Compared to heterosexuality and homosexuality, very little scientific study has been conducted on bisexuality. Based on the studies that have been done, it appears that bisexuals are not people whose orientation is fundamentally homosexual but who have some heterosexual sex on the side. Nor are they people whose orientation is fundamentally heterosexual but who enjoy homosexual sex on the side. Also, fundamentally, they are not people who, at one point in their lives, engage in sexual behavior with persons of one sex and then, at another point, engage in sexual behavior with persons of the other sex, although this pattern of sequential changes is sometimes called transitional bisexuality. Rather, bisexuals are people who are sexually attracted to persons of both sexes during the same general time period in their lives.

It is thought that people develop and experience bisexuality in a number of different ways. For some it begins as a form of experimentation that adds a spark to their sex lives, but it does not become the main arena of sexual activity. For others it is a deliberate choice to participate in whatever feels best at the moment. Three particular sets of circumstances have been thought to be conducive to bisexuality: (a) sexual experimentation in a relationship with a close friend is quite common among women and can also occur between two male friends or a male homosexual may develop a sexual relationship from a previously casual but friendly relationship with a woman. (b) Group sex is another avenue for bisexual experimentation. (c) Finally, some people adopt a bisexual philosophy as an outgrowth of a personal belief system. For instance, some women who have been active in the women's movement find they are drawn closer to other women by the experience and translate this closeness into sexual expression.

Men who are bisexuals are likely to experience homosexual attraction and engage in homosexual experiences before they become aware of their bisexuality. For women, on the other hand, the trend is to experience heterosexuality first.

Although persons with a bisexual orientation do not fit simply into any one mold, there are a few patterns that may apply to many bisexuals. Some men and women seem to alternate their choice of sex partners randomly, depending on availability and circumstances. Some have committed relationships in this fashion, seeking a partner of the alternative sex when the current relationship ends. In other cases, a bisexual person may have simultaneous relationships with a man and a woman. Affairs during a lasting relationship may also be used to express one's bisexual orientation. Most often, whichever of these patterns applies, people with bisexual orientation have a tendency toward more relationships with one gender than the other.

Researchers who have studied female bisexuality note that some women who identify themselves as bisexual say that they have some emotional needs that are best met by men and others that are best met by women. Some bisexual men offered this explanation too, but much more often the male bisexual explains his sexual lifestyle in terms of a need for variety and creativity.

People usually discover their bisexual orientation later in life than either heterosexuals or homosexuals. The majorities of people model the heterosexual lifestyle and drift into bisexual relationships without consciously thinking about it initially. Most individuals who discover their attraction to the same sex try to deny their interest and attempt to fit in with the more socially acceptable heterosexual lifestyle for a while. Usually by adolescence there is increasing internal conflict about their sexual preference that may not be fully resolved until adulthood.

Because it is commonly thought that people are either heterosexual or homosexual, even by people with bisexual interests, these people seem to struggle for a longer period of time trying to conform to one lifestyle or the other. It is common for people to be well into their 20's or 30's before accepting their bisexual orientation. Society's definitions of what is normal, appropriate, right and natural have an enormous influence on how bisexual people feel about their sexual orientation. Given the negative bias toward bisexuality, it is not unusual for women and men with a bisexual orientation to feel alienated from and oppressed by both the heterosexual and homosexual communities. For them, this can raise serious questions about their sexual identity. Bisexual people have problems similar to those that homosexual people have in "coming out" and making their orientation known to family and friends

Someone who is bisexual may often find it harder to start and maintain relationships than people of heterosexual or homosexual orientation. Because bisexual people are both different and often misunderstood, those who do not have a bisexual orientation may be rejecting or feel that a relationship with a bisexual person could not be valid or rewarding. Jealousy, which can be a problem in any relationship, is particularly likely in a relationship in which one partner is bisexual. If both partners are bisexual the possibilities for jealousy may be even greater. The threat of such widespread competition can be very stressful to a couple that is not secure in their relationship.