Homosexuality is one of the three main categories of sexual orientation, along with bisexuality and heterosexuality, within the heterosexual-homosexual continuum (with asexuality sometimes considered a fourth). Scientific and medical understanding is that sexual orientation is not a choice, but rather a complex interplay of biological and environmental factors,[1][3] especially with regard to early uterine environment.[4] While there are those who still hold the view that homosexual activity is "unnatural" or "dysfunctional",[5][6] research has shown that homosexuality is an example of a normal and natural variation in human sexuality and is not in and of itself a source of negative psychological effects.[1][7] Prejudice and discrimination against homosexual and bisexual people, however, have been shown to cause psychological harm.[7][8]
The most common terms for homosexual people are lesbian for females and gay for males, though gay is also used to refer generally to both homosexual males and females. The number of people who identify as gay or lesbian—and the proportion of people who have same-sex sexual experiences—are difficult for researchers to estimate reliably for a variety of reasons.[9] According to major studies, 2% to 10% of people have had some form of same-sex sexual contact within their lifetime.[10][11][12][13][14][15][16][17][18] In a 2006 study, 20% of respondents anonymously reported some homosexual feelings, although only 2-3% identified themselves