USA: Women's sexuality is more "adaptive" than men's, psychologists have found.
They are more likely to become bisexual if opportunities arise to explore same-sex relationships, say researchers.
Men, on the other hand, are more likely to stay on one side of the sexual fence or the other.
US scientists tracked 5,018 women and 4,191 men as they progressed from adolescence to young adulthood, questioning them about their sexual activity.
They found that women were three times more likely than men to change their sexual identities.
Women who were more educated, more physically attractive, and who avoided young motherhood were less likely to experiment with bisexuality.
Lead researcher Dr Elizabeth McClintock, from the University of Notre Dame, Indiana, said: "Women with some degree of attraction to both males and females might be drawn into heterosexuality if they have favourable options in the heterosexual partner market.
"Women who are initially successful in partnering with men, as is more traditionally expected, may never explore their attraction to other women.
"However, women with the same sexual attractions, but less favorable heterosexual options might have greater opportunity to experiment with same-sex partners.
"Women who act on same-sex attraction are more likely to incorporate same-sex sexuality into their sexual identities."
For men, higher levels of education were associated with a lower chance of them seeing themselves as "100% heterosexual" while physical attractiveness had no clear link with sexual orientation.
"Men are less often attracted to both sexes," Dr McClintock added. "Men's sexuality is, in this sense, less flexible.
"If a man is only attracted to one sex, romantic opportunity would little alter his sexual identity."
The findings were presented at the annual meeting of the American Sociological Association in Chicago.