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Studies, theories and social responses

There is no consensus among scientists about the exact reasons that an individual develops a heterosexual, bisexual or homosexual orientation. Although scientists favor biological models for the cause of sexual orientation, they do not believe that the development of sexual orientation is the result of any one factor. They generally believe that it is determined by a complex interplay of biological and environmental factors, and is shaped at an early age. There is considerably more evidence supporting nonsocial, biological causes of sexual orientation than social ones, especially for males. There is no substantive evidence which suggests parenting or early childhood experiences play a role with regard to sexual orientation. Most scientists do not believe that sexual orientation is a choice that can be changed at will.

In 2000, American Psychiatric Association stated: "To date there are no replicated scientific studies supporting any specific biological etiology for homosexuality. Similarly, no specific psychosocial or family dynamic cause for homosexuality has been identified, including histories of childhood sexual abuse." Research into how sexual orientation may be determined by genetic or other prenatal factors plays a role in political and social debates about homosexuality, and also raises fears about genetic profiling and prenatal testing.

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. Simon LeVay has criticized Hirschfeld's theory of an early bisexual stage of development, calling it confusing; LeVay 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 LeVay, 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.

Hirschfeld created a ten-point scale to measure the strength of sexual desire, with the direction of desire being represented by the letters A (for heterosexuality), B (for homosexuality), and A + B (for bisexuality). On this scale, someone who was A3, B9 would be weakly attracted to the opposite sex and very strongly attracted to the same sex, an A0, B0 would be asexual, and an A10, B10 would be very attracted to both sexes. LeVay compares Hirschfeld's scale to that developed by Kinsey decades later.

Sigmund Freud, the founder of psychoanalysis, believed that every human being is bisexual in the sense of incorporating general attributes of both sexes. In his view, this was true anatomically and therefore also psychologically, with sexual attraction to both sexes being an aspect of this psychological bisexuality. Freud believed that in the course of sexual development the masculine side of this bisexual disposition would normally become dominant in men and the feminine side in women, but that all adults still have desires derived from both the masculine and the feminine sides of their natures. Freud did not claim that everyone is bisexual in the sense of feeling the same level of sexual attraction to men and women. Freud's belief in innate bisexuality was rejected by Sándor Radó in 1940 and, following Radó, by many later psychoanalysts. Radó argued that there is no biological bisexuality in humans.

Alan P. Bell, Martin S. Weinberg, and Sue Kiefer Hammersmith reported in Sexual Preference (1981) that sexual preference was much less strongly connected with pre-adult sexual feelings among bisexuals than it was among heterosexuals and homosexuals. Based on this and other findings, they suggested that bisexuality is more influenced by social and sexual learning than is exclusive homosexuality. Letitia Anne Peplau et al. wrote that while Bell et al.'s view that biological factors may be more influential on homosexuality than on bisexuality might seem plausible, it has not been directly tested and appears to conflict with available evidence, such as that concerning prenatal hormone exposure.

Human bisexuality has mainly been studied alongside homosexuality. Van Wyk and Geist argue that this is a problem for sexuality research because the few studies that have observed bisexuals separately have found that bisexuals are often different from both heterosexuals and homosexuals. Furthermore, bisexuality does not always represent a halfway point between the dichotomy. Research indicates that bisexuality is influenced by biological, cognitive and cultural variables in interaction, and this leads to different types of bisexuality.

In the current debate around influences on sexual orientation, biological explanations have been questioned by social scientists, particularly by feminists who encourage women to make conscious decisions about their life and sexuality. A difference in attitude between homosexual men and women has also been reported, with men more likely to regard their sexuality as biological, "reflecting the universal male experience in this culture, not the complexities of the lesbian world." There is also evidence that women's sexuality may be more strongly affected by cultural and contextual factors.

American academic Camille Paglia has promoted bisexuality as an ideal. American Harvard professor Marjorie Garber made an academic case for bisexuality with her 1995 book Vice Versa: Bisexuality and the Eroticism of Everyday Life, in which she argued that most people would be bisexual if not for repression and other factors such as lack of sexual opportunity.