Are Aggressive Sisters an American Cultural Phenomenon?

"Aggression outside the family has to do with what evolutionary biologists call differential parental investment (females across a wide range of species generally contribute more of their physiological resources to the fetus and infant), which leads to relatively higher female selectivity in choosing mates, and thus contributes to sexual selection (males competing among themselves to be chosen, often by intrasexual aggression)."

"But the finding of high aggression among sisters might instead have been explained by something about American culture, such as changing social expectations about female competitiveness in recent years. If the phenomenon has something to do with human evolution, we would expect it to be universal. If, on the other hand, the phenomenon has something to do with American culture, we might expect it to look different in other cultures,"

Males are generally more aggressive across many measures, but sibling dynamics show a different pattern in which sisters in some contexts are as likely as brothers to be aggressive. Evolutionary theory links outside-family aggression to differential parental investment and sexual selection, while within-family aggression reflects competition for shared resources. The high aggression observed among sisters in some samples could stem from cultural changes, such as rising female competitiveness. To distinguish cultural from evolutionary explanations, researchers gathered comparable data from 24 societies worldwide to test whether the pattern is universal or varies with cultural factors like gender equality.

Read at Psychology Today