Kin Selection and Male Homosexual Preference in Indonesia

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.

The Direct Reproductive Cost of Same-Sex Attraction: Evidence from Two Nationally Representative U.S. Samples

Same-sex attraction is associated with a direct reproductive cost, i.e., a reduced number of biological children. The current study aimed to assess this cost for different forms of sexual attraction (i.e., only attracted to opposite sex, mostly attracted to opposite sex, equally attracted to both sexes, mostly attracted to same-sex, only attracted to same-sex), using two large nationally representative datasets (N = 15,208) from the USA. The results indicated that same-sex attraction was associated with substantial loss in direct reproductive output. More specifically, significant differences between the different types of same-sex attraction were found: Exclusive and mostly homosexual orientation identities were associated with the highest direct reproductive cost, while mostly attracted to opposite sex orientation and bisexuality identities were associated with lower direct reproductive costs. In addition, bisexual women did not differ significantly from exclusively heterosexual women in terms of their reproductive output. The implications of these findings for the evolutionary origins of same-sex attraction are further discussed.

Recalled Childhood Separation Anxiety Differs by Anal Sex Role among Gay Men

Male androphilia (i.e., male sexual attraction to adult males) is considered an evolutionary paradox because it is partially influenced by genes and associated with decreased reproduction. Traits associated with attachment to genetic relatives (i.e., kin) could prompt increased kin-directed altruism, thereby offsetting decreased reproduction by helping kin reproduce. These traits include childhood separation anxiety and adulthood neuroticism, which have been associated with feminine gender expression. In prior research, gay men with a receptive (Bottom or Versatile) anal sex role (ASR) reported greater childhood gender nonconformity (GNC) than those with an insertive (Top) ASR. We examined whether ASR groups also differed on recalled childhood separation anxiety and adulthood neuroticism. The Separation Anxiety Scale-Revised and Big-Five Personality Inventory - short form were completed by 350 gay and 146 heterosexual men. For neuroticism, ASR preference groups differed from heterosexual men but not from one another. Gay men who preferred a Bottom or Versatile ASR reported higher recalled childhood separation anxiety than Tops and heterosexual men. Recalled childhood GNC mediated ASR group differences with heterosexual men on childhood separation anxiety. These results indicate that subgroups of gay men delineated by ASR differ on an evolutionarily relevant developmental trait, childhood separation anxiety.

Genomic evidence consistent with antagonistic pleiotropy may help explain the evolutionary maintenance of same-sex sexual behaviour in humans

Human same-sex sexual behaviour (SSB) is heritable, confers no immediately obvious direct reproductive or survival benefit and can divert mating effort from reproductive opportunities. This presents a Darwinian paradox: why has SSB been maintained despite apparent selection against it? We show that genetic effects associated with SSB may, in individuals who only engage in opposite-sex sexual behaviour (OSB individuals), confer a mating advantage. Using results from a recent genome-wide association study of SSB and a new genome-wide association study on number of opposite-sex sexual partners in 358,426 individuals, we show that, among OSB individuals, genetic effects associated with SSB are associated with having more opposite-sex sexual partners. Computer simulations suggest that such a mating advantage for alleles associated with SSB could help explain how it has been evolutionarily maintained. Caveats include the cultural specificity of our UK and US samples, the societal regulation of sexual behaviour in these populations, the difficulty of measuring mating success and the fact that measured variants capture a minority of the total genetic variation in the traits.

Meta-Analyses of Fraternal and Sororal Birth Order Effects in Homosexual Pedophiles, Hebephiles, and Teleiophiles

This study investigated the relations between numbers of older brothers, numbers of older sisters, and the odds of homosexuality in later-born males, including males who are most attracted sexually to prepubescent or early pubescent children (pedohebephiles) and males who are most attracted sexually to adults (teleiophiles). The authors meta-analyzed data from 24 samples of homosexual and heterosexual men, originally reported in 18 studies, and totaling 18,213 subjects. The results confirmed that older brothers increase the odds of same-sex preference in pedohebephiles as they do in teleiophiles. They also replicated the recent finding that older sisters have a similar but weaker statistical association with the odds of homosexuality. These findings have two theoretical implications. First, the findings for older brothers and older sisters indicate some commonality in the factors that influence sexual preference in teleiophiles and those that influence sexual preference in pedohebephiles. Second, the finding for older sisters confirms a prediction stemming from the hypothesis that male fetuses stimulate maternal antibodies that increase the odds of homosexuality in later-born males. Such immunization could result from miscarried as well as full-term fetuses, and number of older sisters should correlate with number of male fetuses miscarried before gestation of the subject.

Direct Reproduction and Sexual Orientation and Gender Diversity in Thailand

Human same-sex sexual attraction is considered to be an evolutionary paradox. This paradox rests on same-sex attracted individuals having lowered direct reproduction, indicating reduced direct fitness of genes that influence same-sex attraction. Yet, relatively few empirical studies have examined the relation between same-sex sexual attraction and direct reproduction. The present study examined direct reproduction (i.e., number of children produced) in a diverse Thai sample (N = 1623) of heterosexual men and women, gay and bisexual men, and lesbian and bisexual women, as well as unique Thai gender categories: sao praphet song (i.e., feminine androphilic third-gender males), toms (i.e., masculine gynephilic third-gender females), and dees (i.e., feminine females who are sexually attracted to toms). In males, heterosexual men were more likely to have produced children than other groups, especially as they aged, and none of the gay men or sao praphet song reported having children. In females, there was some degree of reproduction in each of the groups considered. On average, heterosexual and bisexual women had similar numbers of children, whereas heterosexual women had significantly more children than lesbian women, toms, and dees. Number of children was significantly positively associated with age among heterosexual women, bisexual women, toms, and dees. With age, the discrepancies in average number of children produced between heterosexual women and each of lesbian women and toms became significantly more pronounced. Discussion focuses on how these findings, in conjunction with those of prior studies, inform our understanding of evolutionary selection pressures related to human same-sex sexuality.

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.

Male Homosexual Preference: Femininity and the Older Brother Effect in Indonesia

Male homosexual preference (MHP) is an evolutionary enigma because it is partially heritable and imposes a fertility cost. In occidental societies, homosexual men are feminized at various levels and they have more older brothers than heterosexual men. To evaluate whether femininity and the fraternal birth order (FBO) effect are universal features of MHP or not, we collected original data from homosexual men, heterosexual men, and heterosexual women from Java (Indonesia). Facial photographs were used to test whether homosexual faces are feminized when compared with heterosexual ones. We found that faces manipulated to resemble the average face of homosexual men are perceived as facially feminized, suggesting that homosexual men are facially feminized compared to heterosexual men, although a higher facial femininity was not captured by morphological analyses. Then, family data were used to detect differences in siblings' composition between homosexuals and heterosexuals. Homosexual men displayed a higher number of older brothers than heterosexual men, even when sibship size was controlled for, suggesting that the FBO effect exists in Indonesian populations. Independent of sexual orientation, men with older brothers seem more feminized than those without older brothers, consistent with the immune origin of the FBO effect. In conclusion, MHP in Indonesia is partially feminized and they have more older brothers. Such features are also associated with MHP in other cultural contexts, suggesting a cross-cultural effect of men homosexual preference. An evolutionary explanation is available for the feminizing effect, although the FBO effect remains unexplained even if proximal mechanisms start to be identified.