Table : TABLE 5.1
Biological Correlates of Sexual Orientation
Gay-straight trait differences
Sexual orientation is part of a package of traits. Studies—some in need of replication—indicate that homosexuals and heterosexuals differ in the following biological and behavioral traits:

  • spatial abilities

  • fingerprint ridge counts

  • auditory system development

  • handedness

  • occupational preferences

  • relative finger lengths

  • gender nonconformity

  • age of onset of puberty in males

  • male body size

  • sleep length

  • physical aggression

  • walking style

On average (the evidence is strongest for males), results for gays and lesbians fall between those of straight men and straight women. Three biological influences—brain, genetic, and prenatal—may contribute to these differences.
Brain differences

  • One hypothalamic cell cluster is smaller in women and gay men than in straight men.

  • Anterior commissure is larger in gay men than in straight men.

  • Gay men’s hypothalamus reacts as do straight women’s to the smell of sex-related hormones.

Genetic influences

  • Shared sexual orientation is higher among identical twins than among fraternal twins.

  • Sexual attraction in fruit flies can be genetically manipulated.

  • Male homosexuality often appears to be transmitted from the mother’s side of the family.

Prenatal influences

  • Altered prenatal hormone exposure may lead to homosexuality in humans and other animals.

  • Men with several older biological brothers are more likely to be gay, possibly due to a maternal immune-system reaction.