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

Crusaders, monks and family fortunes: evolutionary models of male homosexuality and related phenomena

The fraternal birth order effect (FBOE) is a well documented preponderance of older brothers among gay men. Until now, it has lacked a proper formal treatment in light of evolutionary theory. In this article, we propose two related mathematical models based on the rigorous application of the inclusive fitness concept. The models examine sibling rivalry and resource allocation within families. One of the models assumes that the laterborn's reproductive ability is not compromised by the manipulation that leads to the exclusive allocation of family resources to the firstborn. The other model posits that this manipulation may lower the laterborn's direct fitness to zero. Both models suggest that the FBOE arises primarily from the older brother's manipulation, as the firstborn's inclusive fitness may increase, while the mother's decreases, but not vice versa. However, under certain conditions, the mother should support an activity that discourages division of family resources, and in extreme cases, the younger brother's homosexuality may be the best available reproductive strategy for his genes. The models also provide insight into the cultural evolution of primogeniture and explain how manipulation between relatives could evolve before the emergence of substantial economic inequality.

Relationship Status Rather than Sociosexuality or Sexual Orientation Predicts Male Sexual Functioning

Male sexual functioning is a prerequisite for conception and consequently for reproduction and is thus a crucial mechanism from an evolutionary and social perspective. Previous studies reported better sexual functioning in coupled compared to single individuals. However, it is not clear whether sexual functioning increases or decreases with a short-term casual sexual strategy, which is another possibility, along with a long-term strategy. Furthermore, sexual orientation may represent yet another mating strategy that may influence sexual functioning. Here, we aimed to test the possible associations between male sexual functioning and sexual strategies, as measured through sociosexuality, relationship status, and sexual orientation. We hypothesize that due to its relationship with the evolved motivation of mate acquisition and mate retention, both stable relationships and sociosexuality would be positively associated with male sexual functioning. We did not expect significant differences in sexual functioning among men with different sexual orientations. We sampled 427 healthy men of different sexual orientations (203 heterosexuals, 77 bisexuals, and 147 gay men) who completed the Male Sexual Function Index (MSFI) and its subscales of desire, arousal, erection, orgasm, and satisfaction; the Sociosexual Orientation Inventory-Revised; and sociodemographic questions. A multivariate general linear model showed that, compared to single individuals, partnered individuals reported greater MSFI-arousal, MSFI-erection, MSFI-orgasm, and MSFI-satisfaction. Sociosexuality and sexual orientation only weakly predicted MSFI domains. Male sexual functioning thus seems to be moderately associated with a long-term mating strategy and can thus serve for relationship maintenance rather than for acquiring new partners.

Occupational Preferences, Childhood Behavior, and Openness: The Role of Sex, Sexual Orientation, and Gender Identity in Iran

Previous research suggests that both same-sex attraction and the personality trait "openness" are associated with sex-atypical preferences and behaviors. Here, we examined the links between adulthood occupational preferences, childhood play behavior, and openness among Iranian cisgender gynephilic males (n = 228), cisgender ambiphilic males (n = 48), cisgender androphilic males (n = 178), transgender androphilic males (n = 58), cisgender androphilic females (n = 226), cisgender ambiphilic females (n = 94), cisgender gynephilic females (n = 31), and transgender gynephilic females (n = 121) from Iran. Cisgender and transgender same-sex attracted males and females exhibited sex-atypical occupational preferences with the latter group showing even more sex-atypicality than the former. The personality trait openness did not differ between cisgender groups. Transgender androphilic males had a significantly higher mean score for openness compared to cisgender androphilic females and transgender gynephilic females, whereas transgender gynephilic females had a significantly lower mean score compared to cisgender androphilic males. In both males and females, childhood sex-atypicality, same-sex attraction, and openness were associated with sex-atypical occupational preferences. Our findings from Iran provides cross-cultural support for interconnectedness of childhood and adulthood sex-atypicality, openness, and same-sex attraction in males and females who are cisgender and transgender.

Fertility of Czech Gay and Straight Men, Women, and Their Relatives: Testing the Sexually Antagonistic Gene Hypothesis

One proposal for the persistence of homosexuality in the human population is the sexually antagonistic gene hypothesis, which suggests that the lower fertility of homosexual individuals, especially men, may be compensated by higher fertility of their relatives of the opposite sex. To test this hypothesis, we have collected data from 7,312 heterosexual men, 459 gay men, 3,352 heterosexual women, and 79 lesbian women mainly from Czechia. In an online survey, participants answered questions regarding their own as well as their parents' and grandparents' fertility. For men, we obtained no significant results except for higher fertility of gay men's paternal grandmothers, but the magnitude of this effect was very small. For the female sample, we recorded lower fertility of lesbian women's mothers and fathers. In line with our expectations, both gay men and lesbian women had lower fertility rates than their heterosexual counterparts. Our results are consistent with recent studies which likewise do not support the sexually antagonistic gene hypothesis.

Genetic variants underlying human bisexual behavior are reproductively advantageous

Because human same-sex sexual behavior (SSB) is heritable and leads to fewer offspring, how SSB-associated alleles have persisted and whether they will remain in human populations are of interest. Using the UK Biobank, we address these questions separately for bisexual behavior (BSB) and exclusive SSB (eSSB) after confirming their genetic distinction. We discover that male BSB is genetically positively correlated with the number of offspring. This unexpected phenomenon is attributable to the horizontal pleiotropy of male risk-taking behavior-associated alleles because male risk-taking behavior is genetically positively correlated with both BSB and the number of offspring and because genetically controlling male risk-taking behavior abolishes the genetic correlation between male BSB and the number of offspring. By contrast, eSSB is genetically negatively correlated with the number of offspring. Our results suggest that male BSB-associated alleles are likely reproductively advantageous, which may explain their past persistence and predict their future maintenance, and that eSSB-associated alleles are likely being selected against at present.

Digit Ratio (2D:4D; Right-Left 2D:4D) and Multiple Phenotypes for Same-Sex Attraction: The BBC Internet Study Revisited

Same-sex attraction may be linked to low prenatal androgen (in men) and high prenatal androgen (in women). Digit ratio (2D:4D) is thought to be a negative correlate of prenatal androgen and right-left 2D:4D (Dr-l) to reflect lateralized differences in sensitivity to prenatal androgen. Lower 2D:4D has been reported for lesbians compared to heterosexuals, but links to high 2D:4D in gay men are less clear. The largest study thus far (the BBC Internet study) found no significant difference between the 2D:4D of lesbians and heterosexual women but a higher 2D:4D in gay men compared to heterosexual men. Here we consider the possibility that low and high prenatal androgen is associated with same-sex attraction in men (n = 108,779) and women (n = 87,742), resulting in more than two phenotypes. We examined the associations between 2D:4D, Dr-l, and same-sex attraction scores in the BBC Internet study. In contrast to the earlier report, which considered sexual orientation in categories, there were positive linear associations in men (right and left 2D:4D, but not Dr-l) and negative linear associations in women (right 2D:4D and Dr-l, but not left 2D:4D). There were no curvilinear relationships for right and left 2D:4D. However, Dr-l showed a U-shaped association with same-sex attraction in men. Thus, (1) high prenatal androgen may be implicated in female homosexuality, while both low and high prenatal androgen may be implicated in male homosexuality, and (2) large side differences in sensitivity to androgen may be associated with elevated same-sex attraction in men.

Male androphilia, fraternal birth order, and female fecundity in Samoa: A 10-y retrospective

Two separate but related literatures have examined familial correlates of male androphilia (i.e., sexual attraction and arousal to masculine adult males). The fraternal birth order effect (FBOE) is a widely established finding that each biological older brother a male has increased the probability of androphilia 20-35% above baseline rates. Other family demographic variables, such as reproduction by mothers, maternal aunts, and grandmothers, have been used to test evolutionary hypotheses that sexually antagonistic genes lead to androphilia among males, lowering or eliminating reproduction, which is offset by greater reproductive output among their female relatives. These proposed female fecundity effects (FFEs), and the FBOE, have historically been treated as separate yet complementary ways to understand the development and evolution of male androphilia. However, this approach ignores a vital confound within the data. The high overall reproductive output indicative of an FFE results in similar statistical patterns as the FBOE, wherein women with high reproductive output subsequently produce later-born androphilic sons. Thus, examination of the FBOE requires analytic approaches capable of controlling for the FFE, and vice-versa. Here, we present data simultaneously examining the FBOE and FFE for male androphilia in a large dataset collected in Samoa across 10 y of fieldwork, which only shows evidence of the FBOE.

Sexual and romantic spectrums: Mostly straights and mostly gays/lesbians

Sexual and romantic orientations and identities exist along a spectrum with varying degrees of compliance to traditional concepts of sex and romance. Recent investigations have focused on individuals who are not exclusive in their sexual and romantic lives-mostly straights and mostly gays/lesbians. Multi-disciplinary research reveals the diversity of individuals' internal and external sexual and romantic feelings and expressions. One corrective strategy is to scrap categorically based assessment tools and replace them with continuum measures that capture the multiplicity of individuals' sexual and romantic lives. Here I suggest several new measures that are more responsive to a spectrum approach.