Infidelity is easy to explain in males. By sleeping around, a guy can potentially impregnate more females and sire more offspring than if he just had one mate. But females cheat, too, even though a woman will only be able to have roughly one baby per year no matter how many male sex partners she has had.

One leading evolutionary hypothesis suggests that a female who mates with multiple males ensures the genetic diversity and quality of her offspring; having higher-quality offspring could theoretically give her more grandchildren later. A 17-year study, published in the June issue of The American Naturalist, challenges this hypothesis.

"This is one of the most careful and most robust studies to explore whether polyandry is adaptive in females," says Tommaso Pizzari, a University of Oxford biologist who was not involved in the research. "The answer is: not really."

Previous studies tested the "quality" hypothesis indirectly. In socially monogamous species, researchers would compare the legitimate and illegitimate offspring of cheating females by asking: Which offspring were larger? Which lived longer? But a better way to understand why female promiscuity evolved, says Jane Reid, a biologist at the University of Aberdeen in Scotland and author on the new study, is to determine whether those illegitimate offspring actually have more babies.

Reid and her team studied the isolated population of wild song sparrows on Mandarte Island in Canada. Like their mainland counterparts, these birds are socially monogamous. Males and females pair up for an entire breeding season or for several breeding seasons, and work together to feed the hatchlings and defend the nest. But they're not always faithful; blood tests showed that in this particular population, 28 percent of hatchlings were fathered by other males.

The researchers studied three generations of song sparrows (including more than 2,300 hatchlings) to see whether the cheating females' illegitimate sons and daughters would have higher reproductive success. It turned out that the bastard hatchlings actually fared worse than their legitimate siblings, producing about half as many offspring, on average. "It's not what people expected," Reid says.

David Westneat, a behavioral ecologist at the University of Kentucky, agrees. "It's essentially saying that there's no evidence for a hypothesis that's been very popular." But the theory is not dead yet, because the results need to be confirmed in other populations and other species. Westneat and Pizzari also expressed some concern over the fact that the study took place on an island—these small and isolated populations have a tendency to evolve in weird ways.

Nevertheless, the scientists agree that female promiscuity does not appear to have reproductive advantages for most of the female song sparrows on Mandarte Island. So what's the point of cheating, if it doesn’t help them to have greater evolutionary success?

Some theories blame it on the boys: Female promiscuity may be an ecological by-product of male promiscuity. If males are under strong selection to mate with multiple females, then in a closed environment females might be forced to mate with multiple males.

Reid is also investigating whether female promiscuity is a genetic by-product of the male version. If there are genes that encourage a male to be promiscuous, he might pass those genes on to his daughters, even though they may not benefit from the behavior. A recent study in zebra finches found support for this hypothesis. If it turns out that genetics have a heavy influence on feminine infidelity, Reid's team would like to try to pin down which genes are contributing to promiscuous behaviors.

"This study throws open the door to understanding why this phenomenon is so widespread," Reid says. "It's still an enigma."