In most fetuses, the center for attraction to the opposite sex developed while the center for attraction to the same sex regressed, but in fetuses that became homosexual, the reverse occurred.Simon Le Vay has criticized Hirschfeld's theory of an early bisexual stage of development, calling it confusing; Le Vay maintains that Hirschfeld failed to distinguish between saying that the brain is sexually undifferentiated at an early stage of development and saying that an individual actually experiences sexual attraction to both men and women.The American Psychological Association states that "sexual orientation falls along a continuum.In other words, someone does not have to be exclusively homosexual or heterosexual, but can feel varying degrees of both.The study also found that "bisexual/unlabeled women had stable overall distributions of same-sex/other-sex attractions." Diamond has also studied male bisexuality, noting that survey research found "almost as many men transitioned at some point from a gay identity to a bisexual, queer or unlabeled one, as did from a bisexual identity to a gay identity." In the 1940s, the zoologist Alfred Kinsey created a scale to measure the continuum of sexual orientation from heterosexuality to homosexuality.Kinsey studied human sexuality and argued that people have the capability of being hetero- or homosexual even if this trait does not present itself in the current circumstances.
Rather, LGB individuals are often raised in communities that are either ignorant of or openly hostile toward homosexuality.
The same study found that 2.8 percent of women ages 18–44 considered themselves bisexual, 1.3 percent homosexual, and 3.8 percent as "something else".
From an anthropological perspective, there is large variation in the prevalence of bisexuality between different cultures.
"found evidence of both considerable consistency and change in LGB sexual identity over time".
Youths who had identified as both gay/lesbian and bisexual prior to baseline were approximately three times more likely to identify as gay/lesbian than as bisexual at subsequent assessments.