Born gay? The psychobiology of human sexual orientation

Sexual orientation is fundamental to evolution and shifts from the species-typical pattern of heterosexuality may represent biological variations. The growth of scientific knowledge concerning the biology of sexual orientation during the past decade has been considerable. Sexual orientation is characterised by a bipolar distribution and is related to fraternal birth order in males. In females, its distribution is more variable; females being less prone towards exclusive homosexuality. In both sexes homosexuality is strongly associated with childhood gender nonconformity. Genetic evidence suggests a heritable component and putative gene loci on the X chromosome. Homosexuality may have evolved to promote same sex affiliation through a conserved neurodevelopmental mechanism. Recent findings suggest this mechanism involves atypical neurohormonal differentiation of the brain. Key areas for future research include the neurobiological basis of preferred sexual targets and correlates of female homosexuality.