Fifteen years ago, Jane (as we’ll call her) considered herself a lesbian and had since an early age. Then, out of the blue, she fell in love with a man.

Their friendship started out platonic—just a guy and girl hanging out—but developed into more, leaving Jane a bit less sure about her sexuality. She was still far more into women than men, but this one guy—she couldn’t get him out of her head. Today, they’re happily married and she considers herself “a lesbian who happened to marry a man.”

Her story begs the question: is Jane gay, straight or lying? According to new research on the science of sexuality, she’s none of the above.

So Much For the Straight and Narrow

For the past fifteen years, Lisa Diamond, Ph.D. a psychologist at the University of Utah, has been following a group of women (including Jane) who are attracted to other women. Her data shows, for the first time, how sexuality develops over a lifetime.At each follow-up (six so far), Diamond asked each woman to label herself as lesbian, bisexual, heterosexual or unlabeled and share details about her love life. Her findings startled even her. Over time, each woman’s chosen labels changed repeatedly, with one noteworthy trend: The older they got, the more likely they were to choose “unlabeled.” In other words, the older they got, the more they felt their sexuality didn’t fit into tidy boxes.

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“We have this idea that sexuality gets clearer and more defined as time goes on,” says Diamond. “We consider that a sign of maturity to figure out who you are. I’ve seen it’s really the opposite.” For her subjects, maturity brought less clarity and definition, not more.

Perhaps even more surprising is the fact that the majority of the men and women who report feeling some same-sex attraction consider themselves heterosexual.

Elizabeth Morgan, professor of psychology at Boise State University, studies same-sex attractions among heterosexuals and finds that straight women often feel more than a friendly affection for other women.

In a study of 484 college students in California who identify as heterosexual, 45 percent of the women in the study had kissed a woman, 50 percent fantasized about women and a full 60 percent reported at least some sexual attraction to women. Broader population data suggests that upwards of 20 percent of women are attracted to other women.

That may have something to do with the way women are socialized. From chatting on the phone for hours to snuggling during chick flicks, women’s friendships are often barely distinguishable from romantic relationships. “Women are encouraged to be emotionally close to each other,” says Morgan. “That provides an opportunity for intimacy and romantic feelings to develop.”

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When otherwise heterosexual women fall for another woman, emotional connection is usually at the core. Diamond points out that many people, with the right person or the right circumstances, are willing to consider being with someone who falls outside their usual pattern (think: the 2001 hit comedy “Kissing Jessica Stein”).

“Even among people who identify as heterosexual, there is a lot of variation in who they fantasize being with, who they’re attracted to and who they actually engage in sexual activity with,” says Morgan. “Sexuality is a continuum.”

What Women Really Want

For women, sexual fluidity—the capacity to be attracted to both men and women—goes back to our sexual wiring.In a 2007 study, Meredith Chivers, Ph.D., assistant professor of psychology at Queen’s University in Ontario, showed participants a series of videos, ranging from nude exercise to homosexual and heterosexual sex to bonobo chimpanzees getting busy. She measured genital arousal and found an interesting pattern: Heterosexual women were turned on by sexual activity, while men and women attracted to women were turned on by gender. In fact, heterosexual women showed a stronger response to bonobo sex than they did to a naked man exercising! (Sorry, guys.)

But psychological arousal didn’t follow. Even though women responded physically to bonobo sex, they didn’t actually feel aroused. (Which, frankly, is a relief.) The reasons for the disconnect are complicated—and still hypothetical. One possibility is that, since women are less able to see and sense physical arousal, they may respond to contextual cues instead. That could actually be a boon, allowing women to make more rational decisions about who to sleep with and when.

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If anything, women’s arousal toward one gender trends female. For heterosexual women who report no same-sex attraction, Chivers has found that images of men and women are equally arousing. For women with even the slightest same-sex attraction, the gender balance tips and they become progressively more aroused by women than men. “For women, there seems to be more gray area, and more potential for same-sex attraction,” says Chivers.Internet behavior—a way of looking at what we do when no one is watching—supports the idea that women’s sexuality has many shades of gray.

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Ogi Ogas, Ph.D., a computational neuroscientist and co-author of “A Billion Wicked Thoughts,” analyzed more than a billion web searches, half a billion search histories and millions of erotic websites and e-books. He found that women were most interested in erotic stories (like fan fiction or romance novels) and that their gender preferences varied, without any clear pattern. Ogas explains that a woman is just as likely to search for “sexy pictures of Jake Gyllenhaal” one moment and “sexy pictures of Keira Knightley” the next.

Of course, that may have more to do with self-comparison (is my stomach as flat as Keira’s?) than sexual arousal. “Women in the media are often sexualized and women constantly get the message that appearance should be important to them, so they’re used to viewing women in a sexualized way,” says Morgan. She points out that women often find other women attractive, but sometimes struggle to figure out whether they’re actually interested in women or simply used to sizing up women’s beauty.

All these shades of gray leave a bit of confusion about what ‘heterosexual’ actually means. Students often come to Morgan saying, “I had a fantasy or a dream about someone of the same sex. Does that mean I’m gay?” Not necessarily, she says. “You can still be heterosexual and have interests, experiences or fantasies with the same sex.”Essentially, ‘heterosexual,’ ‘bisexual,’ or ‘lesbian’ are just labels we choose, usually representing our dominant preference. What those labels actually represent for each individual comes in every color of the rainbow.