Natural Selection of Gender-Bending
Genes that make some people gay may make their siblings fecund.
Homosexuality is at least partly genetic. Studies of identical twins, for example, show that if one of a pair (regardless of sex) is homosexual, the other has a 50% chance of being so, too. How could a trait so at odds with reproductive success survive natural selection?
Brendan Zietsch, of the QIMR, thinks that genes which cause men to be more feminine in appearance, outlook and behavior and those that make women more masculine in those attributes, confer reproductive advantages as long as they do not push the individual possessing them all the way to homosexuality.
Gay men tend to rank higher than straight men in standardised tests for agreeableness, expressiveness, conscientiousness, openness to experience and neuroticism. Lesbians tend to be more assertive and less neurotic than straight women.
Though women prefer traditionally macho men at the time in their menstrual cycles when they are most fertile, at other times they are more attracted to those with feminine traits such as tenderness, considerateness and kindness, as well as those with feminised faces. This suggests that women prefer macho men for breeding but the more feminised phenotype for carers and providers (husbands). And husbands father most of the world’s children.
Zietsch’s researchers asked 4,900 individual twins, not all of them identical, to fill out anonymous questionnaires about their sexual orientation, their gender self-identification and the number of opposite-sex partners they had had during the course of their lives.
They found that the more feminine a man, the more masculine a woman, the higher the hit rate with the opposite sex — though women of all gender identities reported fewer partners than men did (which may reflect male boasting &/or female bashfulness).
Heterosexuals with a homosexual twin tend to have more sexual partners than heterosexuals with a heterosexual twin. The researchers then analyzed the relationships between twins (all genes in common for identical twins; a 50% overlap for the non-identical) and calculated that genes explain 27% of an individual’s gender identity and 59% of the variation in the number of sexual partners. (They also calculated that the genetic component of sexual orientation was 47% — similar to previous studies