People whose sexual desire and activities are strongly channelled toward members of their own sex are a minority of the population (variously estimated to be anywhere from 2% to 10%). Nevertheless, many other people who are in general heterosexual may have mild or occasional interest in members of their own sex.
Conversely, many people who identify themselves as homosexual, or who might prefer homosexual activities or relationships, have engaged in heterosexual activities or even have long-term heterosexual relationships. (Such "heterosexual" behavior by people who would otherwise be homosexual has often been part of being "in the closet", or concealing one's homosexuality, and may be becoming less common as acceptance of homosexuality increases.)
People whose sexual interest and/or activities generally include persons of both sexes are known as bisexuals.
Some studies, notably Sexual Behavior in the Human Male (1948) and Sexual Behavior in the Human Female (1953) by Dr. Alfred C. Kinsey, note that when asked to rate themselves on a continuum from completely heterosexual to completely homosexual, and when the individuals behavior as well as their identify is analyzed, the majority of people appear to be at least somewhat bisexual. Most people have some attraction to either sex, although usually one sex is preferred. Only a minority (5-10%) can be considered fully heterosexual or homosexual. Conversely, only an even smaller minority can be considered "fully" bisexual.