In other words, someone does not have to be exclusively homosexual or heterosexual, but can feel varying degrees of both.
Sexual orientation develops across a person's lifetime–different people realize at different points in their lives that they are heterosexual, bisexual or homosexual." Sexual attraction, behavior and identity may also be incongruent, as sexual attraction or behavior may not necessarily be consistent with identity.
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.
Bisexuality is romantic or sexual attraction to males and females.
The American Psychological Association states that "sexual orientation falls along a continuum.
The 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.
Among some tribes, it appears to be non-existent while in others a universal, including the Sambia of New Guinea and similar Melanesian cultures.
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.
Of youths who had identified only as bisexual at earlier assessments, 60 to 70 percent continued to thus identify, while approximately 30 to 40 percent assumed a gay/lesbian identity over time. suggested that "although there were youths who consistently self-identified as bisexual throughout the study, for other youths, a bisexual identity served as a transitional identity to a subsequent gay/lesbian identity." By contrast, a longitudinal study by Lisa M.
Le Vay compares Hirschfeld's scale to that developed by Kinsey decades later.
Sigmund Freud believed that every human being is bisexual in the sense of incorporating general attributes of both sexes.
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).