Societal Norms Rather Than Sexual Orientation Influence Kin Altruism and Avuncularity in Tribal Urak-Lawoi, Italian, and Spanish Adult Males

Testing Fraternal Birth Order Effects and Antagonistic Effects for Homosexual Men: Power Comparison of Various Methods

Research on the biological determinants of male homosexual preference has long realized that the older brother effect (FBOE, i.e., a higher fraternal birth rank of homosexuals) and the antagonist effect (AE, i.e., more fertile women have a higher chance of having a homosexual son) can both generate family data where homosexual men have more siblings and more older siblings than heterosexual men. Various statistical approaches were proposed in the recent literature to evaluate whether the action of FBOE or AE could be discriminated from empirical data, by controlling for the other effect. Here, we used simulated data to formally compare all the approaches that we could find in the relevant literature for their ability to reject the null hypothesis in the presence of a specified alternative hypothesis (tests based on regression, Bayesian modeling, or contingency tables). When testing for the FBOE, the relative performance of the different tests was different depending on the specific function generating the older brother effect. Even if no tests were found to always perform better than the others, some tests performed systematically poorly, and some tests displayed a systematic high rate of type-I error. For testing the AE, the relative performance of the tests was generally not changed across all parameter values assayed, providing a clear ranking of the various proposed approaches. Pros and cons for each candidate test are discussed, taking into consideration power and the rate of type-I error but also practicability, the possibility to control for confounding variables, and to consider alternative hypotheses.

Recalled Childhood Separation Anxiety in Relation to Sex, Sexual Orientation, and Gender Identity in Thailand

Same-sex sexual attraction is an evolutionary paradox. It is influenced by genes but also associated with reduced reproduction. An altruistic disposition toward kin could increase relatives' reproduction, thus addressing this paradox. Developmentally, such a disposition may manifest as anxiety during separation from kin. This hypothesis has been supported among androphilic (i.e. sexually attracted to males) males, but few similar studies of gynephilic (i.e. sexually attracted to females) females exist. We examined recalled childhood separation anxiety in Thailand and employed the largest and most sexually and gender-diverse sample to date (N = 1403). Among heterosexuals, men recalled less concern for parents' and siblings' well-being during separation than women. Androphilic males evidenced elevated concern for parents' and siblings' well-being during separation, and transfeminine androphilic males known as sao praphet song reported heightened anxiety when separated from parents. Among females, transmasculine gynephilic females known as toms recalled less concern about parents' well-being during separation than feminine females who are attracted to toms and known as dees. Bisexual women recalled less anxiety when separated from parents than dees and lesbian and heterosexual women. These findings suggest a disposition toward heightened kin-directed altruism is evident in childhood among androphilic males, but not gynephilic females.

A Test of the Kin Selection Hypothesis for Female Gynephilia in Thailand

Female gynephilia (i.e., sexual attraction to adult females) is considered an evolutionary paradox because it reduces direct reproduction, yet it is influenced by genetic factors and has persisted over time and across different cultures. The Kin Selection Hypothesis proposes that same-sex attracted individuals offset their lowered direct reproduction by engaging in kin-directed altruism that increases the reproduction of close genetic relatives, thereby enhancing inclusive fitness. Previous research on male same-sex attraction found evidence to support this hypothesis in some cultures. The present study employed a Thai sample to compare altruistic tendencies towards kin and non-kin children in heterosexual women (n = 285), lesbian women (n = 59), toms (i.e., masculine gynephilic females who take on a nonbinary gender identity; n = 181), and dees (i.e., feminine gynephilic females who are attracted to toms; n = 154). The Kin Selection Hypothesis of same-sex attraction predicts that gynephilic groups would show increased kin-directed altruism compared with heterosexual women, but we did not find evidence supporting this prediction. Instead, the tendency to invest more towards kin than non-kin children was more exaggerated in heterosexual women than lesbian women. Also, heterosexual women showed greater dissociation between kin and non-kin altruistic tendencies compared with toms and dees, which may suggest the former's cognition is better attuned for kin-directed altruism. Thus, the present findings were contrary to the Kin Selection Hypothesis for female gynephilia. Alternative explanations regarding the maintenance of genetic factors predisposing individuals to female gynephilia are discussed and require further investigation.

Offspring Production Among the Relatives of Istmo Zapotec Men and Muxes

Male androphilia (i.e., sexual attraction toward adult males) is influenced by biological factors, reliably occurs across diverse cultures, and has persisted over evolutionary time despite the fact that it reduces reproduction. One possible solution to this evolutionary paradox is the sexually antagonistic gene hypothesis (SAGH), which states that genes associated with male androphilia reduce reproduction when present in males but increase reproduction when present in their female relatives. The present study tested the SAGH among the Istmo Zapotec-a non-Euro-American culture in Oaxaca, Mexico, where transgender and cisgender androphilic males are known as muxe gunaa and muxe nguiiu, respectively. To test the SAGH, we compared offspring production by the biological relatives of muxe gunaa (n = 115), muxe nguiiu (n = 112), and gynephilic men (i.e., cisgender males who are sexually attracted to adult females; n = 171). The mothers and paternal aunts of muxe gunaa had higher offspring production than those of muxe nguiiu. Additionally, the relatives of muxe gunaa had more offspring than those of gynephilic men, whereas no such differences were found between the families of gynephilic men and muxe nguiiu. Elevated reproduction by the mothers and, particularly the aunts, of muxe gunaa is consistent with the SAGH. However, the absence of group differences between gynephilic men and muxe nguiiu, and the group differences between the two types of muxes are not predicted by the SAGH. This is the first study to demonstrate reproductive differences between kin of transgender and cisgender androphilic males within the same non-Euro-American culture.

The molecular mechanisms of sexual orientation and gender identity

Differences between males and females are widely represented in nature. There are gender differences in phenotypes, personality traits, behaviors and interests, cognitive performance, and proneness to specific diseases. The most marked difference in humans is represented by sexual orientation and core gender identity, the origins of which are still controversial and far from being understood. Debates continue on whether sexual behavior and gender identity are a result of biological (nature) or cultural (nurture) factors, with biology possibly playing a major role. The main goal of this review is to summarize the studies available to date on the biological factors involved in the development of both sexual orientation and gender identity. A systematic search of published evidence was performed using Medline (from January 1948 to June 2017). Review of the relevant literature was based on authors' expertise. Indeed, different studies have documented the possible role and interaction of neuroanatomic, hormonal and genetic factors. The sexual dimorphic brain is considered the anatomical substrate of psychosexual development, on which genes and gonadal hormones may have a shaping effect. In particular, growing evidence shows that prenatal and pubertal sex hormones permanently affect human behavior. In addition, heritability studies have demonstrated a role of genetic components. However, a convincing candidate gene has not been identified. Future studies (e.i. genome wide studies) are needed to better clarify the complex interaction between genes, anatomy and hormonal influences on psychosexual development.

Familial patterning and prevalence of male androphilia among Istmo Zapotec men and muxes

Male androphilia (i.e., male sexual attraction to other adult males) is known to cluster within families. Some studies demonstrate that male androphilia clusters in both the paternal and maternal familial lines, whereas other studies demonstrated that it clusters only in the latter. Most of these studies were conducted in Euro-American populations where fertility is low and the sexual orientation of male relatives can sometimes be difficult to ascertain. These two factors can potentially confound the results of such studies. To address these limitations, we examined the familial patterning of male androphilia among the Istmo Zapotec of Oaxaca, Mexico--a high fertility, non-Euro-American population where androphilic males are known locally as muxes, a third gender category. The Istmo Zapotec recognize two types of muxes--muxe gunaa and muxe nguiiu--who typify the transgender and cisgender forms of male androphilia, respectively. We compared the familial patterning of male androphilia between muxe gunaa and muxe nguiiu, as well as between gynephilic men and muxes (both cisgender and transgender forms combined). Istmo Zapotec muxe gunaa and muxe nguiiu exhibit similar familial patterning of male androphilia. Overall, muxes were characterized by significantly more muxe relatives than gynephilic men. This familial patterning was equivalent in both the paternal and maternal lines of muxes. The population prevalence rate of male androphilia was estimated to fall between 3.37-6.02% in the Istmo Zapotec. This is the first study that has compared cisgender and transgender androphilic males from the same high fertility population and demonstrated that the two do not differ with respect to the familial patterning of male androphilia.

Elevated Kin-Directed Altruism Emerges in Childhood and Is Linked to Feminine Gender Expression in Samoan Fa'afafine: A Retrospective Study

Androphilia refers to sexual attraction toward adult males, whereas gynephilia refers to sexual attraction toward adult females. The kin selection hypothesis posits that androphilic males help kin increase their reproductive output via kin-directed altruism, thus offsetting their own lowered reproduction and contributing to the fitness of genes underpinning male androphilia. Support for this hypothesis has been garnered in several Samoan studies showing that feminine androphilic males (known locally as fa'afafine) report elevated willingness to invest in nieces and nephews in adulthood. Also, recalled childhood kin attachment and concern for kin's well-being are elevated among Canadian androphilic males (i.e., gay men) and positively associated with childhood feminine gender expression. This study examined whether these childhood patterns were cross-culturally consistent and associated with adulthood kin-directed altruism in a Samoan sample. Samoan gynephilic men, androphilic women, and fa'afafine (N = 470) completed measures of recalled childhood kin attachment and concern for the well-being of kin, recalled childhood gender expression, and willingness in adulthood to invest in nieces and nephews. Fa'afafine recalled elevated anxiety due to separation from kin relative to men and elevated concern for kin's well-being relative to both men and women. Within groups, these characteristics were most robustly associated with childhood feminine gender expression and willingness in adulthood to invest in nieces and nephews among fa'afafine. These findings are consistent with the kin selection hypothesis and the adaptive feminine phenotype model, which proposes that a disposition toward elevated kin-directed altruism among androphilic males is associated with feminine gender expression.

A Problematic Test of the Kin Selection Hypothesis Among the Urak-Lawoi of Ko Lipe, Thailand: Commentary on Camperio Ciani, Battaglia, and Liotta (2015)

Camperio Ciani et al. argued that the Urak-Lawoi people of Ko Lipe island live in a "traditional," "subsistence primitive society" reminiscent of the "ancestral" human past and that their socio-cultural situation is "remarkably similar" to Samoa. On this basis, they asserted that the Ko Lipe Urak-Lawoi are an appropriate population for determining the role that kin selection played in the evolution of male androphilia. The purpose of this commentary is to outline some of our concerns with this characterization and with the statistical analyses conducted by Camperio Ciani et al. in their study of the Urak-Lawoi.

Response: Avuncularity and Kin Selection in Homosexuals: A Problematic Test or a Problematic Hypothesis?

Here we respond to Vasey et al.'s critical comments regarding our article, "Societal norms rather than sexual orientation influence kin altruism and avuncularity in tribal Urak-Lawoi, Italian, and Spanish adult males" (Camperio Ciani, Battaglia, & Liotta, 2015 , JSR doi:10.1080/00224499.2014.993748). The first regards the selection of the Urak-Lawoi population of Ko Lipeh, which is considered too modern and touristic to be adequate to test the kin selection and avuncular hypothesis for homosexuality. We provide historical evidence of the contrary, and show that the population at the inception of our 10 years research was indeed primitive and tribal, and probands actually grew and lived in such a society. Only a few years after the 2004 tsunami, the island was developed and invaded by mass tourism. The second comment regarded the statistical analysis and interpretation of data. We show that we consistently and conservatively considered the effects of all confounding variables, both with comparative tests, and by a series of multivariate regression analyses. This was the orthodox procedure approved by all other reviewers. In conclusion, even addressing these comments, we maintain that the kin selection and avuncularity hypothesis for homosexuality is not supported by empirical data even in this primitive and tribal society.

Human homosexuality: a paradigmatic arena for sexually antagonistic selection?

Sexual conflict likely plays a crucial role in the origin and maintenance of homosexuality in our species. Although environmental factors are known to affect human homosexual (HS) preference, sibling concordances and population patterns related to HS indicate that genetic components are also influencing this trait in humans. We argue that multilocus, partially X-linked genetic factors undergoing sexually antagonistic selection that promote maternal female fecundity at the cost of occasional male offspring homosexuality are the best candidates capable of explaining the frequency, familial clustering, and pedigree asymmetries observed in HS male proband families. This establishes male HS as a paradigmatic example of sexual conflict in human biology. HS in females, on the other hand, is currently a more elusive phenomenon from both the empirical and theoretical standpoints because of its fluidity and marked environmental influence. Genetic and epigenetic mechanisms, the latter involving sexually antagonistic components, have been hypothesized for the propagation and maintenance of female HS in the population. However, further data are needed to truly clarify the evolutionary dynamics of this trait.