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.