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Demographics

In their 2016 literature review, Bailey et al. stated that they "expect that in all cultures... a minority of individuals are sexually predisposed (whether exclusively or non-exclusively) to the same sex." They state that there is no persuasive evidence that the demographics of sexual orientation have varied much across time or place. Men are more likely to be exclusively homosexual than to be equally attracted to both sexes, while the opposite is true for women.

Surveys in Western cultures find, on average, that about 93% of men and 87% of women identify as completely heterosexual, 4% of men and 10% of women as mostly heterosexual, 0.5% of men and 1% of women as evenly bisexual, 0.5% of men and 0.5% of women as mostly homosexual, and 2% of men and 0.5% of women as completely homosexual. An analysis of 67 studies found that the lifetime prevalence of sex between men (regardless of orientation) was 3–5% for East Asia, 6–12% for South and South East Asia, 6–15% for Eastern Europe, and 6–20% for Latin America. The International HIV/AIDS Alliance estimates that worldwide between 3 and 16% of men have had some form of sex with another man at least once during their lifetime.

According to major studies, 2% to 11% of people have had some form of same-sex sexual contact within their lifetime; this percentage rises to 16–21% when either or both same-sex attraction and behavior are reported.

According to the 2021 United States Census, there were about 1.2 million same-sex couple households. In the United States, according to a report by The Williams Institute in April 2011, 3.5% or approximately 9 million of the adult population identify as lesbian, gay, or bisexual. A 2013 study by the CDC, in which over 34,000 Americans were interviewed, puts the percentage of self-identifying lesbians and gay men at 1.6%, and of bisexuals at 0.7%.

In October 2012, Gallup started conducting annual surveys to study the demographics of LGBT people, determining that 3.4% (±1%) of adults identified as LGBT in the United States. It was the nation's largest poll on the issue at the time. In 2017, the percentage was estimated to have risen to 4.5% of adults, with the increase largely driven by millennials. The poll attributes the rise to greater willingness of younger people to reveal their sexual identity.

Measuring the prevalence of homosexuality presents difficulties. It is necessary to consider the measuring criteria that are used, the cutoff point and the time span taken to define a sexual orientation. Many people, despite having same-sex attractions, may be reluctant to identify themselves as gay or bisexual. The research must measure some characteristic that may or may not be defining of sexual orientation. The number of people with same-sex desires may be larger than the number of people who act on those desires, which in turn may be larger than the number of people who self-identify as gay, lesbian, or bisexual.

Reliable data as to the size of the gay and lesbian population are of value in informing public policy. For example, demographics are of help in calculating the costs and benefits of domestic partnership benefits, of the impact of legalizing gay adoption, and of the impact of the U.S. military's former Don't Ask Don't Tell policy. Further, knowledge of the size of the "gay and lesbian population holds promise for helping social scientists understand a wide array of important questions—questions about the general nature of labor market choices, accumulation of human capital, specialization within households, discrimination, and decisions about geographic location."