Human sexual orientation has a heritable component.

We present an overview of behavioral genetics research on homosexual and heterosexual orientation. Family, twin, and adoptee studies indicate that homosexuality and thus heterosexuality run in families. Sibling, twin, and adoptee concordance rates are compatible with the hypothesis that genes account for at least half of the variance in sexual orientation. We note observations of homosexual behavior in animal species, but the analogy to human sexual orientation is unclear. We discuss the reproductive disadvantage of a homosexual orientation and present possible mechanisms that could maintain a balanced polymorphism in human populations.

Molecular investigations into complex behavior: lessons from sexual orientation studies.

A review of recent molecular genetic investigations into sexual orientation is presented and used as a model to identify unique challenges encountered in research on complex behavior. Five main themes are presented that are generally applicable to most behavioral genetics investigations: (1) A precise definition of the phenotype that, as much as possible, unambiguously differentiates the study group from the control group must be achieved; (2) environmental and social contributions to a trait may exhibit significant sex differences; (3) subsets within the study sample may reveal important information about genetic and environmental contributions to the trait that otherwise would be missed; (4) the high degree of specificity required to define a study sample severely restricts the application of results to general populations; and (5) studies attempting to replicate data must strictly parallel the original methodology if valid comparisons are to be made.