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.

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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.

The psychologist Jim Mc Knight writes that while the idea that bisexuality is a form of sexual orientation intermediate between homosexuality and heterosexuality is implicit in the Kinsey scale, that conception has been "severely challenged" since the publication of Homosexualities (1978), by Weinberg and the psychologist Alan P. Kinsey's 1948 work Sexual Behavior in the Human Male found that "46% of the male population had engaged in both heterosexual and homosexual activities, or 'reacted to' persons of both sexes, in the course of their adult lives".

From an anthropological perspective, there is large variation in the prevalence of bisexuality between different cultures.

Among some tribes, it appears to be non-existent while in others a universal, including the Sambia of New Guinea and similar Melanesian cultures.

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.Magnus Hirschfeld argued that adult sexual orientation can be explained in terms of the bisexual nature of the developing fetus: he believed that in every embryo there is one rudimentary neutral center for attraction to males and another for attraction to females.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.In a longitudinal study about sexual identity development among lesbian, gay, and bisexual (LGB) youths, Rosario et al."found evidence of both considerable consistency and change in LGB sexual identity over time".The American Psychiatric Association stated: "To date there are no replicated scientific studies supporting any specific biological etiology for homosexuality.