For the past two generations, extensive research has been conducted on the determinants of homosexuality. But, until now, scant attention has been paid to what is perhaps the most mysterious--and potentially illuminating--variation of human sexual expression, bisexuality. Today, as ignorance and fear of AIDS makes greater awareness of all forms of sexual behavior an urgent matter of private and public consequence, leading sex researchers Martin Weinberg, Colin Williams, and Douglas Pryor provide us with the first major study of bisexuality. Weinberg, Williams, and Pryor explore the riddle of dual attraction in their study of 800 residents of San Francisco. Fieldwork, intensive interviews, and surveys provided a wealth of data about the nature of bisexual attraction, the steps that lead people to become bisexual, and how sexual preference can change over time. They found that heterosexuals, more often than homosexuals, become bisexual; that bisexual men and women differ markedly in their sexual behavior and romantic feelings; that most bisexuals ultimately settle into long-term relationships while continuing sexual activity outside those relationships; and they also explain why transsexuals often become bisexual. Moreover, the authors discovered that as the AIDS crisis unfolded, many bisexual men entered into monogamous relationships with women, and bisexual women into more lesbian relationships.