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 (Zeitsch et al 2008 – our seminar reading – is a good example, but see also 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)* * In reading list Homosexuality - Biological Hypotheses

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”.

Kin Selection Hypothesis Wilson then argued that “this kin- selection 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…

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 same sex 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?

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

1.Mothers (1) 2.Mothers 1 st (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 mat grps Class or relatives: Mothers (1) Mothers of first borns (1) Maternal aunts (0.75) Maternal uncles (0.25) Maternal grandparents (0.5) Sons & daughters of maternal grandparents (0.25-1) Paternal aunts (0) Paternal uncles (0) Paternal grandparents (0) Sons & daughters of paternal grandparents (0) Maternal line and paternal line fecundity of the two sexual orientation groups (likelihood of sharing X-chromosomes) Iemmola, F. & Ciani, A. C. (2009)

Immune Response Theory of Male Homosexuality Bogaert, A. F. (2006). Biological versus nonbiological older brothers and men’s sexual orientation. Proceedings of the National Academy of Sciences, 103,

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% (1/7 th ) 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. Immune Response Theory of Male Homosexuality