Some people might conclude that men overestimate the number of sexual partners they've had because they're pigs.

But Norman Brown, a University of Alberta professor of psychology, has other theories.

He says there is important information to be gathered from his survey of approximately 1,100 heterosexual Canadians, who were asked to recall how many sexual partners they had had during their lives.

Two-thirds of women said they knew precisely how many partners they had and reported an average of 6.2. Men on the other hand were twice as likely to say they had no idea and on average guessed 11.9.

"Every time a man has sex with a woman, a woman has to have sex with a man. So either there are some very lucky joes out there or someone is not getting their numbers right," Brown said.

Brown is a cognitive psychologist, which means he studies how people acquire, organize, and utilize real-world knowledge. So after they answered the question, they were asked how they came up the number they gave.

By asking respondents how they came up with their number, Brown was able to work out what methods men and women were more likely to use when calculating the number of sexual partners.

He found that men were twice as likely as women to use a rough approximation method, while women were more likely to think about individual incidents and calculate the number this way. People who had had few sexual partners were the most likely to say they just knew.

It remains unclear how male overestimation could result in such a wide disparity in the reported number of partners.

Brown has three theories, none of which are very flattering to men. One is that men cannot be bothered to remember individual lovers. The next is that men are lazy or don't have the patience to think seriously when answering a telephone poll. The third is his 'Don Juan' theory that there might in fact be a number of men in Alberta who have been, in Brown's words, "lucky joes."

Brown said his findings have applicability in a number of areas, including strategies for dealing with people who engage in high-risk sexual behavior.