Usually, when a woman cheats on her husband, the whole family suffers. But in societies where extramarital relations are common, it turns out that female infidelity often translates into benefits for a man's nieces and nephews, one researcher claims.


In a paper forthcoming in the journal Proceedings of the Royal Society B, University of Utah anthropologist Alan Rogers lays out a mathematical proof for just how much infidelity equals a doting uncle.


Caring for a sister's children is like hedging your genetic bets: you definitely know that there's a relationship there, so you are helping to ensure that a piece of your genes survives.


"In many societies where extramarital mating is the norm, men may not share genes with the children of wives. There is less doubt about relatedness to sisters' children,” Rogers explained in a statement on Monday. “This suggests an interesting hypothesis: perhaps natural selection has shaped this practice, by encouraging males to direct investment toward genetic relatives.”


Previously, scientists had thought a man would show extra care for a sister's children if he was likely to have fathered less than a quarter of his wife's children. Now, with some new fancy mathematical footwork, Rogers says that a man will tend to care for nieces and nephews if he's unsure about the paternity at least half of his wife's children.


Rogers arrived at this new result by relaxing some of the assumptions made in previous studies.


Other scientists had assumed both that “women are equally receptive to extramarital affairs and that each has an infinite number of paramours," Rogers says. "These assumptions both lower estimates of relatedness between men and the children of their sisters. Relaxing either assumption increases our estimate of the fitness payoff to men who invest in children of sisters."


Rogers' piece is the latest twist on a relatively old evolutionary concept called kin selection, which is an evolutionary explanation for why people are willing to help relatives despite the risk of harm or even death. Generally, we would expect people to perform selfless and very dangerous acts only for very close relatives. Or, as the British evolutionary biologist J.B.S. Haldane once quipped, he "would lay down my life for two brothers or eight cousins."


While we tend to think of female infidelity as a somewhat rare occurrence in present-day society, there's evidence that it is not all that uncommon in isolated hunter-gatherer communities in South America and Africa today. 


It's hard for researchers to accurately gauge the percentage of cheating wives in the industrialized world. Randomized surveys might underestimate the rate, since people might be reluctant to even anonymously admit they've cheated on their spouse; the kinds of surveys in women's magazines where participants know the questions are about adultery may overestimate the rate, since participants are self-selecting.


Even if adultery isn't the norm in modern America, it's far from uncommon: according to the University of Chicago's General Social Survey, in any given year about 12 percent of married men and 7 percent of married women admit to having sex outside of marriage.


We may assume to know that the consequences of these affairs, but in some cases, the pull of genetics may mean that some family members may actually be helped by infidelity.


"People are nice to relatives all over the world, and I think selection has something to do with that," Rogers said.