This journal article proposes that homosexuality can be explained by a polygenetic trait influenced by multiple genes. During development, these genes shift male brain development in a more feminine direction. While single alleles may produce homosexuality, carriers of these alleles who are heterosexual tend to be better fathers and more attractive mates. There is a balanced polymorphism where the feminizing effects in heterosexuals offsets the reproductive disadvantages of these alleles contributing to homosexuality. The birth order effect on homosexuality may be a byproduct of a mechanism that shifts later-born sons' personalities more femininely, reducing competition with brothers.