Psychology was one of the first disciplines to study homosexuality as a discrete phenomenon. In the late 19th and early 20th centuries, pathological models of homosexuality were standard. Psychologists later began responding to the needs of gay, lesbian, and bisexual people including, most visibly, responses to the AIDS pandemic of the 1980s and 1990s. Major psychological research on homosexuality, which has been carried out predominantly in America, can be divided into five categories: