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Homosexuality - Biological Hypotheses If we assume that historically homosexual individuals have had lower reproductive success than heterosexual individuals, how is that homosexuality is present as a surprisingly common (1-10%?) trait in the human population? Difficult to explain as an evolutionary adaptation. Homosexuality - Biological Hypotheses 1. Kin selection hypothesis (Wilson 1978) 2. Pleiotropy hypothesis (best example Iemmola & Ciani 2009 – slide 7) 3. Biological (non-adaptive) side effect Example: mother immune response (fraternal birth order) hypothesis 4. Alternative strategy – pursued when prospects of RS by conventional strategy is low. 5. Reciprocity in same-sex alliances (Kirkpatrick 2000) 6. Exotic becomes erotic (Bem 1997) Kin Selection Hypothesis Kin Selection Hypothesis E. O. Wilson, the author of Sociobiology, noted that homosexuality seems to be a universal in human societies, found throughout history and in all cultures, albeit at relatively low levels (~ 1 to 10%). Working from animal analogies – the sterile castes of eusocial insects (honeybees et al) and bird “helpers at the nest” – Wilson proposed that homosexual individuals help collateral relatives (siblings, nephews and nieces, cousins) so that “homosexual genes proliferate through collateral lines of descent, even if the homosexuals themselves do not have children”. Wilson then argued that “this kinselection hypothesis would be substantially supported if some amount of predisposition to homosexuality were shown to be inherited” (1978). He then cites evidence from twin studies which suggests the existence of genes for homosexuality (e.g., monozygotic twins are more likely to be concordant for the trait than are dizygotic twins). But there are problems with this hypothesis… 1 Problems with Wilson Theory 1. Wilson’s human model does not fit the animal models which suggested the kin-selection hypothesis! In the eusocial insects and the birds and mammals with ‘helpers’, the non-reproductive helper is not genetically different from the reproductives s/he helps! Helping (or sterility) is a facultative trait in these species – it has zero heritability. For example, Queen and worker are different not because of different genes they have but because of different developmental histories. 2. Wilson does not indicate why samesex sexual orientation needs to go along with helping (it doesn’t, for example, in the animal models). Why not just help and skip sex altogether? Genetic Effects can be Complex • Pleiotropy = single gene affects multiple traits • Epistasis = multiple genes interact to affect a trait • multiple traits interact to produce fitness • therefore, natural selection for gene combinations Heritability of Homosexuality Bailey and Pillard (1991): occurrence of homosexuality among brothers • 52% of identical (monozygotic) twins of homosexual men were likewise homosexual • 22% of fraternal (dizygotic) twins were likewise homosexual • 11% of adoptive brothers of homosexual men were likewise homosexual Bailey and Pillard (1993): occurrence of homosexuality among sisters • 48% of identical (monozygotic) twins of homosexual women were likewise homosexual (lesbian) • 16% of fraternal (dizygotic) twins were likewise homosexual • 6% of adoptive sisters of homosexual women were likewise homosexual Maternal line and Class or relatives: paternal line fecundity • Mothers (1) of the two sexual • Mothers of first borns (1) orientation groups • Maternal aunts (0.75) (likelihood of sharing • Maternal uncles (0.25) • Maternal grandparents (0.5) X-chromosomes) • Sons & daughters of maternal grandparents (0.25-1) • Paternal aunts (0) • Paternal uncles (0) • Paternal grandparents (0) • Sons & daughters of paternal grandparents (0) 1. Mothers (1) 2. Mothers 1st (1) 3. Mat aunts (0.75) 4. Mat uncs (0.25) 5. Mat grdps (0.5) 6. s&d of mat grps 7. Pat aunts (0) 8. Pat uncs (0) 9. Pat grdps (0) 10.s&d of pat grps Iemmola, F. & Ciani, A. C. (2009) 2 Immune Response Theory of Male Homosexuality Immune Response Theory of Male Homosexuality The maternal immune hypothesis was never intended to account for the sexual orientation of all homosexual men. Half or more of all homosexual men have zero older brothers, demonstrating that other etiological factors must be in play… Bogaert, A. F. (2006). Biological versus nonbiological older brothers and men’s sexual orientation. Proceedings of the National Academy of Sciences, 103, 10771-10774. Bogaert, A. F. (2006). Biological versus nonbiological older brothers and men’s sexual orientation. Proceedings of the National Academy of Sciences, 103, 10771-10774. Immune Response Theory of Male Homosexuality Immune Response Theory of Male Homosexuality Note: This is from original paper (Blanchard & Bogaert 1996). Subsequent research has removed the large-family confound and shown that older brothers but not older sisters have this effect. G estimates the likelihood that if you are a homosexual male, it can be attributed to your having had older brothers. And the more older brothers you had, the more likely it is. Overall they have estimated that as a statistical predictor of homosexuality in males, the older brother factor accounts for 14% (1/7th) of the variance. This is certainly compatible with the immune response theory, for other factors could protect the mother or the female fetus against the immune reaction. 3 Exotic becomes Erotic (Bem 1996) • provides same basic account for both opposite-sex and same-sex erotic desire, and for both men and women. • proposes that biological variables do not code for sexual orientation per se but for childhood temperaments that influence a child’s preferences for sex-typical or sexatypical activities. • These preferences lead children to feel different from opposite-sex or same-sex peers—to perceive them as “exotic.” • This, in turn, produces heightened physiological arousal that subsequently gets eroticized to that same class of peers: Exotic becomes erotic. • The theory claims to accommodate both the empirical evidence of the biological essentialists and the cultural relativism of the social constructionists. Exotic becomes Erotic Exotic becomes Erotic The temporal sequence of events leading to sexual orientation for most men and women according to Bem. A → B. Role of genes is to set temperament. B → C. Temperament influences activity preferences. Exotic becomes Erotic C → D. Gender-conforming children will feel different from opposite-sex peers, and gendernonconforming children will feel different from same-sex peers. D → E. These feelings of being different produce heightened physiological arousal. For male-typical child, it may be felt as antipathy or contempt in presence of girls (“girls are yucky”); for the female-typical child, as timidity or apprehension in presence of boys. Theory claims that every child – conforming or nonconforming – experiences heightened, nonspecific physiological arousal in presence of peers from whom he/she feels different. E → F. Exotic becomes erotic not too much data… 4