heteroflexible, homoflexible, men who have sex with men and women who have sex with women may also be used to describe sexual identity or identify sexual behavior.Some sources state that bisexuality encompasses romantic or sexual attraction to all gender identities or that it is romantic or sexual attraction to a person irrespective of that person's biological sex or gender, equating it to or rendering it interchangeable with pansexuality.Rather, LGB individuals are often raised in communities that are either ignorant of or openly hostile toward homosexuality.Bisexuality as a transitional identity has also been examined.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.
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.
According to Le Vay, Hirschfeld believed that in most bisexual people the strength of attraction to the same sex was relatively low, and that it was therefore possible to restrain its development in young people, something Hirschfeld supported.
and environmental factors (including fraternal birth order, where the number of older brothers a boy has increases the chances of homosexuality; specific prenatal hormone exposure, where hormones play a role in determining sexual orientation as they do with sex differentiation; The American Psychological Association has stated that "there are probably many reasons for a person's sexual orientation and the reasons may be different for different people".
It further stated that, for most people, sexual orientation is determined at an early age.
A 2002 survey in the United States by National Center for Health Statistics found that 1.8 percent of men ages 18–44 considered themselves bisexual, 2.3 percent homosexual, and 3.9 percent as "something else".