Bisexuality is romantic attraction, sexual attraction, or sexual behavior toward both males and females, or to more than one sex or gender. It may also be defined as romantic or sexual attraction to people of any sex or gender identity, which is also known as pansexuality.The term bisexuality is mainly used in the context of human attraction to denote romantic or sexual feelings toward both men and women, and the concept is one of the three main classifications of sexual orientation along with heterosexuality and homosexuality, all of which exist on the heterosexual–homosexual continuum. A bisexual identity does not necessarily equate to equal sexual attraction to both sexes; commonly, people who have a distinct but not exclusive sexual preference for one sex over the other also identify themselves as bisexual.Scientists do not know the exact cause of sexual orientation, but they theorize that it is caused by a complex interplay of genetic, hormonal, and environmental influences, and do not view it as a choice. Although no single theory on the cause of sexual orientation has yet gained widespread support, scientists favor biologically-based theories. There is considerably more evidence supporting nonsocial, biological causes of sexual orientation than social ones, especially for males.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 coined in the 19th century.