Although causal theories of sexual orientation generally fall into two categories, biological/genetic and social/environmental, sexual orientation, as a complex, multidimensional construct, cannot be [Page 1322]explained fully by either alone. Most theories are constrained by their operational definitions of sexual orientation (e.g., self-identification, sexual attraction, and/or sexual activity); evidence that sexual orientation is fluid, especially for women; and their emphasis on explaining homosexuality rather than heterosexuality, often grouping homosexuality and bisexuality together. Although women's sexual orientation has been the focus of scholarship, theories tend to focus on men's sexual orientation.