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".
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.
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".
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.