Paul L. Vasey

Homosexual behavior in primates: A review of evidence and theory

Homosexual behavior is defined as genital contact, genital manipulation or both between same-sex individuals. Available data indicate that this behavior is phylogenetically widespread among the anthropoid primates, but totally absent among prosimians. The majority of the 33 species that demonstrate homosexual behavior do so rarefy, but for a substantial number (N =12) it appears to be a more common pattern under free-ranging conditions. I summarize data on homosexual behavior as it relates to form, living condition, age, sex, social organization, and ecological context, and discuss hormonal, demographic, and sociosexual theories for primate homosexual behavior. Among adult primates, the behavior is not the product of abnormal excesses or deficiencies in androgens. Prenatal excesses of androgens may have some effect on the expression of female homosexual behavior, but these effects might vary over the life span, and data are equivocal at present. Demographic processes that result in skewed sex ratios can favor the expression of homosexual behavior in a population, which causes intraspecific variation. I examine several sociosexual explanations, including (a) proceptivity enhancement, (b) receptivity reduction, (c) dominance assertion, (d) practice for heterosexual copulation, (e) tension regulation, (f) reconciliation, and (g) alliance formation. An evolutionary scenario highlights the transformations this behavior underwent during the evolution of the anthropoid primates. I suggest exaptation as a theoretical framework for interpreting homosexual behavior and conclude that future consideration of sexual selection among primates should address homosexual components of this process.

Relatedness threshold for nepotism in Japanese macaques

Abstract Although nepotism has been evidenced in various areas of primate behaviour, relatively little is known on how it varies with degree of relatedness (r). For example, nepotism might decrease proportionally and asymptotically with relatedness, or it might decrease up to a certain relatedness threshold, beyond which it plummets. This issue was investigated by analysing one type of beneficent behaviours, agonistic support (third-party interventions in conflicts), among laboratory-housed female Japanese macaques, Macaca fuscata. In previous experiments, mothers and sisters were tested in situations in which they could help their daughters and younger sisters to outrank peers, which they did successfully. In the present experiments, grandmothers and aunts were tested using similar protocols. In the presence of their grandmother, granddaughters outranked all dominant peers, but in the presence of their aunt, nieces could not. These combined results point to a relatedness threshold for effective nepotistic aiding above the aunt–niece level of relatedness. Analysis of 1002 interventions received by juvenile females from various categories of adult female kin in the unmanipulated, complete group confirmed this hypothesis. Finally, 11 years of data on the distribution of female homosexual activity according to relatedness provided complementary evidence for the threshold hypothesis. Non-kin females commonly engaged in homosexual interactions, as did aunts and nieces, but closer kin never did, suggesting that aunts and nieces do not recognize each other as kin for the purpose of sexual interactions. Together, these results point to a relatedness threshold for nepotistic aiding at r=0.25 and suggest that this threshold is not behaviour-specific but generalized.

Is gender identity disorder in children a mental disorder

Empirical studies were evaluated to determine whether Gender Identity Disorder (GID) in children meets the Diagnostic and Statistical Manual of Mental Disorders-4th Edition (DSM-IV, American Psychiatric Association, 1994) definitional criteria of mental disorder. Specifically, we examined whether GID in children is associated with (a) present distress; (b) present disability; (c) a significantly increased risk of suffering death, pain, disability, or an important loss of freedom; and if (d) GID represents dysfunction in the individual or is simply deviant behavior or a conflict between the individual and society. The evaluation indicates that children who experience a sense of inappropriateness in the culturally prescribed gender role of their sex but do not experience discomfort with their biological sex should not be considered to have GID. Because of flaws in the DSM-IV definition of mental disorder, and limitations of the current research base, there is insufficient evidence to make any conclusive statement regarding children who experience discomfort with their biological sex. The concluding recommendation is that, given current knowledge, the diagnostic category of GID in children in its current form should not appear in future editions of the DSM.

Sexual Orientation, Controversy, and Science

Summary Ongoing political controversies around the world exemplify a long-standing and widespread preoccupation with the acceptability of homosexuality. Nonheterosexual people have seen dramatic surges both in their rights and in positive public opinion in many Western countries. In contrast, in much of Africa, the Middle East, the Caribbean, Oceania, and parts of Asia, homosexual behavior remains illegal and severely punishable, with some countries retaining the death penalty for it. Political controversies about sexual orientation have often overlapped with scientific controversies. That is, participants on both sides of the sociopolitical debates have tended to believe that scientific findings—and scientific truths—about sexual orientation matter a great deal in making political decisions. The most contentious scientific issues have concerned the causes of sexual orientation—that is, why are some people heterosexual, others bisexual, and others homosexual? The actual relevance of these issues to social, political, and ethical decisions is often poorly justified, however.

Avuncular Tendencies and the Evolution of Male Androphilia in Samoan Fa’afafine

The kin selection hypothesis for male androphilia holds that genes for male androphilia can be maintained in a population if the fitness costs of not reproducing directly are offset by enhancing indirect fitness. Kin share some proportion of genes identical by virtue of descent. Theoretically speaking, androphilic males can increase their fitness indirectly by allocating altruistic behavior toward kin, which, in turn, allows kin to increase their reproductive success. Research conducted in Independent Samoa has shown that androphilic males (known locally as fa’afafine) report significantly higher avuncular tendencies relative to gynephilic men. Here, we replicate this sexual orientation difference, using a larger, independent sample, suggesting that the documented sexual orientation difference in avuncular tendencies in Independent Samoa is genuine. We also extend previous research by showing that fa’afafine exhibit significantly higher avuncular tendencies even when compared to a more closely matched control group that also lacks direct parental care responsibilities (i.e., gynephilic men with no children). Although the greater avuncular tendencies of fa’afafine relative to gynephilic men are consistent with the predictions of the kin selection hypothesis for male androphilia, further research is needed before deeming male androphilia an adaptation for promoting elevated avuncularity. Likewise, more research is needed before deeming elevated avuncularity in fa’afafine an evolved adaptation for promoting indirect fitness. We discuss these findings in the context of alternative evolutionary explanations for male androphilia (i.e., an evolved by-product of an adaptation).

Men's Preferences for Women's Breast Morphology in New Zealand, Samoa, and Papua New Guinea

Sexual selection via mate choice may have influenced the evolution of women’s breast morphology. We conducted an image-based questionnaire quantifying and comparing the preferences of men from Papua New Guinea (PNG), Samoa, and New Zealand (NZ) for images of women’s breast size, breast symmetry, areola size, and areolar pigmentation. Results showed that men from PNG preferred larger breasts to a greater extent than men from Samoa and NZ, providing some support for the hypothesis that men from subsistence living cultures have a greater preference for morphological cues indicative of caloric reserves. Symmetrical breasts were most attractive to men in each culture. However, preferences were highest among NZ men, followed by men from Samoa, and were lowest among men from PNG. These results did not support the hypothesis that people living in higher pathogen environments have a greater preference for traits indicative of pathogen resistance and developmental stability. Large areolae were preferred among men from PNG, and to a lesser extent in Samoa, while in NZ men preferred medium-sized areolae. Thus, men’s preferences for women’s areolar size appear to be highly culturally specific. Darkly pigmented areolae were most attractive to men from Samoa and PNG, whereas men from NZ preferred areolae with medium pigmentation. These findings suggest that areolar pigmentation indicative of sexual maturity is preferred by men rather than lighter pigmentation, which may signal that a woman is in the early years of reproductive maturity. This study highlights the importance of cross-cultural research when testing the role of morphological cues in mate choice.

Female choice and inter-sexual competition for female sexual partners in Japanese macaques

Observations of a captive colony of Japanese macaques (Macaca fuscata) were carried out with the goal of documenting (1) competition between males and females for female sexual partners and (2) choice of same-sex sexual partners by females, despite the presence of motivated, opposite-sex alternatives. Data were collected during 21 homosexual consorts hips involving 14 females living in a mixed-sex group of 37 individuals. Intersexual competition for female sexual partners was manifested when a sexually motivated male and female (competitors) simultaneously sought exclusive access to the same infertile or post-conception female (focus of competition). This occurred during male intrusions on female homosexual consortships and counter-challenges by female competitors against intruding males. Inter-sexual competition for female sexual partners took the form of approaches and solicitations directed at the focus of competition, as well as displacements and aggression directed at ones competitor. Females did not acquire alloparental care for their immature offspring from their same-sex consort partners. Thus, female competitors appeared to engaged in potentially risky competition for same-sex sexual partners in the absence of any obvious reproductive benefit ( e.g. insemination or alloparental care). Following these interactions, females which were foci of competition chose to mount with the female competitors significantly more often than not.

An Adaptive Cognitive Dissociation Between Willingness to Help Kin and Nonkin in Samoan Fa’afafine

Androphilia refers to sexual attraction and arousal to adult males, whereas gynephilia refers to sexual attraction and arousal to adult females. Previous research has demonstrated that Samoan male androphiles (known locally as fa’afafine) exhibit significantly higher altruistic tendencies toward nieces and nephews than do Samoan women and gynephilic men. The present study examined whether adaptive design features characterize the psychological mechanisms underlying fa’afafine’s elevated avuncular tendencies. The association between altruistic tendencies toward nieces and nephews and altruistic tendencies toward nonkin children was significantly weaker among fa’afafine than among Samoan women and gynephilic men. We argue that this cognitive dissociation would allow fa’afafine to allocate resources to nieces and nephews in a more economical, efficient, reliable, and precise manner. These findings are consistent with the kin selection hypothesis, which suggests that androphilic males have been selected over evolutionary time to act as “helpers-in-the-nest,” caring for nieces and nephews and thereby increasing their own indirect fitness.

Male Androphilia in the Ancestral Environment

The kin selection hypothesis posits that male androphilia (male sexual attraction to adult males) evolved because androphilic males invest more in kin, thereby enhancing inclusive fitness. Increased kin-directed altruism has been repeatedly documented among a population of transgendered androphilic males, but never among androphilic males in other cultures who adopt gender identities as men. Thus, the kin selection hypothesis may be viable if male androphilia was expressed in the transgendered form in the ancestral past. Using the Standard Cross-Cultural Sample (SCCS), we examined 46 societies in which male androphilia was expressed in the transgendered form (transgendered societies) and 146 comparison societies (non-transgendered societies). We analyzed SCCS variables pertaining to ancestral sociocultural conditions, access to kin, and societal reactions to homosexuality. Our results show that ancestral sociocultural conditions and bilateral and double descent systems were more common in transgendered than in non-transgendered societies. Across the entire sample, descent systems and residence patterns that would presumably facilitate increased access to kin were associated with the presence of ancestral sociocultural conditions. Among transgendered societies, negative societal attitudes toward homosexuality were unlikely. We conclude that the ancestral human sociocultural environment was likely conducive to the expression of the transgendered form of male androphilia. Descent systems, residence patterns, and societal reactions to homosexuality likely facilitated investments in kin by transgendered males. Given that contemporary transgendered male androphiles appear to exhibit elevated kin-directed altruism, these findings further indicate the viability of the kin selection hypothesis.

Homosexual behaviour in birds: frequency of expression is related to parental care disparity between the sexes

Homosexual behaviour occurs in over 130 species of birds, yet explaining its maintenance in evolutionary terms appears problematic at face value, as such sexual behaviours do not seem in immediate pursuit of reproductive goals. Parental care sexual conflict theory predicts that release from parental care translates to an increased propensity towards polygamous sexual behaviour. We hypothesized that homosexual behaviour(s) may be expected to increase in frequency for the sex that invests less in parental care and potentially enjoys increased mating opportunities. Consistent with our predictions, lower relative contribution to parental care for a particular sex is related to increased frequency of occurrence of homosexual behaviour. For males, highly polygynous species with minimal male parental investment exhibit higher frequencies of male homosexual behaviour, including male–male mounting and especially courtship. In socially monogamous species, male parental investment is greater, and the expression of male homosexual behaviour is lower. Similarly, among pair-bonding species, frequencies of male–male pair bonding increase with decreases in male contribution to care relative to females. When females of socially monogamous species provide less care than males, they exhibit higher frequencies of homosexual behaviour, namely pair bonding and courtship activities. Conversely, when females of polygynous species provide the bulk of parental care, female–female sexual behaviour is infrequently expressed. Homosexual behaviour in birds is more likely to occur under scenarios of enhanced mating opportunity without necessarily influencing reproductive success and thus may exist neutrally, or alternatively provide a behavioural template co-opted for adaptive design.