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Sexual Orientation and not necessarily manifested by overt sexual behav- Sexuality ior, associated with self-identification of label, or aligned with affectional bond- Jaroslava Varella Valentova and Marco Antonio ing (love). Correa Varella Department of Experimental , Institute of Psychology, University of São Paulo, Cidade Introduction Universitária São Paulo, SP, Brazil The majority of males and predominantly prefer opposite- sexual and/or romantic part- Synonyms ners. Such sexuality has been accepted as a stan- dard default, possibly because it is easier to see its ; ; ; biological/evolutionary relevance. Indeed, in sex- Same-sex sexuality; ; Sexual ually reproducing species, preference for and sex- minorities; ual activities with opposite-sex partners evolved as a mechanism for combining genomes through complementary gametes gaining genetic variabil- Definition ity and direct fitness. Thus, scholars have argued that homosexuality is an evolutionary puzzle Sexual orientation refers to a psychological mech- because it impedes the reproductive success of anism directing one’s sexuality towards individ- their owners. However, on the phylogenetic uals based on their apparent sex. Individuals form level, evolutionary fitness is more than direct fit- a continuum between extreme point of exclusive ness, and on the ontogenetic level, sexuality is heterosexuals, i.e., oriented exclusively towards more than fertile heterosexual penile-vaginal individuals of the opposite sex, and exclusive intercourse. Although direct reproduction is the homosexuals, i.e., oriented exclusively towards ultimate evolutionary force behind most sexual individuals of the same sex, with bisexuals being activities, sexuality in general gained many other similarly attracted to both and other individ- proximate functions during its evolution, e.g., as a uals being attracted to both sexes in various social and pair bonding mechanism or means of degrees (e.g., predominantly heterosexual with resource acquisition. Such functions are interme- some potential to same-sex attraction). Sexual diate goals to achieve direct reproduction, e.g., by orientation refers more to attraction or desire means of survival and alliance formation and/or than behavior or identity, because orientation is indirect reproduction by helping kin, also known

# Springer International Publishing AG 2016 T.K. Shackelford, V.A. Weekes-Shackelford (eds.), Encyclopedia of Evolutionary Psychological Science, DOI 10.1007/978-3-319-16999-6_3622-1 2 Sexual Orientation and as inclusive fitness. Therefore, many forms of predominantly heterosexual, bisexual, predomi- sexuality, such as oral or , , nantly homosexual, etc.). Similarly, a severe clin- sexual preferences for different species, prefer- ical depression or severe forms of autism are most ences of same-sex individuals, or individuals out- probably not adaptive; they are not only neutral side of reproductive age, cannot per se offer direct but rather maladaptive. However, they both are reproductive success of the individual, but still extremes on a continuum of adaptive variation, these nonfertile forms of sexuality can offer such as the capacity of sadness and systemizing other adaptive sociosexual functions (e.g., pair abilities, respectively (for an overview about bonding, alliance formation, resource acquisition, adaptive individual differences see, Buss and well-being, etc.) that might indirectly foster future Greiling 1999). In the same line, bisexuality or reproduction, regardless of sexual orientation. It is some degree of homosexual tendencies among important to note that personal motivations for predominantly heterosexual individuals as well sexual activities, homosexual or heterosexual as some heterosexual tendencies among predom- ones, are quite different from their possible socio- inantly homosexual individuals could be viewed sexual functions or possible evolutionary adaptive as adaptive variation of sexual orientation. From values. Sexual relations are mostly motivated by this perspective, even if exclusive homosexuality pleasure, love, self-esteem/revenge, and/or does not have any possible adaptive value, it obtaining resources. Such personal motivations would be a mistake to assume that the same is triggering homo/heterosexual behavior should true for the whole continuum of sexual not be confused with the socioecological or evo- orientation. lutionary reasons that maintain sexual activity in the population. Further, sexual orientation is a psychological Evolutionary Theories of Sexual mechanism that generates a continuous array of Orientation individual variation and not a dichotomous psy- chological trait (e.g., Kinsey et al. 1948; Petterson Sexual orientation has been described as a mech- et al. 2015). In this sense, exclusive homosexual- anism navigating or directing sexuality towards ity and exclusive heterosexuality present the individuals of the opposite sex, same sex, or in extreme opposite poles of the continuum, while varying degrees to both sexes (Bailey 2009; Bai- the majority of individuals fall in between, ley et al. 2016; Savin-Williams 2016; Vrangalova although closer to the heterosexual extreme. Fol- and Savin-Williams 2012). Similar to some het- lowing this line, homosexuality or nonhetero- erosexual practices (e.g., extra-pair affairs), atti- sexuality (including bisexuals) is more than tudes toward same-sex sexuality can vary greatly exclusive homosexuality, and sexual orientation between cultures. In some cultures, same-sex sex- does not equal homosexuality. One possibility uality constitutes an integral part of the sociocul- would be to focus on the adaptive value of the tural system; in others such sexuality is mechanism itself (i.e., sexual orientation); the criminalized and persecuted (for review see, Bai- other would be to explore the possible adaptive ley et al. 2016). For these reasons, individuals can value of one of its manifestations (e.g., exclusive vary in their overt manifestation of sexual prefer- homosexuality). The adaptiveness of the mecha- ence, from a tendency hidden from public or even nism itself is usually not the focus of the debate. self to open preferences and behaviors. There are The debate is mostly centered on whether and also other motives why sexual preferences, behav- how the individual differences generated by this iors, and identities do not need to be aligned. mechanism are adaptive. A mistake would be to Nonpreferred sexual behavior can be manifested, extrapolate a possible nonadaptiveness of an for example, under conditions without access to extreme point within the continuum (exclusive preferred sexual partners; it can be done for finan- homosexuality) to the other variations along the cial motives or simply to satisfy one’s partner. whole continuum of sexual orientation (e.g., Similarly, romantic affection can be largely Sexual Orientation and Human Sexuality 3 disconnected from sexual desires (Diamond success of transgendered male homosexuals 2003). For example, a person can repeatedly fall (males who adopt look and behavior) is in love only with individuals of the same sex, but close to zero (Vasey et al. 2014), this does not prefer and actively seek sexual partners of both need to apply to other forms of nonheterosexual sexes. Thus, sexual orientation refers to sexual individuals. In one study, 32.8 % of men and desires, attraction, and preferences rather than to 65.4 % of women reported a potential for homo- overt sexual behavior, identity, or affective sexual response, suggesting that a huge percent- feelings. age of predominantly heterosexual people do have Sexual orientation has a genetic component a propensity to experience same-sex sexual attrac- that explains approximately one third of the vari- tion and/or interaction (Santtila et al. 2008). In this ation between individuals (e.g., Zietsch study, the potential for homosexual response was et al. 2012; for review, see Bailey et al. 2016). based on responses (quite impossible, very This suggests that sexual orientation is a complex unlikely, quite unlikely, not likely, not unlikely, trait that is influenced by genetic and environmen- relatively likely, very likely) on the following tal factors. Sex are one of the environ- question: “If a, in your opinion, handsome mental factors that influence the development of [to male participants]/beautiful [to female sexual orientation. According to the organiza- participants], whom you like, suggested sexual tional hypothesis, the neural system of homosex- interaction with you, how likely would you be ual individuals is influenced by sex-atypical able to do it (if you could define the nature of the levels during early developmental interaction and nobody else would know about phases. Systematic differences in neuroanatomy it)?”. In line with this finding, a large-scale study between homosexual and heterosexual individ- demonstrated that almost 10 % of heterosexually uals suggest that compared to their heterosexual identified men reported that they have engaged in counterparts homosexual men were, on average, same-sex sexual activities (Pathela et al. 2006). less androgenized and homosexual women more This applies even more to non-Western androgenized during early, probably prenatal, populations, such as , where the majority development (for review, see Bailey et al. 2016; of predominantly heterosexual men engage in sex LeVay 2010; Wilson and Rahman 2005). Neither with both women and males called genetic nor hormonal influences suggest that sex- Fa’afafine (Petterson et al. 2015). Further, 20 % of ual orientation is necessarily fixed and rigid over homosexually identified white American men the lifespan. Sexual orientation develops during reported having been married to a woman at ontogeny whereas can change over some point in their life, and 50 % of homosexual time, particularly among women (Diamond men reported having produced at least one child 2008). Such a fluidity of sexual orientation does (Bell and Weinberg 1978). According to the more not equal to a conscious choice of different sexual recent demographic data (2013) of the USA, 37 % desires and attractions (Diamond 2008). Thus, of LGBT-identified individuals report having at sexual orientation is not a choice nor a cultural least one child. Keeping in mind these assump- invention; it has genetic and neurohormonal tions, we can outline some of the empirically underpinnings and is not necessarily immutable. supported evolutionary theories explaining the Furthermore, occasional same-sex behavior and adaptiveness of human homosexuality and stable same-sex preferences have been nonheterosexuality. documented along a number of nonhuman animal species (Sommer and Vasey 2006). Thus, a varia- 1. Homosexuality as an adaptation: Kin selection tion on the sexual orientation continuum is not a theory human specific and has its phylogenetic roots. Moreover, it is commonly accepted that homo- One of the oldest and partly supported theories of sexuals do not have any reproductive success. homosexuality as an adaptation was postulated by Although it has been shown that reproductive E. O. Wilson (1978), inspired by Hamilton’s kin 4 Sexual Orientation and Human Sexuality selection theory. Homosexual individuals, who do preferences of none of the partners. Both not invest into their own exclusive reproduction, age-stratified and -stratified styles of may propagate their indirectly by same-sex relationships exist in the modern West- supporting reproduction of close relatives, with ern culture, although the most frequent style of whom he/she shares own genes. Studies relationships is egalitarian between two individ- conducted in the USA, Great Britain, or Japan uals who identify as homosexuals and are of sim- did not support the theory (for review, see Bailey ilar , status, education, and age. This et al. 2016), which can be explained by persisting latter style of same-sex relationships seems to be relatively negative attitudes toward homosexual specific for contemporary Western populations. individuals in these countries. On the other hand, On the other hand, the other two styles of same- systematic investigations of Fa’afafine, who are sex relationships appear in populations that share officially recognized as “” or a trans- more common sociocultural characteristics with gender type of male homosexuals (individuals human ancestral populations (e.g., geographical who are biologically males but have feminine connectedness of the kin and strong familiar characteristics and adopt feminine gender roles) bonds), and the evolutionary strategy can thus among the population of Samoa, supported the still be adaptive in the present as it could have theory of kin selection. In particular, Fa’afafine been advantageous in the ancestral environment show higher avuncular tendencies (support of (VanderLaan et al. 2014). Importantly, evolution- nieces and nephews) compared to men who prefer ary strategy of transgender homosexuals can dif- women as sexual partners, which supposedly fer from the one adopted by increases their fitness (e.g., Vasey and homosexuals or nonheterosexuals. In other VanderLaan 2010). words, some evolutionary hypotheses (e.g., kin Samoan Fa’afafine fit a specific subgroup of selection) can be supported in one specific sub- homosexual men, the transgender type of male group of homosexual individuals (e.g., in trans- homosexuals that is a specific although still het- gender homosexual men) in specific society erogeneous group. This subgroup of transgender contexts (e.g., large families, close kin bonds, individuals whose sexuality is aimed at individ- geographical proximity) but not in others (e.g., uals of the same biological sex is frequent in many in cisgender homosexual men in big anonymous other societies, such as India (), Thailand cities with prevalent ). (), or native North American tribes (Berdache). The transgendered homosexuals 2. Homosexuality as a by-product of mostly prefer same-sex relationships, although sex-atypicality usually with cisgender (i.e., biological sex aligned with gender role) men. Sexual and/or romantic Miller (2000) suggested that male homosexuality relations between partners of different gender per se is not adaptive, but rather it is a by-product roles are documented in numerous populations of another adaptation. He proposed that some around the world (Crapo 1995). Furthermore, characteristics that are, on average, more typical age stratified type of same-sex relationships (i.e., for women than men, such as lower aggressive- one of the partners is much younger than the ness, higher cooperation, and social skills in gen- other) has been documented across populations, eral, became adaptive also in men during the such as ancient Greece, Middle Ages Japan, or changes in social structure of relatively recent contemporary New Guinea (Crapo 1995). Such human evolution. Futher, more feminine charac- relationships, as opposed to gender-stratified teristics in men can be typical for slow life ones, do not need to imply one transgendered of men who invest more in relationship commit- and one cisgender partner, rather both of them ment and parental care than in mating success are cisgender but of different age categories, (Jeffery 2015). Thus, men who have more femi- although such interactions can be temporary and nine and fewer masculine characteristics can do not need to imply predominant same-sex thrive better in a more complex and Sexual Orientation and Human Sexuality 5 predominantly monogamous society, including most frequent strategy would be exclusive or at having relatively higher reproductive success.In least predominant heterosexuality with males of this scheme, male homosexuality would thus be rather masculine characteristics, while bisexuality an extreme nonadaptive by-product of a general and predominant homosexuality linked to adaptive male shift to feminization. Gender non- increased in men would represent a conformity (lower in men) has a sneaking sexual strategy. From this point of strong genetic component (for review, see Bailey view, feminine men who have some potential for et al. 2016), and women find femininity in men opposite-sex activities can be successful in oppor- attractive (Perrett et al. 1998). Furthermore, tunistic heterosexual activities and secure direct numerous studies indicate that nonheterosexual reproductive success. This can happen in particu- men are, in comparison to heterosexuals, more larly because feminine characteristics were shown feminine (for review, see Bailey et al. 2016). to be attractive to women (e.g., Perrett et al. 1998), Homosexual men would thus form an extreme at least under certain conditions. At the same time, point on a continuum between male masculinity more feminine bisexual or predominantly homo- and male femininity, while relatively feminine sexual men can be perceived as weak rivals to heterosexual men would have some adaptive more masculine exclusively or predominantly het- advantages, at least in urban, middle class Western erosexual men and can thus reduce intrasexual groups.In line with the kin selection theory male-male competition. Consequently, feminine outlined above, this theory also aims to explain predominantly homosexual men can be accepted the existence and maintenance of feminine male more as friends of heterosexual men’s female homosexuals in the population. This theory, none- partners, thus offering opportunities to sneak cop- theless, does not apply to masculine male homo- ulations.High numbers of predominantly homo- sexuals and other nonheterosexuals nor to female sexual individuals reporting some opposite-sex homosexuals. Rice et al. (2013) suggested that attraction, behavior, and also biological offspring homosexual orientation is a by-product of (see above) would tend to support this theory. The sex-atypical epigenetic marks that feminize theory thus does not attempt to explain exclusive males and masculinize females during embryonic homosexuality or transgender homosexual men development. Thus, the opposite logic of the the- who do not have any interest in sexual encounters ory could also be applied to explain masculine with opposite-sex partners. It rather shows that female homosexuals who would represent an various degrees of bisexuality and predominant extreme pole of a general female masculinization. homosexuality can serve as an alternative and This might have been adaptive for some women successful reproductive strategy.This theory need during the evolutionary past, for example, by con- not apply only to men because bisexual and pre- centrating more social and physical power, lead- dominantly homosexual women can also show ing to higher independence, better self-defense some opposite-sex attraction and behavior, as and defense of their kin, and higher resource well as some reproductive success. On average, acquisition. Such characteristics might have been women are more masculine than hetero- advantageous in the ancestral and also recent soci- sexual women (Bailey and Zucker 1995), and ety, where mortality in men has been relatively masculine heterosexual women report higher higher than in women. (tendency for uncommitted sexual variety) than feminine women (Mikach and Bai- 3. Predominant homosexuality as an adaptation: ley 1999). We can thus speculate that masculine Sneaking and conditional sexual strategy predominantly homosexual women can secure direct reproductive success without the necessity Following the previous line of reasoning, predom- of creating and maintaining long-term bonds with inant homosexuality in feminine males can be also men but rather with other women.Compared to understood as a sneaking sexual strategy (Jeffery predominant female homosexuality or bisexual- 2015). In , it has been suggested that the ity, a fluid sexual orientation might have helped 6 Sexual Orientation and Human Sexuality ancestral women secure resources and care for peacemaking and reconciliation, or resource their offspring under conditions without a primary gains. male partner by receiving parental investment from other women (Kuhle and Radtke 2013). 5. Homosexuality as a by-product: Overdomi- According to this theory, fluidity of female sexual nance and sexually antagonistic orientation can be understood as a conditional hypothesis sexual strategy, meaning that variations between opposite-sex and same-sex sexuality can be adap- Both behavioral and molecular genetic studies tive in different conditions, such as different age have shown that the male homosexuality is linked, or life phases. among others, to the X chromosome and is thus inherited from the maternal lineage (Sanders 4. Nonheterosexuality as an adaptation: Same- et al. 2014). Further, female relatives of male sex alliance formation homosexuals have more children and grandchildren than control group of women with This theory holds that various degrees of sexual no homosexual individuals among relatives (e.g., feelings and behaviors aimed toward both males Ciani and Pellizzari 2012; for review, see Bailey and females offer an individual advantage over an et al. 2016). It is thus suggested that greater fecun- exclusive orientation toward only one sex dity and fertility of female relatives of homosex- (Kirkpatrick 2000). The main reason towards ual men can explain the persistence of a nonheterosexual interactions would be formation nonadaptive form of male sexuality through sex- and maintenance of same-sex alliances that are ually antagonistic selection (e.g., Ciani and crucial in many other and in particular Pellizzari 2012; Zietsch et al. 2008). In other in the human evolutionary past. The author shows words, sexually antagonistic genes can bring anthropological and primatological evidence advantages for one sex (e.g., higher fecundity in supporting the view that strong and emotional women) but can be principally disadvantageous same-sex alliances can offer a reproductive for the other sex (e.g., can cause exclusive homo- advantage to the individual. More specifically, sexuality in men).However, as shown by recent same-sex alliances reduce competition and models, the frequency of only female carriers increase cooperation between members of the cannot explain a stable proportion of exclusive same sex, while maintained access to partners of male homosexuals in the population but a large the opposite sex ensures potential reproductive proportion of both nonhomosexual female and success. In this perspective, heterosexual sex male carriers can (Chaladze 2016). Based on among stable partners outside the fertile period behavioral genetic studies, a considerable propor- (i.e., outside the fertile window within the female tion of men carrying the homosexual genes are not menstrual cycle and after menopause) or any sex- exclusively or predominantly homosexual. It was ual activities that do not lead to fertilization (e.g., suggested that homosexual phenotype can only be oral or anal sex) serve the same function, i.e., expressed in a homozygous form of the underly- reducing disputes, increasing cooperation, and ing alleles, while individuals with a heterozygous thus maintaining the stable dyad (Bártová and form (only one allele of the gene) carry the phe- Valentová 2012).The question is whether notype but the phenotype is not expressed (for nonheterosexual behavior was originally selected review, see McKnight 1997). Such heterozygous for alliance formation or whether alliance forma- carriers are supposed to possess some advanta- tion is rather a newly acquired sociosexual func- geous characteristics that can increase reproduc- tion of same-sex sexual behavior. In this sense, tive success of their owners.So far, this theory was nonheterosexuality would serve proximate socio- supported by a study showing that heterosexual sexual functions for which nonprocreative sexu- twins of homosexual men, who most probably ality in general was co-opted, such as alliance share the genetic component of homosexuality formation, dyad maintenance, lowering stress, although not homosexual orientation, report Sexual Orientation and Human Sexuality 7 higher number of female sexual partners than evolutionary paradox. However, when essentialist heterosexual men with a heterosexual twin categorical thinking is discarded, it might become (Zietsch et al. 2008). Thus, heterosexual men clearer that the majority of variation on the con- who carry the homosexuality-linked genetic com- tinuum of sexual orientation can offer adaptive ponent, potentially in a heterozygous form, do advantages for their carriers. This view can be have higher mating success (considered a proxy supported by changing the perspective of sexual- of a reproductive success) than the control group ity equaling only with reproduction, and by of heterosexual men, which can explain its rela- acknowledging the fact that at least some homo- tively stable prevalence in the population. sexual and nonheterosexual individuals within the The theories of by-product thus focus more on whole spectrum of sexual orientation do repro- explanation of the extreme exclusive form of male duce and raise their offspring. homosexuality, rather than the rest of the Most outlined theories present adaptive rea- nonheterosexual continuum. The theories can sons for the evolution of nonheterosexual orienta- apply to other forms of nonheterosexuals, though, tions, either by stressing indirect reproduction via because they can also be carriers of the homosex- kin selection or direct reproduction via sneak cop- ual genotype. It has been shown that a potential ulations or same-sex alliance formation that can for homosexual response has a stronger genetic increase survival and future direct reproductive component than homosexual behavior or self- capacity. Even by-product theories offer plausible labeled identification (Santtila et al. 2008). Thus, evolutionary reasoning for the origins and main- any degree of nonheterosexual tendencies might tenance of nonheterosexual orientations. In this be more suitable for genetic analyses than exclu- sense, nonheterosexual orientations would have sive homosexuality with no heterosexual been passed on throughout generations together potential. with the adaptive trait of sex-atypicality, advan- Furthermore, the theories of homosexuality as tages of an increased fertility in the other-sex kin, a by-product originally did not apply to women, or in carriers who do not express the homosexual although in principle they can work similarly, phenotype. These theories are not mutually exclu- since genetic component has been documented sive, and together they can explain a bigger pro- also in female sexual orientation (for review, see portion of the sexual orientation continuum. Bailey et al. 2016), and the potential to homosex- Moreover, these theories offer a stronger case ual response is even more frequent in women than against the argument that it is impossible for any men (Santtila et al. 2008). Thus, exclusive male form of homosexuality to have evolved. and female homosexuality indeed can be a Finally, it is important to realize that the by-product of otherwise adaptive variation of sex- outlined theories are not just-so-stories nor naïve ual orientation. panadaptationism as critics often portrait it. The theories offer specific hypotheses with predictions that can be empirically tested. We attempted to Conclusion provide the existing empirical support for each theory, although we realize that more research During the last century, human and other animal using a variety of sources of evidence and (vertebrates and invertebrates) sexual orientation methods is still needed. Only convergent evidence has been extensively studied within many scien- of numerous empirical tests can give decisive tific areas. Despite important findings in the areas support and scientific credibility to a particular of psychology, anthropology, medicine, neurosci- adaptive hypothesis. ence, , endocrinology, and development, Sexual orientation is a complex psychological evolutionary reasoning has suffered numerous mechanism that can profit from analysis within an conceptual and terminology issues. These mis- evolutionary framework, where otherwise hetero- conceptions have led to the view that homosexu- sexuality might be taken for granted. 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