Psychiatric theories of homosexuality have included the following: that homosexuality is a regression to the earliest (oral) stage of development; that most families of homosexuals are characterized by an overprotective mother and an absent father; or that homosexuals fear engulfment by a dominant mother in the pre-Oedipal phase. Some authorities have suggested that homosexuality may be an expression of nonsexual problems, such as fear of adult responsibility, or may be triggered by various experiences, such as having sexual relationships with members of one's own sex at an early age that prove to be very satisfying. Arguments regarding the roots of lesbianism include disappointing heterosexual love experience, a father who displays distaste for men who express interest in his daughter, and memories of abusive relationships with men.
Many of these theories have been discredited in recent years, particularly by those who cite biological causation. Some researchers have contended that a disruption in the hormonal processes of the mother while she is pregnant may be one explanation. Simon Levay, a neurobiologist at the Salk Institute, has suggested that homosexuality may be related to brain functioning, as part of the hypothalamus in homosexual men is about a quarter to half the size it is in heterosexual men. Subsequent studies have shown that homosexual men react to certain substances believed to be human pheromones differently from heterosexual men. Several studies have pointed to a genetic predisposition governed by one or more genes on the X chromosome.
Other recent studies, while not directly supporting biological explanations for homosexuality, suggest that it may be a predisposition that can be detected at an early age among children who do not appear to have traditional gender identification. Whether it can be easily detected or not, most theorists agree that homosexual orientation tends to arise at an early age. Substantially fewer studies of homosexuality have been performed among lesbians, perhaps because of the greater stigma which is often attached to male homosexuality in many Western cultures.
The American Psychiatric Association no longer considers homosexuality a disorder, unless sexual orientation becomes an object of distress for the individual. In such cases, the individual—referred to by psychologists as ego-dystonic—may choose to seek psychiatric treatment. Also, beginning in the late 20th cent., biologists more openly examined and discussed the occurrence of homosexual behaviors among animals, which has been documented in several hundred species. Such behaviors, which may include courtship, sexual contact, bond formation, and the rearing of young, are found both in wild and captive animals. Many gay-rights activists have criticized the various theories which try to "explain" homosexuality, particularly those that treat it as an illness in need of treatment.