Homosexuality, birth order, and evolution: Toward an equilibrium reproductive economics of homosexuality

The survival of a human predisposition for homosexuality can be explained b y sexual orientation being a polygenetic trait that is influenced by a numb er of genes. During development these shift male brain development in the f emale direction. Inheritance of several such alleles produces homosexuality . Single alleles make for greater sensitivity, empathy, tendermindedness, a nd kindness. These traits make heterosexual carriers of the genes better fa thers and more attractive mates. There is a balanced polymorphism in which the feminizing effect of these alleles in heterosexuals offsets the adverse effects (on reproductive success) of these alleles' contribution to homose xuality. A similar effect probably occurs for genes that can produce lesbia nism in females. The whole system survives because it serves to provide a h igh degree of variability among the personalities of offspring, providing t he genotype with diversification and reducing competition among offspring f or the same niches. An allele with a large effect can survive in these circ umstances in males, but it is less likely to survive in females. The birth order effect on homosexuality is probably a by-product of a biological mech anism that shifts personalities more in the feminine direction in the later born sons, reducing the probability of these sons engaging in unproductive competition with each other.