A University of Utah study produced new mathematical support for a theory that explains why men in some cultures often feed and care for their sisters’ children: where extramarital sex is common and accepted, a man’s genes are more likely to be passed on by their sister’s kids than by their wife’s kids. 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.
