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, including many gay people not openly identifying as such due to homophobia and heterosexist discrimination.[10] Homosexual behavior has also been documented and is observed in many non-human animal species