Men are more honest, accurately reporting their true number of partners despite social pressure to look like a Don Juan.

The findings offer the best explanation yet for a paradox that has baffled psychologists since sex surveys began in the 1960s. Heterosexual men routinely report they have slept with more partners than do women: British men, for example, claim an average of thirteen partners over their lifetimes, compared with nine for women.

These figures, however, are statistically impossible. Unless hundreds of thousands of British men are having sex with foreign women — which is most unlikely given similar findings elsewhere — the average for both genders should be about the same.

Scientists have resolved this by assuming that both sexes must be lying, with men exaggerating and women underreporting their experience. The new study, however, shows only the latter to be true.

Women, it found, are serial liars when it comes to sex. Not only do they “forget” about certain partners: they also add years to the age at which they lost their virginity, and refuse to tell the truth about masturbating or using pornography.

The results, which are published in the Journal of Sex Research, suggest that the sexual proclivities of men and women are in fact much closer than traditional research has indicated.

“Women are so sensitive about being labelled ‘sluts’ or ‘whores’ that they are very reluctant to be honest about their sexual behaviour, even in supposedly anonymous surveys,” said Terri Fisher, who led the study at Ohio State University.

“You would assume that when a heterosexual man has sex, a woman is having sex at the same time, but the statistics always suggest otherwise. That can’t be true. We thought males would be overinflating their experience while females underreport it, but that’s not what we found.”

In the study, Dr Fisher and Michele Alexander, of the University of Maine, asked 96 men and 105 women, all of whom were heterosexual students between the ages of 18 and 25, to answer questions about their sex lives under three different conditions.

One group was left alone in a room and told their answers would be completely anonymous. A second group filled in the same questionnaire, but were told that a researcher was watching and could match their face to their form.

With the final group, the scientists attached electrodes to the volunteers’ necks, forearms and hands, and told them they were being wired to a lie detector test.

The women in the three groups responded very differently according to the testing conditions.

Those who thought they were being watched gave an average of 2.6 partners, compared with 3.4 in the anonymous group.

Those who thought they were attached to a lie detector were still more forthcoming, giving an average of 4.4 partners.

There was no such spread among the men. Those on the dummy polygraph claimed 4.0 partners, compared with 3.7 for the other groups — a difference that is not statistically significant.

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