Bisexuality refers to sexual behavior with or physical attraction to people of both genders (male and female), or a bisexual orientation. People who have a bisexual orientation "can experience sexual, emotional, and affectional attraction to both their own sex and the opposite sex"; "it also refers to an individual's sense of personal and social identity based on those attractions, behaviors expressing them, and membership in a community of others who share them." It is one of the three main classifications of sexual orientation, along with a heterosexual and a homosexual orientation. Individuals who do not experience sexual attraction to either sex are known as asexual.

According to Alfred Kinsey's research into human sexuality in the mid-20th century, many humans do not fall exclusively into heterosexual or homosexual classifications but somewhere between. The Kinsey scale measures sexual attraction and behavior on a seven-point scale ranging from 0 (exclusively heterosexual) to 6 (exclusively homosexual). According to Kinsey's study, a substantial number of people fall within the range of 1 to 5 (between heterosexual and homosexual). Although Kinsey's methodology has been criticized, the scale is still widely used in describing the continuum of human sexuality.

Bisexuality has been observed in various human societies and elsewhere in the animal kingdom throughout recorded history. The term bisexuality, however, like the terms hetero- and homosexuality, was only coined in the 19th century.

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Despite misconceptions, bisexuality does not require that a person be attracted equally to both sexes. In fact, people who have a distinct but not exclusive preference for one sex over the other may still identify themselves as bisexual. A recent study by researchers Gerulf Rieger, Meredith L. Chivers, and J. Michael Bailey, which attracted media attention in 2005, purported to find that bisexuality is extremely rare in men. This was based on results of controversial penile plethysmograph testing when viewing pornographic material involving only men and pornography involving only women. Critics state that this study works from the assumption that a person is only truly bisexual if he or she exhibits virtually equal arousal responses to both opposite-sex and same-sex stimuli, and have consequently dismissed the self-identification of people whose arousal patterns showed even a mild preference for one sex. Some researchers say that the technique used in the study to measure genital arousal is too crude to capture the richness (erotic sensations, affection, admiration) that constitutes sexual attraction. The study, and The New York Times article which reported it, were subsequently criticized as flawed and biphobic.National Gay and Lesbian Task Force (July 2005). The Problems with "Gay, Straight, or Lying?" (PDF) Retrieved July 24, 2006. FAIR also criticised the study.FAIR (July 8, 2005). New York Times Suggests Bisexuals Are "Lying": Paper fails to disclose study author's controversial history.