Genetic, hormonal, neurological, and other biological factors need to be taken into account to fully understand sexual orientation. This work represents the latest research and theory on causes of variation in sexual orientation. It looks at sexual orientation as a cross-species phenomenon with numerous determining factors. This work is a collection of chapters by some of the leading researchers in the scientific study of sexual orientation. The theory that many genetic, hormonal, neurological, and other biological factors need to be taken into account to fully understand sexual orientation is espoused in this book. It presents much of the latest research on the causes of variation in sexual orientation and related phenomena. It views sexual orientation as a cross-species phenomenon with both biological and environmental determinants.