Most surveys about sex find impossibly that men have had far more partners than women, typically two to four times as many.

Either there are a bunch of phantom females out there, or somebody is lying.

Or perhaps men just have lousy memories.

Psychologist Norman R. Brown at the University of Michigan has done several studies on the apparent flaw in these surveys. The latest was a web-based survey of 2,065 heterosexual non-virgins with a median age in their late 40s.

The women reported on average 8.6 lifetime sexual partners. The men claimed 31.9.

Rather than let it go at that, Brown and his colleagues later in the survey asked the participants to rate the truthfulness of their response. About 5 percent said they lied. In addition, more than 10 percent said they knew their answer wasn't accurate.

"They gave an answer and then two minutes later admitted they had lied about the answer," Brown said.

But there's more to the discrepancy. Men and women use different methods to calculate their past dalliances.

Women rely on a raw count, a method Brown says is known to result in underestimation.

"They tend to say, 'I just know,' and if you ask them to explain how they know, they say, 'Well, there was John, Tom, etc.'"

Men also rely on a flawed strategy.

"Men are twice as likely to use rough approximation to answer the question," Brown said. "And rough approximation is a strategy known to produce over-estimation."

Then again, maybe Brown's study is flawed, too.

His next survey will be done by telephone, to find out if people lie and fudge as much in that medium, or if the Web-based surveys invite such behavior. The self-proclaimed liars "could be liars who lie about lying," he said.

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