Men, it seems, are bad at keeping track of their bedtime activities. When asked how many partners they had slept with, men overestimated their sexual history. Norman Brown, of the University of Alberta, discovered a reason for this exaggeration: men couldn't remember and would instead roughly estimate their number of lovers. Two out of three women remembered each partner they had or they kept a running tally. They reported an average of 6.2 people. Men, however, were twice as likely not to remember; leaving their calculations to guesswork. They reported an average of 11.9 women.

Do men really sleep around more than women? "For a long time, this has been a bit of a puzzle; kind of an Achilles' heel," says Brown. "Every time a man sleeps with a woman, a woman sleeps with a man." Brown believes the disparity is really a result of how men and women make mental calculations.

To prove his point, he found that both genders responded similarly on other questions in his study. When asked about their sexual activity in the past year, men and women had the same number of partners. They also had sex the same number of times in the previous month. In addition, both sexes engaged in equal levels of high-risk behavior.

Brown concedes that his study does not fully explain the disparity between sexual partners. It's possible that a few very active men, the "Don Juans" according to Brown, could have skewed the scale for men. His research is similar to another study from the State University of New York, at Buffalo, that found both men and women underestimate their partner's sexual past.

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