Infidelity helps kids get closer to uncles

Men in some cultures often feed and care for their sisters' children, where extramarital sex is common and accepted, as a man's genes are more likely to be passed on by their sister's kids than by their wife's kids, researchers have claimed.

The theory previously was believed valid only if a man was likely to be the biological father of less than one in four of his wife's children - a number that anthropologists found improbably low.

But in the new study, University of Utah anthropology Professor Alan Rogers shows mathematically that if certain assumptions in the theory are made less stringent and more realistic, that ratio changes from one in four to one in two, so the theory works more easily.

In other words, a man's genes are more likely to be passed by his sisters' children if fewer than half of his wife's kids are biologically his - rather than the old requirement that he had to sire fewer than a quarter of his wife's kids, according to the study.

"Imagine a mutation that encourages its bearers, if they are men, to be helpful and invest resources in the children of their sisters," Rogers said.

"If that man lives in a society where most of his wife's children were fathered by other men, then this gene may not be in many of his wife's children. A man really doesn't know if any of his wife's kids were fathered by him, but he knows he and his sister have the same mom. So this gene may, in fact, be in more of his sisters' children."

"Thus, over time, the frequency of this gene increases because men are increasing the survival and fitness of their sisters' children - the ones more likely to carry the gene," he added.

The new study "shows that it is much easier than we thought for your niece to be a closer relative than your wife's daughter," Rogers said.

The study is published online in the journal Proceedings of the Royal Society B.