Homosexuality refers to sexual attraction or sexual behavior between people of the same sex, or to a sexual orientation. As an orientation, homosexuality refers to "an enduring pattern of or disposition to experience sexual, affectional, or romantic attractions primarily to" people of the same sex; "it also refers to an individual’s sense of personal and social identity based on those attractions, behaviors expressing them, and membership in a community of others who share them."
Homosexuality is one of the three main categories of sexual orientation, along with bisexuality and heterosexuality, within the heterosexual-homosexual continuum. The number of people who identify as homosexual — and the proportion of people who have same-sex sexual experiences — are difficult for researchers to estimate reliably for a variety of reasons. In the modern West, major studies indicate a prevalence of 2% to 13% of the population.
A 2006 study suggested that 20% of the population anonymously reported some homosexual feelings, although relatively few participants in the study identified themselves as homosexual.
Psychology was one of the first disciplines to study homosexuality as a discrete phenomenon. In the late 19th and throughout most of the 20th centuries, pathological models of homosexuality were standard. On December 15, 1973, the American Psychiatric Association, removed homosexuality from its official list of mental disorders. The American Psychological Association Council of Representatives adopted the same measure on January 24-26, 1975.
The first attempts to classify homosexuality as a disease were made by the fledgling European sexologist movement in the late 19th century. In 1886 noted sexologist Richard von Krafft-Ebing listed homosexuality along with 200 other case studies of deviant sexual practices in his definitive work, Psychopathia Sexualis. Krafft-Ebing proposed that homosexuality was caused by either "congenital [during birth] inversion" or an "acquired inversion".
There is no consensus among scientists about the exact reasons that an individual develops a heterosexual, bisexual, gay, or lesbian orientation. The main reasons cited include genetic and environmental factors, likely in combination. Other factors that may play a role include prenatal hormone exposure, where hormones play a role in determining sexual orientation as they do with sex differentiation; and prenatal stress on the mother.The American Academy of Pediatrics has stated that "sexual orientation probably is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences".
No, homosexuality is not an illness, a mental disorder, or a disease.