Homosexuality Paradox, The
KeywordsReproductive Success Sexual Attraction Female Stimulus Homosexual Woman Male Homosexuality
The heritable components of human homosexuality are not immediately explicable according to basic evolutionary principles because they entail reduced average individual reproductive success relative to alternative genotypes.
Evolutionary psychology generates research questions by considering the ancestral survival and reproductive challenges faced by humans and human ancestors. At the heart of evolutionary thinking is reproductive success, as every trait informed by genetics must allow for its own replication in the next generation. For example, a genetic mutation that causes early death or sterility will not replicate under normal circumstances and is thus quickly removed from the population. Less extreme cases of limited reproductive success also tend to be outcompeted by prevailing alternatives. If a mutation merely reduces fertility (i.e., lower probability of fertilization at each copulation, all else equal), one may expect it to be outcompeted by alternatives and exit the gene pool after some number of generations. Homosexuality appears to be a trait that, like sterility or reduced fertility, reduces the likelihood of reproduction. One can think of exclusive homosexuality as analogous to sterility and non-exclusive homosexuality as analogous to reduced fertility. This has led researchers to wonder how homosexuality continues to appear at a relatively consistent rate in the human population. Strictly speaking, a paradox is an inconsistent or logically contradictory premise; the maintenance of homosexuality is better described as a puzzle or a mystery, since a solution exists, even if it is not forthcoming. Before looking at how researchers have responded to this puzzle, it is necessary to consider whether it is appropriate to apply evolutionary thinking to homosexuality in the first place.
Williams (1966) encouraged evolutionary thinkers to avoid invoking evolutionary theory whenever simpler mechanisms are sufficient in explaining a phenomenon. If the expression of homosexuality is not influenced by genetics or other heritable elements, then evolution will not help explain its existence. If the expression of homosexuality is not common or culturally universal, then evolution may be unnecessary to explain its existence. Likewise, if the expression of homosexuality is not an ancestral human condition, evolutionary theory may be unnecessary. Finally, if the expression of homosexuality does not reduce reproductive success relative to alternatives, then it is not fundamentally mysterious according to evolutionary theory. Evidence regarding each of these conditions is discussed ahead.
Does Homosexuality Have Heritable Components?
Heritability estimates (h 2) describe the proportion of phenotypic variance that is attributable to genetic relatedness. These estimates are calculated by looking at the probability of expressing a particular phenotype (in this case, homosexual identification) as a function of genetic relatedness to others who express that phenotype (in this case, homosexual relatives), compared to the probability of expressing the phenotype in the general population. In other words, a trait is heritable to the extent that it is more likely to reoccur within families than between unrelated people. Especially useful for these estimates is data on identical twins who were reared apart and thus did not also share identical developmental environments. Depending on the methods and statistical assumptions used, twin studies provide heritability estimates between roughly h 2 = 0.3 and 0.6 for men and h 2 = 0.2 and 0.7 for women (e.g., Alanko et al. 2010). These are considered moderate to large estimates for psychological variables; personality variables cluster around h 2 = 0.5 and intelligence is between h 2 = 0.5 and 0.8. As long as a trait is heritable, it can be subject to natural selection; thus, it is irrelevant in this context where the genes that contribute to homosexuality are located or what they do. That said, one popular hypothesis for males is that differences in genes on the X chromosome of mother and child cause developmental feminization of male brain regions involved in sexual behavior (Mustanski et al. 2005). No specific genetic hypotheses are available for females, although prenatal androgen exposure appears to be important.
Is Homosexuality Universal and Frequent?
Rates of homosexual identification vary by region, but anthropological studies in every region of the world find homosexual expression of some description. These expressions can differ wildly. Egalitarian homosexuality is the predominant form in modern Western societies, wherein two adult, gender-typical individuals enter into short- and long-term sexual relationships. However, transgendered forms of homosexuality, wherein homosexuals adopt opposite-sex gender roles and appearance from a young age, have also been well documented in populations like the fa’afafine of Samoa, the hijra of India, the kathoey of Thailand, and the berdache of Native America. Some researchers argue that transgendered homosexuality represents the more ancestral form of male homosexual expression, as evidenced by its preponderance in societies with more ancestral cultural practices (VanderLaan et al. 2013). Others argue that the oldest and most widespread form of male homosexual behavior is transgenerational, wherein young men become sexual apprentices to older men until they reach maturity. This form of male homosexual behavior is likely distinct from male homosexuality as a preference for same-aged male partners but has been documented as a common and socially sanctioned practice in cultures ranging from Ancient Greece and Rome; to the Azande and Zulu tribes; to India, Pakistan, and Persia; to Imperial China, Japan, and Korea; to Melanesia, Polynesia, and aboriginal Australia (Greenberg 1988).
Unfortunately, female homosexuality is less well documented by anthropological and historical sources, partly because in many cultures no terminology was developed to define female homosexuality and (relatedly) because female homosexuality has been marginalized by male historical perspectives that disbelieve or discredit female homosexual preferences. Despite these limitations, female homosexuality is understood to be universal and diverse. Native American women frequently inhabited a similar transgendered homosexual role as homosexual men. For example, the hwame of the Mohave possessed unique spiritual faculties, and their marriage to women was viewed as normative (Blackwood 1984). Similar practices are observed in many African societies in Nigeria and Kenya, where women may take on the role of husband and father. Women in Lesotho are known to develop intimate sexual friendships with other women in which they engage in the full range of sexual acts but do not consider their acts sexual because no penis is present. In the West and elsewhere, female homosexuality is expressed in long- and short-term relationships, and females who practice homosexual behaviors range from highly masculine to highly feminine and everything in between.
Like heritability estimates, global rates of homosexuality are imprecise as a result of differences in methods and populations. The best estimates for the number of men who identify as homosexual are between 2 % and 5 %, although somewhere between 5 % and 15 % of men report some attraction or sexual behaviors toward the same sex (e.g., Bagley and Tremblay 1998). Between 1 % and 3 % of women identify as homosexual, although somewhere between 10 % and 15 % report some attraction or sexual behaviors toward the same sex (Mercer et al. 2007). Non-heterosexual women are more likely to report being bisexual or queer, whereas non-heterosexual men are more likely to report being exclusively homosexual. This has led to the suggestion that females experience greater “sexual fluidity” than males (Diamond 2009). Regardless, rates of both male and female homosexuality are greater than would be expected to be the result of random mutation, and the breadth of cultures in which homosexuality is found recommends that it is not the result of particular environments.
Does Homosexuality Reduce Reproductive Success?
Western male homosexual identification is associated with roughly a third the number of offspring that are had by heterosexual males. Males who identify as homosexual are much more likely than heterosexual males to never have offspring, and when they do, they tend to have fewer across the life span (Schwartz et al. 2010). Data on females suggests that homosexuality is associated with about a 20–30 % chance of at least one offspring, compared to an 80–90 % chance of motherhood in the general population (Morris et al. 2002). However, when thinking about the evolution and maintenance of a trait, one must not confuse current reproductive outcomes with the ancestral reproductive outcomes that prevailed for the vast majority of human existence. Measuring reproductive success in industrial cultures across the last several decades says almost nothing about the ancestral conditions in which the trait was designed by natural selection to replicate itself. For example, modern obese Americans have shorter life spans than the nonobese, but this does not cause scientists studying obesity to rule out adaptive theories for the development of genes that promote slow metabolism and the accumulation of body fat. Rather than recording current reproductive success as a benchmark of adaptiveness, considering how a trait may have been expressed and the outcomes of that expression in ancestral environments can be more illuminating.
One window into ancestral reproductive success is simply to look at patterns of sexual attractions that would have encouraged or discouraged mating in ancestral environments. Male sexual desire is usually considered category specific, strongly favoring either one sex or another (Chivers et al. 2005), such that there is a negative correlation between sexual attraction toward one sex and toward the other sex and men of all orientations with higher sex drives have more sex with a single preferred sex (Lippa 2007). This is not true of females, who show a weak positive correlation between attraction to one sex and the other (Lippa 2007). Looking across women of all orientations, the number of past sexual partners of one sex is positively correlated with the number of past sexual partners of the other sex (in contrast to a negative correlation in men; Kanazawa 2016). This may suggest that women who express same-sex attraction and engage in more same-sex activities also report greater opposite-sex attraction and engage in more opposite-sex activities, potentially improving reproductive outcomes. However, this description may not apply to exclusively homosexual women, who show reduced attraction to males as a function of their attraction to females (Lippa 2007). In general, homosexual males express sexual attractions and behaviors that can be expected to have reduced reproductive success in ancestral environments, but it is less clear whether ancestral females would have suffered reduced reproductive success as a result of same-sex attractions.
Is Homosexuality an Ancestral Condition?
Comparative research has identified thousands of species that engage in homosexual acts, from house flies, beetles, and dragonflies; to shorebirds, geckos, and sunfish; to dogs, dolphins, and elephants. The particular practices, apparent purposes, frequencies, and phylogenies vary as widely as one could expect for a set of behaviors that span so many taxa. In almost every case, however, individual homosexual acts represent an infrequent, contextual, or strategic deviation from a typically heterosexual repertoire. The few exceptions to this include the domesticated male sheep (rams), of which only about 75 % show interest in mating with females, about 15 % show no mating interest whatsoever, and about 10 % are strictly interested in mounting other males. Among the apes, homosexual contact is frequent, particularly in our closest relatives, the chimpanzee and bonobo. Male-male cohorts dominate chimpanzee social structures and female-female cohorts dominate bonobo social structures; in both species, homosexual contact is used to maintain relationships, facilitate cooperation, and signal social dominance or submission (de Waal 1982). A recent phylogenetic analysis discovered that the frequency of same-sex genital interactions in primates is associated with social complexity and that male and female same-sex genital interactions may evolve independently (Fernandes et al. in press).
Deciding whether the character of human homosexual expression has changed substantially across the past several thousand years requires some speculation, but that humans have expressed some form of homosexual attraction and behavior since our common ancestor with the extant primates is nearly indisputable. Debate surrounds the translation and interpretation of texts and artworks that appear to describe homosexuality, but the debate centers on the exact nature of those sexual relationships, their exclusivity, and how they were perceived, not whether same-sex relationships occurred. It is an open question, for example, just how exclusively homosexual men and women may have been in ancestral hunter-gatherer societies. Across recorded history there have existed cultural entities that form in celebration of homosexual desire (e.g., the sixteenth century mignons of France, the seventeenth century fanchonos of Portugal, and the seventeenth through twentieth century mollies of England; Fone 2000). The term “homosexual” was coined at the emergence of the first large-scale homosexual cultural movement in the late nineteenth century in protest of German anti-sodomy statutes and has served to unite individuals under a broad conception of sexuality. When thinking about ancestral forms of homosexuality, cultural identities and sexual norms may not be applicable. For example, in ancestral communities on the scale of 20–80 individuals, perhaps no more than a couple of homosexuals of either sex would be present. These men and women would have been disconnected from an understanding of sexual variance and identity. Sexual norms likely included sexual fidelity and consent, but one can only speculate whether concepts of categorical sexual preferences or disgust toward same-sex activity were ancestrally significant.
As described, female sexual fluidity may suggest that the genes associated with homosexuality in females may not have suffered greatly reduced reproductive success in ancestral environments. A promising hypothesis for female homosexuality will be outlined ahead, but the high frequency of apparently exclusive homosexuality among males poses a greater challenge. The following two sections present evidence regarding whether human male sexual orientations are fundamentally exclusive, i.e., psychological and behavioral data may help illustrate how male sexuality would have been expressed in ancestral environments.
Evidence Against Male Sexual Fluidity
Western men’s reported sexual orientation skews toward exclusivity, producing a bimodal distribution clustering on exclusive heterosexuality and exclusive homosexuality. This distribution is corroborated by implicit, physiological, and neural measures of sexual attraction in men. Self-reported sexual identity tends to match implicit associations of male and female stimuli as sexual targets in the implicit association task, with homosexual men showing rapid associations between sexual categories and male, more than female, stimuli (in contrast to heterosexually identified men; Snowden and Gray 2013). Several studies show that self-reported identity also aligns with penile arousal responses, with homosexual men showing larger responses to male stimuli than female stimuli and bisexual men showing no difference in response to male and female stimuli (Chivers et al. 2005). Regarding neural responses, the broad activity patterns of self-identified homosexuals exposed to male stimuli measured in an fMRI are similar to those of heterosexuals exposed to female stimuli, and differences in activity between preferred and non-preferred stimuli are similar in magnitude for heterosexuals and homosexuals (this study also found no evidence for conscious inhibition). Additionally, a taxometric analysis of sexual orientation in a sample of 14,118 American men demonstrates that homosexuality is a nonarbitrary sexual taxa defined by broad uniformity in sexual behaviors and attractions as well as mental health variables (Norris et al. 2015). Heterosexual and homosexual men report similarly stable sexual attractions across 6- and 10-year periods, whereas homosexual women and bisexual men and women show less stable sexual attractions (Savin-Williams et al. 2012).
Evidence in Support of Male Sexual Fluidity
Individuals of all orientations report overlapping and inconsistent sexual identities, behaviors, and attractions, with anonymous and non-categorically framed studies showing higher reported identity-inconsistent sexual feelings. Just over half of exclusively heterosexual men and all homosexual men in a college sample reported questioning their sexual identity, indicating experiences ranging from hypothetical thoughts to sexual exploration (Morgan et al. 2010). Several studies have reported that just over half of self-identifying homosexual men report having had heterosexual sex at some time (e.g., 64 %, Whisman 1996). Whisman also found that 45 % of homosexually identified men report having had a romantically meaningful heterosexual relationship. In a longitudinal study of New Zealanders, among men who reported same-sex attraction at age 21, only half reported same-sex attraction at the final assessment at age 26 (Dickson et al. 2003). In a large retrospective study assessing lifetime sexual partners, 90 % of the 5.8 % of men who had at least one same-sex sexual partner since puberty also had at least one opposite-sex sexual partner during that time (Laumann et al. 1994). Although only 0.58 % of sexually active men studied by Laumann et al. reported exclusively homosexual sex since puberty, more than four times that percentage (2.4 %) reported exclusive attraction to the same sex, and 2.0 % reported a homosexual identity. These data suggest that behavioral bisexuality is common among men who identify as homosexual even if such behavior is transient or contextual.
Thus, homosexual and heterosexual identities are, for modern Western men, statistically and personally reliable and honest predictors of sexual attractions and behaviors. Yet the evolutionary and developmental sources of these patterns remain unclear, and when the boundaries of sexual identities are probed, inconsistency and diversity can be found. There is no consensus regarding how to interpret male sexual orientation; some researchers imply that ancestral male sexualities resembled modern categorical identities, and others argue that ancestral male sexuality was likely to be more fluid than modern identities suggest.
Is There an Evolutionary Puzzle?
Homosexuality in humans is heritable (Alanko et al. 2010; Mustanski et al. 2005) and currently responsible for reduced fecundity (Schwartz et al. 2010). In addition, human homosexuality is prevalent (Bagley and Tremblay 1998), and same-sex attractions and behaviors are ancient and likely to be deeply ancestral. These facts constitute an evolutionary puzzle, one that has generated fascinating evolutionary hypotheses and fruitful research avenues.
There are several compelling theories for the maintenance of human homosexuality. Some of these theories describe homosexuality as the result of genetic abnormalities, developmental instability, social irregularity, or a pathogen. These hypotheses are interesting, if speculative, but the purpose of this discussion will be to review testable evolutionary hypotheses for human homosexuality. While male and female homosexuality both appear to have genetic components, the associated genes and etiology of homosexuality are believed to differ between the sexes. In particular, there exist reliable prenatal and childhood predictors of homosexuality in men, for example, greater numbers of older brothers and more female-typical interests are positively associated with homosexual identification (Bogaert 2006). Female homosexuality is more difficult to predict, has greater variability in its expression, and may evolve independently from male homosexuality (Alanko et al. 2010; Fernandes et al. in press). Differences of this kind may be attributable to differences in the selection pressures faced by ancestral males and females or differing modern cultural pressures (or both). Consequently, evolutionary theory may be applicable to both male and female homosexuality, but there is little reason to expect identical theoretical considerations. Thus, male and female hypotheses are presented separately. It could be argued that male homosexuality is more difficult to explain in an evolutionary framework. This is not only because females show higher levels of sexual fluidity but also because male mating strategies benefit more than female sexual strategies from rapid, repeated, and diverse opposite-sex copulations. Thus, homosexual men who would refuse sex with women may be further removed from their maximally successful reproductive strategy than are homosexual women. Perhaps as a consequence, there exist more diverse hypotheses for the evolution of male homosexuality, although this may also be because there are more male researchers investigating these topics.
The sexual antagonism hypothesis proposes that the genes that contribute to homosexuality in men facilitate reproductive success when expressed in women. In this view, the expression of male homosexuality is a costly by-product of selection for reproductive adaptations in women. This hypothesis was inspired by the finding that the female relatives of homosexual men show greater fecundity than the female relatives of heterosexual men, particularly in the maternal line (Iemmola and Camperio-Ciani 2009). Several studies have failed to replicate these findings (e.g., Schwartz et al. 2010), but the hypothesis remains tenable.
The overdominance or heterosis hypothesis describes the possibility that homosexuality is reported by men who carry homozygous recessive alleles that, when expressed in the more common heterozygotic form, confer selective advantages (e.g., such men are more cooperative, more lingual, and less aggressive; Zietsch et al. 2008). Here, androphilic men represent the tail of a distribution experiencing selection for a linked set of traits that produce female-typical behavior. In theory, only a small heterozygote advantage is necessary to produce a stable proportion of homozygotes, leading to the persistent, low rate of exclusive homosexuality. Sexual antagonism and overdominance are often supported via computer modeling techniques that allow researchers to set the parameters on a population and to simulate the gene frequencies that unfold over generations. Critics of these techniques argue that such models demonstrate only that the hypotheses are possible given the tidy parameters included in the model, parameters which are not always empirically validated.
The genes that contribute to male homosexuality may survive via kin selection by supporting the reproductive success of kin who carry copies of those genes. If, by providing care to close relatives, a man enhances the overall reproductive success of his kin group, he may recoup the cost of his abstinence from reproductive sex. Evidence for elevated investment into kin among male homosexuals has been directly demonstrated on the island of Samoa among the fa’afafine (Vasey and VanderLaan 2010) but is not found in Western cultural environments (Rahman and Hull 2005). VanderLaan and colleagues point to elevated childhood separation anxiety in Western homosexual men as a sign that Western men are concerned for kin but may be culturally constrained from providing care. Kin-investing, transgendered forms of male homosexuality are also more frequent in cultures with more ancestral characteristics (smaller communities, simpler agriculture), which may suggest that ancestral male homosexual expressions were maintained via kin-directed investment in the context of transgendered homosexuality (VanderLaan et al. 2013).
The coalitional mating hypothesis emphasizes the comparative evidence for benefits gained by male chimpanzees from male-male alliances built upon sexual favors. The alliance or affiliation hypothesis thoroughly develops the human anthropological evidence for peripheral males gaining status through their sexual relationships with other men (Muscarella 2007). The homosocial hypothesis describes homosexual behavior as a natural extension of male-male and female-female cooperation serving to reduce violent mating rivalries. Underlying these hypotheses is the suggestion that direct, individual, and ultimately reproductive benefits can be gained via sexual behavior as a form of social influence. In other words, these hypotheses recommend that ancestral homosexual expressions were reproductively beneficial in certain social arrangements. Hypotheses of this kind have remained largely untested but are indirectly supported with reference to comparative and anthropological research. For example, high-status male chimpanzees have been observed allowing low-status male grooming partners to mate with estrus females (de Waal 1982). Grooming between males frequently results in arousal and penile erection, and subordinate males can placate aggression by handling the genitals of dominant males. Similar patterns are observed among female bonobos. Anthropological research provides examples of lower-status human males gaining resources, status, and reproductive access via sexual relationships with higher-status males (Greenberg 1988). These relationships are typically intergenerational or transgendered in context, elevating younger or more feminine males in the mating market through sexual relationships with higher status men.
Evolutionary theories for female homosexuality take a common form, closely resembling the affiliation hypotheses described for men just above. It is typically argued that female same-sex attractions aided ancestral females in creating and maintaining cooperative partnerships with other females. The function of these partnerships may have varied from interpersonal and sexual defense, to alloparenting and reciprocal child care, to status management and mate retention (Diamond 2009; Kanazawa 2016; Kuhle and Radtke 2013). Diamond points to the independence between arousability, or the capacity to become aroused, and proceptivity, or the motivation to initiate sex, in apes that lack conspicuous estrus (signs of fertility). If arousability is decoupled from proceptivity and continuous throughout the menstrual cycle, there is little evolutionary reason for it to discriminate between sexual stimuli of different sexes, i.e., becoming aroused by female stimuli will usually not inflict opportunity or energetic costs. Diamond argues that female sexual fluidity is thus a consequence of reduced pressure to curtail arousability in homo lineages (although she offers no direct adaptive benefits; Diamond 2009). Kuhle and Radtke (2013) highlight the benefits to child-rearing that can be gained by females who are willing to recruit female sexual partners. Bonobo females provide the clearest nonhuman example of lifelong female-female sexual coalitions, the members of which provide significant care to the unrelated offspring of their partners. Bonobo offspring receive basic care such as food sharing, defense, and transportation. These relationships involve sexual activities including genito-genital rubbing which frequently leads to orgasm. Human offspring are particularly altricial, requiring a lengthy and costly period of development before becoming self-sufficient. Without some form of outside care, researchers have argued that ancestral human mothers would have found it extremely difficult to raise offspring, particularly in cases where the father was not present. In modern hunter-gatherer societies, alloparental care from unrelated females (usually other mothers) is common and is associated with improved offspring survival. Sexual activity and especially orgasm promotes associations between the sexual partner, positive sensations, and social-bonding hormones including oxytocin. It is hypothesized that ancestral females took advantage of this capacity to manufacture social commitment in order to facilitate reciprocated child care. Recently, Kanazawa (2016) underscored the conflict resolution functions of same-sex interactions, proposing that in ancestral polygynous arrangements, females who found reliable ways to diffuse conflict would have been better able to collaborate for their own benefit. In other words, women who lacked a capacity for sexual fluidity may have suffered from an inability to function in recurrent polygynous mating situations. Each of these theoretical perspectives seeks to explain female sexual fluidity without necessarily addressing the appearance of exclusive female homosexuality. Exclusive female homosexuality is uncommon, often transient, and appears to be linked to higher than average prenatal exposure to androgens (Savin-Williams et al. 2012). This leads many researchers to consider it inappropriate or unlikely to fit a directly adaptive theory to the phenomenon. Again, one can only speculate as to the influence that social environments and identities may have upon modern sexual orientations, but such effects may be worth considering.
Human homosexuality has puzzled evolutionary thinkers for decades, its existence challenges basic principles of natural selection, and this problem has inspired creative thinking on the topics of ancestral mating patterns, comparative sexuality, reciprocal and kin altruism, and genetic inheritance. Many advances in our understanding of sexuality and mating in general are owed to the study of homosexuality, and it is a research avenue that continually reveals new questions and invites original answers. The study of homosexuality also has unique implications in the social and political world, as homosexuals around the world suffer immeasurably for harboring desires deemed taboo. One hopes that a growing understanding of homosexuality, particularly its evolutionary and developmental origins, will help to illustrate how deeply ancestral, universal, and ultimately healthy same-sex attractions and behaviors are. Progress on this topic is encouraging and indicates that public perception can be effected by conversations in science and that the public are interested in learning more about human sexual variance.
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