Two new studies find that women may be genetically predisposed to cheating on their partners.
One study published today by the University of California, Los Angeles Center on Behavior, Culture, and Evolution and the University of New Mexico says women have evolved to cheat on their mates during the most fertile part of their cycle, but only when those mates are less sexually attractive than other men.
The study in the Journal of Hormones and Behavior examined 38 coeds from one large, unidentified U.S. university.
"We found that women were most attracted to men other than their primary partner when they were in the high fertility phase of the menstrual cycle," said Dr. Martie Haselton, a UCLA researcher. "That's the day of ovulation and several days beforehand."
A related study, which will be published in Evolution and Human Behavior, finds that women are more likely to fantasize about men other than their mates, but only when they don't consider their mates to be particularly sexy. That UCLA study examined 43 normally ovulating women.
"We're claiming the desire to cheat is what evolved in women, that they may notice they have these desires at a certain point in their cycle," said Elizabeth Pillsworth, co-author of the study and an assistant professor of communication and psychology at UCLA. "Whether they translate into unfaithful behaviors is a matter of their own choosing. Cheating is a choice."
A 2004 ABC News poll found that men were almost twice as likely to cheat as women, with 21 percent of men admitting to cheating compared to 11 percent of women.
"The exception was women who have very sexually attractive partners," Pillsworth said. "These women did not flirt with other men when they were at high fertility."
Pillsworth said that the cheating was linked to humans' ancient past when women looked for men with strong characteristics, and strong genes, to carry on the human race.
The studies also suggest that males are able to sense, on some level, when women are more likely to cheat and that they become more jealous. If a man's partner is physically attractive, however, he is in a jealous and "mate-guarding" mode all the time, regardless of her cycle.
"Women who are most attractive are most fertile, and they also tend to be the targets of other men to steal them away," Pillsworth said.
Pillsworth said she hoped the studies helped women to understand their feelings.
"I hope the message women get is that they can use this information to realize their biology is toying with their desires and to ask themselves, 'Am I going to let that run my life, my sexual decision-making?' " Pillsworth said. "For the men I would say not to be too fearful of these findings. While women may notice other men during this part of their cycle, unfaithful behavior is relatively rare."