By Anna David

For years there's been a rumor floating around that women don’t cheat as much as men. If you believe that, have fun looking for Bigfoot and working on your "9/11 was a government conspiracy" manifesto. The truth is, girls are not quite as innocent as we seem.

Take it from a reformed female philanderer who spent her entire college career betraying one boyfriend after another. Following a karmic pos-college decade sent dateless and alone, I cleaned up my act, but not before I learned firsthand the freaky appeal and downsides of illicit hookups. And I'm not alone: These days women are increasingly willing to venture outside their relationships for sex. In fact, in a 2007 MSNBC.com/iVillage survey of more than 70,000 adults, 50 percent -- that's men and women -- admitted to being unfaithful at some point in their lives.

According to Judy Kuriansky, clinical psychologist and author of The Complete Idiot's Guide to a Healthy Relationship, this uptick in female infidelity makes sense. "We're not dealing with the 'electrician coming to the house' scenario anymore. Women are more financially independent, so they have more opportunities to meet people at work and outside the home." Plus, we're exposed to a 24-hour news cycle that keeps us abreast of rampant cheating by celebrities and politicians -- hello, Mr. Edwards! -- which makes betraying you seem, if not expected, then at least somewhat normal.

Hillary*, 27, who works for a nonprofit, claims to have cheated on every partner she's ever had. "My boyfriend thinks I'm this good girl. Most men just don't assume women are capable of what we're capable of." Not every girl's behavior is this extreme, of course. But it's time you found exactly what we're doing, why we're doing it, and what could be in it for you.

Why We Stray
Conventional wisdom says men cheat to affirm their virility and satiate their libidos, while women are unfaithful because they crave romance and feel neglected. That may be true occasionally, but women's sexual appetites can be just as strong as yours. So, sometimes, we cheat for the same reason Brett Favre unretires every year: because we can. Says Rebecca, a 32-year-old New York-based editor who betrayed her man on a recent work trip to L.A.: "I never would have done it in New York, but everything feels sort of surreal when you're away from home -- like no one's going to know about it."

Unlike guys, many women also use infidelity as a form of revenge. Sara, a 31-year-old bartender, had a one-night fling because she didn't like the way her boyfriend reacted to her attempt to spice up their sex life. "I started taking pole dancing classes and got fully waxed, and he told me it was trashy," she recalls. "So I thought, Fuck him, I'm going to find someone who appreciates this. I invited over my ex. Two bottles of wine and one lap dance later, we were screwing on the couch. Frankly, it felt great. My ex made me feel sexy, not embarrassed."

In fact, if a girl feels really unhappy in a relationship, she may even start an affair as a way out. "My boyfriend kept pressuring me to marry him, and I didn't want to," says Carine, a 27-year-old law student. "The more we fought about it, the more I escalated a flirtation with a classmate. Finally, late one night, after a round of 'strip-studying,' we had sex on a deserted floor of the school library. When I told my boyfriend, he broke up with me on the spot. I felt guilty but also, truthfully, kind of relieved."

How We Pull It Off
So why haven't you been wise to women's cheating ways before now? For one thing, it's quite possible that we're uniquely suited to it. There are a few decidedly feminine qualities -- our intense attention to detail, the way we consider a given situation from every angle -- that make us pretty good at the game of deception. "Women are better at covering their tracks than men," says Ian Kerner, Ph.D., sex therapist and author of She Comes First. "They really think about what a suspicious guy would look for."

We also employ many of the same tactics that men have used on us for years, such as the time-honored "accuse you just as you accuse me" approach. When Chelsea, a 28-year-old boutique owner, went to a crowded bar with her boyfriend and "ended up running into" the guy she liked (translation: she knew he'd be there), she took off with her crush for an hour. "He lived upstairs, so we headed to his apartment, taking each other's clothes off in the stairwell on the way up," she says. "It was extra dirty knowing my boyfriend was just two floors away." When she returned to the bar and her boyfriend asked where she'd been, Chelsea turned the tables. "I said, 'I don't know what you're talking about. I've been looking for you everywhere. Where the hell have you been?"

Of course, we don't fool you every time. Gwen, 30, knew she was busted when her boyfriend spied an enormous scratch she'd gotten during a drunken hookup the night before. "I was fucking this guy standing up in his bathroom, and I scraped my back against the towel rack," she recalls. Her weak excuse -- that there'd been "a whole lot of crap" on top of her bed when she passed out on it -- didn't pass muster with her furious (and soon-to-be former) guy.

Degrees of Transgression
For every woman who gets off on jumping out of your bed, into another man's, and then back into yours while you're none the wiser, there's another who's tormented with guilt. The problem is, even if she wants to call things off, that doesn't always mean she can or will.

One night after work, Samara, a 28-year-old waitress, confronted the restaurant's sous chef about hoe they had to stop cheating on their partners together. The next thing she knew, they were having sex on a dining room chair, "completely getting off on the fact that we were in a space that coworkers and customers walked into and out of workers all day. It was illicit on two levels, which just made it more of a turn-on."

Indeed, the exhilaration of doing something bad can make for volcano-temperature sex, and that can make cheating a hard habit to break. As Kerner explains, "Secretiveness creates a lot of the thrill. Often, once people have been caught, the person they were cheating with is no longer interesting to them."

Eventually, the stress of infidelity begins to outweigh the excitement for most women. But sometimes it can have a positive outcome, according to Kuriansky. . "Cheating can end up being constructive if it causes a couple to ask why it happened and make a new commitment to each other." As unbelievable as that may sound, Naomi, a 26-year-old yoga instructor, can back it up: "I felt awful after I cheated. I realized I'd only fucked the other guy because my boyfriend and I had been fighting a lot. I went home, confessed, and bawled my eyes out. He was pissed but ended up seeing how we both had contributed to the situation. We stayed together, and things have been great since."

Sin Signals
Before you start tailing your girl like a paranoid freak, realize that most of the time the signs she's cheating are pretty freakin' obvious. A few tip-offs: She's not putting out; she slams her computer shut or clicks out of window when you approach; she's suddenly "crashing at Lisa's" all the time (despite the fact that she kissed slumber parties goodbye after sixth grade); and, most peculiar, she's no longer interested in having those once-cherished long, emotional conversations with you about what your relationship means or where it's going.