A biologic perspective on sexual orientation.

Sexual orientation may be defined as the sustained erotic attraction to members of one's own gender, the opposite gender, or both-homosexual, heterosexual, or bisexual, respectively. Interest in sexual orientation is as old as the science of psychology, yet many fundamental issues remain unresolved. This article reviews research in the development and psychopathology of sexual orientation as well as the results of family and twin studies. Research in genetic linkage, sex hormones, and brain differences also is discussed.