Six homosexual siblings

Sperm competition and the persistence of genes for male homosexuality

Homosexuality is increasingly recognized as having a genetic component. Why then does it persist, when common sense suggests that it should result in fewer offspring? Monozygotic-twin studies permit a rough estimate of the importance of genetics (70%) in the development of male homosexuality, and the proportion of homosexuals remains constant: Fisher's Theorem then tells us there is an advantage to the heterozygote, which we find need be no greater than 2%. Behavior and sperm competition suggest what this advantage might be.

Homosexuality in monozygotic twins reared apart

We describe six pairs of monozygotic twins, in which at least one member of five pairs were homosexual, and one of the remaining pair was bisexual, from a series of 55 pairs, reared apart from infancy; all the female pairs were discordant for homosexual behaviour. This and other evidence suggest that female homosexuality may be an acquired trait. One male pair was concordant for homosexuality, while the other was not clearly concordant or discordant; this suggests that male homosexuality may be associated with a complex interaction, in which genes play some part.

Is homosexuality familial? A review, some data, and a suggestion

This report summarizes evidence that sexual orientation is familial. Family studies report that homosexual subjects have more homosexual siblings than do heterosexual subjects and more than would be expected given population frequencies. Twin studies find in general a higher concordance in sexual orientation among monozygotic than among dizygotic twins. Evidence for associated psychopathology in homosexual subjects or their relatives is inconclusive. Possible sources of bias in these reports are discussed and a suggestion is offered that family studies should be undertaken in which the sexual orientation of relatives would be directly ascertained by interview.