You've heard the stories: A seemingly straight woman falls for another woman (hey there, Cynthia Nixon); an erstwhile lesbian winds up with a man (howdy, Anne Heche). The switch-hitter invariably offers some explanation about "falling in love with the person, not the gender." Weird and rare, right? No, according to Lisa M. Diamond, a psychologist at the University of Utah and one of the foremost sexuality researchers in the country. In the first study of its kind, Diamond interviewed the same 100 women over the course of 10 years about their sexual leanings. She reports in her new book, Sexual Fluidity, that the traditional model of straight, bisexual, and gay — which is most often used to define men and boys (who are the focus of most studies on sexual orientation) — doesn't necessarily apply to women. The upshot? Most of us are capable of being somewhat fluid with our emotions. A few more revelations:

AGE IS A NONISSUE. "Contrary to the notion that you struggle with your sexuality, come out, embrace your new identity, end of story, the majority of the women I interviewed kept changing their labels over time," says Diamond. While men tend to be less flexible, women unintentionally keep their options open as they get older.

SO IS GENDER (SORT OF). "I have never met a woman over 30 who suddenly experiences same-sex attraction unless it involves a particular woman," says Diamond. A straight woman can experience "just one" same-sex attraction, just as a lesbian can have "just one" opposite-sex attraction, because for women, it's the emotional bond with a particular person that precipitates the attraction in the first place.

WE'RE ONE-WOMAN WOMEN. "When we talk to girls about sexuality, we talk about it only being appropriate in relationships," says Diamond. "It's not that women don't ever want casual sex, but most of the research suggests that relationships play a more organizing role for women's eroticism than for men's."

IT'S NATURAL. In the past, women who had one or two same-sex attractions were dismissed as crazy, in denial, unable to find a man, or repressed. But Diamond found that occasional attraction to other females is common. Among the women she interviewed, 30 percent claimed a different sexual label since the interview two years before. When the decade was up, two-thirds had changed their sexual identity at least once since her study began.

BUT NOT UNIVERSAL. "Not all women are equally fluid, so the question is why," says Diamond. "That is the question that wakes me up at 3 in the morning."

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