Kin selectionOne of the first and most influential evolutionary theories of homosexuality is based on observations of altruistic behaviour in the sterile worker castes of social insects. The ‘kin selection hypothesis’ for male homosexuality was formulated by Wilson in 1975 and states that while homosexual men cannot reproduce directly, genes for homosexuality will be propagated indirectly through family members, whose survival and reproductive success is increased with the provision of aid and resources by a homosexual relative.
A number of criticisms have been made of the kin selection theory. The degree of altruism directed toward family members (so-called “collateral nepotism”) would need to be extremely large to offset the evolutionary cost created by the loss of direct reproduction. It has been suggested that homosexuals are unable to donate a sufficient proportion of their resources toward their kin because they expend so much energy in pursuit of non-reproductive relationships (Bobrow and Bailey, 2001). However the primary flaw in the kin selection theory is that it is based upon the assumption that there is a genetic basis to male homosexuality, something that has yet to be confirmed; it instantly “backfires if the genetic speculation is wrong” (Gould, 1978).
Despite its flaws, a number of experiments have been designed in recent years to test the kin selection hypothesis empirically. In one such experiment, a sample of 57 heterosexual and 66 homosexual men from America completed family relationship questionnaires that would assess the level of generosity, affinity to family members and willingness to receive. Following non-significant results on all scales analysed, it was determined that “homosexual men were no more likely than heterosexual men to channel resources towards family members” (Bobrow and Bailey, 2001). Furthermore, when compared to heterosexual men, homosexual men were found to give
Exaptation theory
To date, there is little support, empirical or otherwise, for the kin selection hypothesis as an explanation for the way in which homosexuality has come to evolve (Bobrow and Bailey, 2001; Rahman and Wilson, 2003). It has recently been proposed that the somewhat fluid concept of ‘homosexuality’ should be avoided, and instead homosexual should be examined (Muscarella , 2001). The most popular theory states that homosexual behaviour be selected for, as it acts to reinforce same-sex alliances between individuals (a so-called ‘non-conceptive function’). Such alliances arise from the selection for reciprocal altruism at the individual level, which decreases inter-male aggression and increases resource exchange. Subsequently, the formation of same-sex alliances, reinforced by occasional homosexual behaviour, increases survival and in an indirect way, reproductive success (Kirkpatrick, 2000). In Darwinian terms, if a particular characteristic confers differential reproductive success in this way, the organism will survive the process of natural selection, reproduce, and the trait (in this case male homosexuality) will persist.
Kirkpatrick’s model (2000) has a number of strengths. Same-sex alliances have no direct role in reproduction and so the contemporary paradox of decreased reproductive success amongst homosexuals is excluded from this theory. Additionally, same-sex alliances, unlike the kin selection theory, do not require that members of the extended family benefit from the partnership (Kirkpatrick, 2000). This theory is further reinforced by the results of studies examining same-sex alliances (or ‘homosocial behaviour’) in both humans (Ross and Wells, 2000) and primates (Vasey, 1995 cited in Muscarella , 2001). Both these studies suggest that homosexual behaviour has evolved as an ‘exaptation’ of homosocial behaviour. An exaptation is a feature that arises not from natural selection, but as a neutral variable of a behaviour that demonstrates fitness-enhancing potential, and so in time, is acted upon by natural selection.
Anthropological and historical investigations have studied South Asian village societies in an attempt to confirm the exaptation theory (Ross and Wells, 2000). Cross-cultural studies such as this are essential to highlight the evolutionary benefits of male homosexuality that might have been present over the past three million years of human development. Indeed, it has been stated (Bobrow and Bailey, 2001; Rahman and Hull, 2005) that homosexuality cannot easily be explained in evolutionary terms when set against the backdrop of modern, Westernised society.
The exaptation theory is by far the most convincing proposal for the evolution of male homosexuality. However, it too has its weaknesses. It goes only as far as explaining the persistence of homosexual and does not suggest how a more permanent homosexual orientation might subsequently develop in association with psychological or environmental factors.
Back to the genome
Research published in 2004 has yet again shifted the focus of evolutionary theories for male homosexuality. Camperio-Ciani and colleagues (2004), provided empirical evidence that female maternal relatives of homosexual men have increased fecundity when compared to female maternal relatives of heterosexual men, based on a very large sample of over 4,600 individuals. The results confirm previous reports of a maternally transmitted, X-linked gene for male homosexuality (Hamer et al., 1993a). The researchers also claim to have resolved the Darwinian paradox, stating that their findings reveal “hitherto unexpected, reproductive advantages associated with male homosexuality” (Camperio-Ciani et al., 2004) that might make such a trait evolutionarily viable in Darwinian terms.
