We all know the stereotypes about adultery -- men are always looking for an excuse to cheat while women dutifully keep the home fires burning, right?

Not if you're willing to believe Dr. Helen Fisher, a trusted expert in the field of biological anthropology, and the chief scientific advisor for Chemistry.com. And her revelations about the gender divide (including the fact that women like to get laid on the side just as much as men) are based on biology, not hearsay.

Read on about the underrated female libido and the fallacies of modern adultery -- and prepare to get really stressed out about where your girlfriend really goes during those "weekly book club meetings."

Women -- now just as awful as men!
For a long time, Dr. Fisher explains, adultery was a man's game. That's because women depended on their husband for their livelihood and support; if they strayed from the marital bed, they might lose that security. (In previous hunting and gathering societies, where both genders contributed more equally to the family, women were more likely to cheat.)

The modernized economy has changed the stats. Recent surveys show that for those under 40, adultery rates are the same for men and women. "Now, as women are piling back in the job market and regaining what they have lost -- economic power -- they are expressing their sexuality," Dr. Fisher says. "They can risk it."

Why we cheat
And there's more bad news: Men and women may cheat equally now, but they have different goals. For a dude, it might be about "sex and adventure" -- also known as the novelty of seeing a pair of boobs that do not belong to your wife. For women, adultery tends to signify a deeper dissatisfaction. She's not sleeping around because she wants a one-night thrill; she's probably looking for a whole new life.

"54 percent of men in one study who'd been adulterous said they were perfectly happy with their wife," Fisher says, compared to only 36 percent of women. "Women are more likely to go out when they're looking for something else -- they're looking for a new partner."

The Tiger Woods brain
Dr. Fisher says science supports the stereotype that men are "more able to divide sex and love." It might come down to simple biology. "If the brain is being primed by a lot of testosterone [in the womb], it tends to build a brain that is more compartmentalized," she says. "There's fewer connections between the left and right hemisphere in men, on average."

At the risk of gross oversimplification, your actual brain architecture might make it easier for you to stray -- affairs are simply relegated to a different compartment of the brain. This could all just be a scientific excuse to be a pig, of course. But if it's true, that means certain brains -- like those belonging to infamous golf pros -- sure have a hell of a lot of compartments.

The tables are turning
"I think men and women are in collusion on this matter. Both sexes want to believe that men cheat more," says Dr. Fisher, noting that an entire "cottage industry" has sprung up around the supposed prevalence of the adulterous man. "Women want men to think that they are not cheaters. Men want to think that women are not cheaters. The fact is -- every time a man's sleeping around, he's sleeping around with a woman."

Dr. Fisher's many years of research on sexuality help fuel Chemistry.com, where you can find a girlfriend who will probably cheat on you anyway.