When leaving your wealth to your sister's children makes sense

In most human societies, men pass on their worldly goods to their wife's children. But in about ten percent of societies, men transfer their wealth to their sister's sons, a process called matrilineal inheritance. A new study published in Proceedings of the Royal Society B this week has produced insights into this practice.

"Matrilineal inheritance is puzzling for anthropologists because it causes tension for a man caught between his sisters and wife," explains Santa Fe Institute researcher, Laura Fortunato, who has used game theory to study matrilineal inheritance. "From an evolutionary perspective it's also puzzling because you expect an individual to invest in his closest relatives – usually the individual's own children."

For decades, research on the practice of matrilineal inheritance focused on the probabilities of a man being the biological father of his wife's children – probabilities that lie on a sliding scale depending on the rate of promiscuity or whether polyandrous marriage (when a woman takes two or more husbands) is practiced.

Previous studies have thus looked at the probability that a man is more closely related to his sister's children than to his wife's children. Below this "paternity threshold" a man is (genetically) better off investing in his sister's offspring, who are sure to be blood relatives, rather than his own wife's children.

In her work modelling the evolutionary payoffs of marriage and inheritance strategies, Fortunato looked at what payoffs there were for men and women in different marital situations – including polygamy.

"What emerges is quite interesting," says Fortunato. "Where inheritance is matrilineal, a man with multiple wives 'wins' over a man with a single wife." That's because wives have brothers, and those brothers will pass on their wealth to the husband's offspring; so more wives means more brothers-in-laws to invest in your children.

The model also shows an effect for women with multiple husbands. The husband of a woman with multiple husbands is unsure of his paternity, so he may be better off investing in his sister's offspring. "A woman does not benefit from multiple husbands where inheritance is matrilineal, however," Fortunato explains, "because her husbands will invest in their sisters' kids."

Family structure determines how societies handle relatedness and reproduction issues, Fortunato says. Understanding these practices and their evolutionary implications is a prerequisite for a theory of human behaviour.