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Marina Lacroix

How homosexuality may have evolved | Gender bending | The Economist - 0 views

www.economist.com/...displaystory.cfm

homosexuality nature-nurture

shared by Marina Lacroix on 17 Nov 08 - Cached
  • THE evidence suggests that homosexual behaviour is 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.
  • In a paper to be published soon in Evolution and Human Behavior, they suggest the advantage accrues not to relatives of the opposite sex, but to those of the same one. They think that genes which cause men to be more feminine in appearance, outlook and behaviour 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.
  • Other evidence does indeed show that homosexuals tend to be “gender atypical” in areas beside their choice of sexual partner. Gay men often see themselves as being more feminine than straight men do, and, mutatis mutandis, the same is true for lesbians. To a lesser extent, homosexuals tend to have gender-atypical careers, hobbies and other interests.
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  • Personality tests also show differences, with gay men ranking 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.
  • Dr Zietsch and his colleagues tested their idea by doing a twin study of their own. They asked 4,904 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.
  • Their first observation was that the number of sexual partners an individual claimed did correlate with that individual’s “gender identity”. 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.
  • When the relationships between twins were included in the statistical analysis (all genes in common for identical twins; a 50% overlap for the non-identical) the team was able to show that both atypical gender identity and its influence on the number of people of the opposite sex an individual claimed to have seduced were under a significant amount of genetic control. More directly, the study showed that heterosexuals with a homosexual twin tend to have more sexual partners than heterosexuals with a heterosexual twin.
  • According to the final crunching of the numbers, genes explain 27% of an individual’s gender identity and 59% of the variation in the number of sexual partners that people have. The team also measured the genetic component of sexual orientation and came up with a figure of 47%—more or less the same, therefore, as that from previous studies. The idea that it is having fecund relatives that sustains homosexuality thus looks quite plausible.
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