Male homosexual preference (MHP) challenges evolutionary thinking because the preference for male-male relationships is heritable, implies a fertility cost (lower offspring number), and is relatively frequent in some societies (2-6% in Western countries) for a costly trait. It has been proposed that individuals with a MHP counterbalance reproductive costs through the transfer of resources to kin, thereby improving their indirect reproduction through kin's reproductive success. This kin selection hypothesis is not supported in Western countries and Japan, although consistent evidence has been obtained in Samoa. In this study, data from Java (Indonesia) were obtained to assess the avuncular tendencies of men with contrasting sexual orientation to measure possible resource transfer. Consistent with the kin selection hypothesis, males with a homosexual orientation reported an increased willingness to transfer resources toward nephews and nieces and declared having transferred more money to nephews and nieces. We developed a method to quantitatively estimate the contribution of kin selection on inclusive reproduction associated to sexual orientation, taking into account various possible biases. Kin selection reduced the direct reproductive cost of homosexual men by 20%, so suggesting that kin selection alone is insufficient to explain the maintenance of male homosexuality. Other potential factors are discussed, as well as the limitations of the study and the social determinant operating for the expression of increased avuncular tendencies of homosexual men.
Keywords: Darwinian paradox; Kin selection hypothesis; Male homosexual preference; Sexual orientation.