gay men and lesbians sampled in multiple studies reported "substantially more" cross-sex-typed behavior in childhood than heterosexual subjects. Sexual orientation sees greater intricacy when non-binary understandings of both sex and gender are considered. Sociologist Paula Rodriguez Rust (2000) argues for a more multifaceted definition of sexual orientation: Relationships outside of orientation Gay and lesbian people can have sexual relationships with someone of the opposite sex for a variety of reasons, including the desire for a perceived traditional family and concerns of discrimination and religious ostracism. While some LGBT people hide their respective orientations from their spouses, others develop positive gay and lesbian identities while maintaining successful heterosexual marriages. Coming out of the closet to oneself, a spouse of the opposite sex, and children can present challenges that are not faced by gay and lesbian people who are not married to people of the opposite sex or do not have children. Fluidity Often, sexual orientation and sexual orientation identity are not distinguished, which can impact accurately assessing sexual identity and whether or not sexual orientation is able to change; sexual orientation identity can change throughout an individual's life, and may or may not align with biological sex, sexual behavior, or actual sexual orientation. Sexual orientation is stable and unchanging for the vast majority of people, but some research indicates that some people may experience change in their sexual orientation, and this is more likely for women than for men. A recent evolutionary theory of female sexual fluidity suggests that women may not have a sexual orientation in the same sense as men, and that women's apparent sexual orientation may instead be a byproduct of their sociosexual orientation. Compared to males, father absence increases female same-sex sexual behavior in adulthood more significantly. The American Psychological Association distinguishes between sexual orientation (an innate attraction) and sexual orientation identity (which may change at any point in a person's life). Causes The exact causes for the development of a particular sexual orientation have yet to be established. To date, much research has been conducted to determine the influence of genetics, hormonal action, development dynamics, social and cultural influences—which has led many to think that biology and environment factors play a complex role in forming it. Biology Research has identified several biological factors which may be related to the development of sexual orientation, including genes, prenatal hormones, and brain structure. No single controlling cause has been