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.
Kinsey himself disliked the use of the term bisexual to describe individuals who engage in sexual activity with both males and females, preferring to use bisexual in its original, biological sense as hermaphroditic, stating, "Until it is demonstrated [that] taste in a sexual relation is dependent upon the individual containing within his anatomy both male and female structures, or male and female physiological capacities, it is unfortunate to call such individuals bisexual." The Janus Report on Sexual Behavior, published in 1993, showed that 5 percent of men and 3 percent of women considered themselves bisexual and 4 percent of men and 2 percent of women considered themselves homosexual.
More modern studies estimating the demographics for bisexuality have varied.
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.
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".