and the cooperative dynamics of kinship.
Although terms like tung ting and kathoey are unique to Thailand, the stories they evoke—the male child who dances while others roughhouse, who feels most at home among the feminine rather than the masculine—echo far beyond Southeast Asia. From the sunny shores of Samoa to the windy plains of the Zapotec Isthmus in Mexico, we find accounts of gender-diverse people born male who, from childhood, display interests and behaviors more typical of girls. In addition to these feminine traits, it is also common to hear such individuals described as caretakers, providers, and supporters of their families’ well-being. These observations are not confined to anecdote; they have been documented systematically in ethnographic and quantitative research across diverse cultural contexts, particularly among same-sex-attracted individuals assigned male at birth who adopt culturally specific, non-binary gender labels, including the Samoan fa’afafine, Istmo Zapotec muxes, Thai sao praphet song, and Indonesian waria. The recurrence of kin-directed behavior across diverse cultural settings has prompted scholars to propose that these patterns may reflect universal features of male same-sex sexuality that are in line with the kin selection hypothesis.
Yet, how universal are these patterns truly? To answer this question, we must look not only at whether same-sex-attracted males display kin-directed altruism, but also at how these patterns take shape through development and across cultures. Here, we detail that developmental and cross-cultural lenses not only help to refine evolutionary theory but also reveal the ways in which freedom of sexual and gender expression can benefit families and communities.
Despite growing evidence that same-sex-attracted males often display kin-oriented traits, early tests of the kin selection hypothesis yielded largely inconclusive results. Most of these investigations were conducted in Western or industrialized nations such as the United States, the United Kingdom, Italy, Spain, Canada, and Japan, where researchers typically compared gay and heterosexual men on their self-reported willingness to help relatives in hypothetical scenarios. Relying primarily on self-report questionnaires and urban convenience samples such as university students, community volunteers, or online respondents, these studies seldom found group differences in kin-directed altruism. Some of this work even considered whether contextual factors such as geographical distance from relatives, as examined in Canada, or broader collectivistic values, as explored in Japan, might help explain why earlier research had not revealed differences in kin-directed behavior. While such inquiries were important, they tended to isolate single factors rather than consider how these ecological and social features operate together.
These inconclusive findings reflect what might be considered a kind of disciplinary myopia. By focusing primarily on Western or other similarly industrialized contexts such as Japan, early research overlooked how social and ecological conditions shape the expression of kin-directed altruism. In societies where people have fewer children, relatives live farther apart, and households are economically independent, opportunities for kin-directed behaviors are naturally limited for individuals of any orientation. In contrast, in developing or transitional economies, parents often have children at younger ages and depend on extended family members for practical and emotional support. In such settings, a sexually and/or gender-diverse individual who remains within or nearby the same household in which they grew up can meaningfully assist with childcare, household finances, or the care of ageing parents—roles that become particularly beneficial in settings where families depend on extended kin networks and gain measurable advantages from every additional caregiver or contributor within the household.
Ethnographic and quantitative data alike reveal this pattern. Among the Istmo Zapotec of southern Mexico, for instance, muxes have a long-standing reputation as caretakers who remain close to their parents and contribute to family welfare. Consistent with this reputation, the sisters of muxes report receiving more childcare help from their muxe siblings than women whose brothers are heterosexual. In Samoa, fa’afafine are similarly renowned for their devotion to kin and are among the most empirically documented cases of kin-directed altruism. They not only express a stronger willingness to help nieces and nephews but also engage in tangible acts of childcare, household labor, and monetary support. In Indonesia, same-sex-attracted males also show elevated altruistic tendencies, reporting both greater willingness to assist nieces and nephews and higher levels of actual resource transfer to them, consistent with the predictions of the kin selection hypothesis. In northern Thailand, gay men and sao praphet song likewise report heightened willingness to assist kin, reflecting the same underlying prosocial orientation.
In light of these cross-cultural contrasts, the question becomes not whether greater kin-directed altruism by same-sex-attracted individuals exists, but rather under what circumstances it is expressed. The adaptive feminine phenotype model helps reconcile the cross-cultural discrepancies. This model proposes that kin-directed altruism among same-sex-attracted males arises alongside a cluster of feminine traits that emerge early in development, including empathy, nurturance, and sensitivity to family relationships. Such traits are thought to reflect an evolved predisposition toward prosocial behavior that, in ancestral environments, may have contributed to the survival of kin. Yet, femininity alone is not sufficient for these altruistic patterns to emerge. Their expression depends on ecological and cultural conditions that make them both possible and meaningful. These would include contexts in which families live in close proximity, rely on each other for childcare and material support, and tolerate or even value male femininity. In societies with rich histories of collective caregiving, these dispositions can be channeled into acts that enhance family welfare. In contrast, in cultures where kin networks are fragmented or where male femininity is stigmatized, such behaviors may be suppressed or redirected toward non-kin. In this way, the adaptive feminine phenotype model links biology and context; it suggests that both inherited prosocial tendencies and the environments that afford their expression jointly shape the development of kin-directed altruism.
Evidence consistent with this model can be found in the developmental pathways that connect early emotional dispositions to later kin-directed behavior. One of these pathways involves the expression of childhood separation anxiety—that is, feelings of worry or distress when away from parents or caregivers. In addition to being more commonly observed among girls than boys, cross-cultural, retrospective, and longitudinal research indicates that feminine male children who grow up to be same-sex-attracted also display elevated indicators of childhood separation anxiety. Although often interpreted in clinical contexts as a sign of psychopathology, recent research suggests that these anxieties may instead reflect early emerging tendencies toward empathy and sensitivity toward others’ well-being. Indeed, closer examination of these patterns reveals that this anxiety tends not to be self-focused. Rather than worrying about their own safety or comfort, same-sex-attracted males recall being more concerned about the well-being of their parents and siblings. This kin-focused concern often appears alongside other feminine traits, such as interest in nurturing play with dolls, that together point to an early orientation toward empathy and care for others.
Research in Canada, Samoa, and northern Thailand indicates that these early-emerging, feminine tendencies may develop into adult forms of kin-directed altruism. In each of these countries, stronger childhood concern for family well-being and greater childhood femininity among same-sex-attracted males are each associated with increased willingness in adulthood to help care for nieces and nephews or to contribute to their families’ needs. In Samoa, this association is especially robust: fa’afafine who recalled greater childhood concern for kin’s wellbeing and feminine behaviors and interests also reported higher adulthood levels of kin-directed altruism. In contrast, in Canada, where feminine expression is often discouraged, both gay men’s femininity and kin-directed altruism appear to attenuate over time—a pattern that is consistent with the adaptive feminine phenotype model. Research in northern Thailand reveals similar dynamics among gay men and sao praphet song. Taken together, these findings suggest that kin-directed altruism among same-sex-attracted males arises from a cluster of feminine traits—empathy, nurturance, and sensitivity to family relationships—that begin in childhood and, when culturally permitted, mature into enduring prosocial commitments to kin. Across these cultures, then, what at first glance may seem pathological could in fact represent the developmental roots of prosocial tendencies.
Thus, rather than being set aside after earlier studies failed to find support, the kin selection hypothesis appears to be entering a new phase—one informed by cross-cultural evidence, developmental insight, and an appreciation of the environments that allow care for kin to flourish. In such settings, kin-directed behavior can have tangible effects on family well-being and, by extension, on the evolutionary maintenance of same-sex sexuality. The challenge now is not to decide whether kin selection applies, but to understand the conditions under which it operates and how it interacts with other adaptive processes. Longitudinal studies that follow individuals across development could clarify how early emotional dispositions translate into adult patterns of kin investment, and whether gender-diverse children already display these altruistic tendencies. Cross-cultural projects that systematically compare societies with different family structures, gender norms, and degrees of economic interdependence would help identify the environments where these patterns are most likely to emerge. Equally, quantitative models capable of estimating how much kin-directed altruism offsets the reproductive cost of lower fertility are needed to test the hypothesis’ explanatory scope. Early estimates from Indonesia suggest that helping kin increases the number of nieces and nephews overall, though not enough to offset the lack of one’s own offspring completely, implying that kin altruism likely functions alongside other adaptive mechanisms. In this view, the kin selection hypothesis remains central, not as a stand-alone explanation but as part of a broader evolutionary account of how human cooperation, attachment, and sexual and gender diversity intertwine.
On that warm evening in Chiang Mai, the mother’s description of her tung ting child was not framed in terms of pathology or concern, but in terms of acceptance and hope for a future in which the child’s uniqueness could help the family grow and endure, rather than diminish them. Her words reflect, in simple form, the logic of the kin selection hypothesis: that diversity in sexuality and gender has long contributed to our evolutionary history by fostering patterns of cooperation and caregiving. Reflecting upon her story not only advances evolutionary theory but also underscores how cultural contexts create the social conditions for altruistic dispositions to take root. Ultimately, it is in societies that make space for diverse expressions of sexuality and gender and value interdependence that these prosocial dispositions can flourish, helping to sustain kin networks, strengthen intergenerational bonds, and enrich the collective well-being of families and communities.
Suggested Resources
Ghiasvand, P., & VanderLaan, D. P. (2025). Evolutionary perspectives on same-sex attraction [Video]. Discover Psych YouTube Channel. https://www.youtube.com/watch?v=1QbfLKyBQeM
Gómez Jiménez, F. R., Vasey, P. L. (2020). Kin-directed altruism and the evolution of male androphilia among Istmo Zapotec muxes. Evolution and Human Behavior. 43(3), 224–233. https://doi.org/10.1016/j.evolhumbehav.2022.02.002
Hu, D. Z., Gómez Jiménez, F. R., Saokhieo, P., Kaewthip, O., Supindham, T., Chariyalertsak, S., & VanderLaan, D. P. (2025). Kin-directed altruism and male androphilia in Thailand: Investigating the roles of femininity and neuroticism. Archives of Sexual Behavior, 54, 2861–876. https://doi.org/10.1007/s10508-025-03233-z
Nila, S., Barthes, J., Crochet, P. A., Suryobroto, B., & Raymond, M. (2018). Kin selection and male homosexual preference in Indonesia. Archives of Sexual Behavior, 47, 2455–2465. https://doi.org/10.1007/s10508-018-1202-y
VanderLaan, D. P., Petterson, L. J., & Vasey, P. L. (2017). Elevated kin-directed altruism emerges in childhood and is linked to feminine gender expression: A retrospective study of Samoan fa’afafine. Archives of Sexual Behavior, 46, 95–108. https://doi.org/10.1007/s10508-016-0884-2
VanderLaan, D. P., Petterson, L. J., & Vasey, P. L. (2016). Femininity and kin-directed altruism in androphilic men: A test of an evolutionary developmental model. Archives of Sexual Behavior, 45, 619–633. https://doi.org/10.1007/s10508-015-0632-z
Vasey, P. L., Petterson, L. J., Semenyna, S. W., Gómez Jiménez, F. R., & VanderLaan, D. P. (2020). Kin selection and the evolution of male androphilia. In L. Workman, W. Reader, & J. H. Barkow (Eds.), Cambridge Handbook of Evolutionary Perspectives on Human Behavior (pp. 366–377). Cambridge: Cambridge University Press.
Header Image: Lukas Avendaño is a muxe performer from Oaxaca, Mexico. Photo by Mario Patinho via Wikimedia Commons.
