While it is still difficult to explain how male homosexuality has risen naturally, as these men are much less likely to produce offspring than heterosexual men, scientists believe they probably convey an indirect benefit by enhancing the survival prospects of close relatives.
According to the 'kin selection hypothesis', while homosexual individuals do not directly pass on their genes to successive generations by having children, they indirectly spread their genes through their families.
By acting altruistically toward nieces and nephews, homosexual men would perpetuate the family genes, including some of their own.
Lead researchers Paul Vasey and Doug VanderLaan of the University of Lethbridge, Canada tested this idea for the past several years on the Pacific island of Samoa.
They chose Samoa because males who prefer men as sexual partners are widely recognized and accepted there as a distinct gender category-called fa'afafine-neither man nor woman.
Past research has also shown that the fa'afafine are much more altruistically inclined toward their nieces and nephews than either Samoan women or heterosexual men. They are willing to babysit a lot, tutor their nieces and nephews in art and music, and help out financially-paying for medical care and education and so forth.
In a new study, the scientists set out to unravel the psychology of the fa'afafine, to see if their altruism is targeted specifically at kin rather than kids in general.
They found that compared to Samoan women and heterosexual men, the fa'afafine showed a much weaker link between their avuncular or uncle like - behavior and their altruism toward kids generally.
This cognitive dissociation, the scientists argue, allows the fa'afafine to allocate their resources more efficiently and precisely to their kin-and thus enhance their own evolutionary prospects.
To compensate for being childless, each fa'afafine would have to somehow support the survival of two additional nieces or nephews who would otherwise not have existed.
"If kin selection is the sole mechanism by which genes for male same-sex sexual attraction are maintained over time," the fa'afafine must be "super uncles" to earn their evolutionary keep, explains Vasey.
Consequently, Vasey suggests "that the fa'afafine's avuncularity probably contributes to the evolutionary survival of genes for male same-sex sexual attraction, but is unlikely to entirely offset the costs of not reproducing."
The findings appear in journal Psychological Science.