A study by Jan Havlíček in the newest issue of academic journal Biology Letters lends credence to the theory of mixed-mating strategies — the idea that single women search for a nurturing man to nest with and, once that need is met, will want to go to bed with the man with the best genes.

The author is quick to point out his study merely finds an attraction to dominant males during women's most fertile time and not necessarily a predisposition to infidelity.

"It doesn't mean women are programmed to cheat," he said. "They may simply fantasize more this time of month.

Besides, points out Havlíček, "Documenting the tendency doesn't say whether it's right. Xenophobia is deeply rooted in humans. That doesn't mean we should be xenophobes."

Havlíček's research breaks new ground: It is the first to link dominant traits in men to their body odor, and it's also the first study to show a change in female sexual preference based on both a woman's relationship status and her state of fertility.

In the study, 48 male students filled out surveys with questions such as "Do you try to outdo others?" Havlíček then gave male subjects cotton pads to wear under their arms for 24 hours. Female students who had given information on their menstrual cycles and partnership status smelled the pads and rated them for sex appeal.

Single women and women with partners but not ovulating showed no significant preference for the scent of men who scored high on the survey. Ovulating women with partners, however, showed a strong preference for dominant men.

Havlíček's study adds to a large body of research in recent years on changes in human attraction during the course of a woman's menstrual cycle. Havlíček co-authored a study last year that found men rated pictures of fertile women as more attractive than pictures of the same women at different times of the month. Other studies have shown ovulating women prefer men with symmetrical faces. Scientists believe symmetry, like dominant social behavior, is a proxy for good genes.

"[Havlíček's study] is new because it finds different responses for single women and women in relationships," said Nick Neave, a doctor of Evolutionary Biology at Northumbria University in the United Kingdom.

Conventional wisdom says men are inclined to spread their genes far and wide — or bed as many women as possible — while women search for one good man to help them build a home and raise children.

But surveys on infidelity, though they vary widely, consistently show that women do cheat, if more rarely than men. And surely, evolutionary scientists reason, women should have an evolutionary incentive to secure the best genes for their children.

Hence the theory on mixed-mating strategy. It predicts a drive by women to land the best possible man as a caregiver, and once that has been accomplished, have the urge to sneak around with the most powerful, or dominant, man they can find.

In the late 1980s and early '90s, studies of birds thought to be monogamous found up to 30 percent of offspring in some species were sired by males not paired off with the mothers. Those observations fit the mixed-mating theory. Evolutionary biologists began to look more closely at whether the theory applied to humans.

"We've seen, for example, that men have different preferences for women they want as wives, and women they want to have occasional sex with," said Daniel Frynta, an evolutionary biologist at Charles University.

Humans are caught in a mating no-man's-land, he argues: inclined toward monogamy, but with the biological drive to cheat.

"Humans live in families, but are also highly social. We have a lot of opportunities for extra-monogamous mating, but also face risks. We're left with this tension," he said. "Some species, like monkeys, which live in small, closed families, are almost exclusively monogamous. They don't have many opportunities not to be. The male keeps a close eye on the female."

The mixed-mating theory is widely accepted among evolutionary scientists, but far from proven. Frynta said the results of Havlíček's study will have to be duplicated and other studies in the same vein conducted.

Meanwhile, Havlíček is working on a survey of jealousy and infidelity. He says he has found that 60 percent of men and 57 percent of women have cheated on a partner.

"That seems high," he said. "Then again, it could just be the culture in Prague."