While scientists have not discovered an excuse for infidelity, they can at least offer an explanation for why some women are inclined to cheat: their fertility cycle. In a small study, researchers found that women were more likely to flirt with men when their chance for getting pregnant was at its highest.
The handsomest of the bunch, as always, have little to worry about. The urge to be unfaithful was mostly felt by women who rated their partners low in "sexual attractiveness relative to investment attractiveness." In other words, the average boyfriend or husband is nice to have at home, but the guy across the bar looks better.
Dr. Martie Haselton, the lead author of the study, says that such shallow desires act as a stand-in for a girl's reproductive best interest.
"Women can't see a man's genetic make up," says Haselton. "So, they come to prefer good genes on the outside."
You may fall for the nerdy type, but your reproductive urges favor the masculine and attractive, regardless of whom you happen to be dating at the time.
"Women have a biology that can lead them to stray," says Haselton.
These urges served a purpose in ancient times, when a man's scruffy good looks conferred an evolutionary advantage to a woman who wanted the strongest offspring possible. But in the age of private eyes and divorce lawyers, those who sneak around with the alluring hunter-gatherer type are now likely to find trouble.
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"It can be costly to engage in that behavior," says Haselton.
For the study, which was published in the journal Hormones and Behavior, Haselton had 38 college women fill out a daily diary of their sexual interests and feelings. The majority of them had a boyfriend or husband. When the women neared their time of ovulation, they reported feeling more attractive and more interested in going out to meet men, even if they were already in a relationship.
Flirting also increased, much to the dismay of their partners. As a result, men were more likely to become jealous and possessive during their girlfriend's peak reproductive days. The unattractive guys were on guard all the time, especially if the women they were dating were seen as better looking than them.
Haselton says that some women are clearly slipping past their protective mates. About two to four percent of all pregnancies, according to some estimates, are the result of a one-time fling or affair.
"Men are not the only ones who are cheating," she says.
The sanctity of marriage is not hopeless, however. Indeed, any biological urge to cheat must compete with a natural impulse for monogamy. With the extended time it takes to care for a child, Haselton says that women must balance their basic reproductive desires against the need to find a responsible partner.
"The push for monogamy is also strong," she says.
In the end, it may be natural to find others attractive, but that is no reason to wreck a good relationship.
"It's their choice," Haselton says.
