Are Aggressive Sisters an American Cultural Phenomenon?

I recently wrote about a surprising finding among American participants: Contrary to all the data on violent crime, playground aggression, and other measures of aggressive behavior, sisters are every bit as likely to hit, kick, or yell at their siblings as are brothers ("Are sisters as aggressive as brothers"). The general pattern, and the exceptions, were presumed to have something to do with evolved influences on human behavior. 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). Aggression inside the family is, according to evolutionary biologists, a different game, in which both sexes are competing for the same resources.

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, and perhaps the gender reversal inside the family would be more pronounced in societies with greater gender equality.

To address this question, a team of researchers headed by Michael Varnum and myself (and including 49 other researchers from around the world) compared sibling aggression to aggression between non-siblings in 24 different societies (ranging from Bolivia and Columbia in South America to Thailand and South Korea in Asia, and Senegal in Africa, as well as several European and North American countries).

The questions (answered by 4,013 participants across those different countries) included things like “Have you ever hit or slapped a brother?” “Have you ever hit or slapped a male friend?” “Have you ever hit or slapped a female acquaintance?” (including all the combinations of sex with sibling, friend, acquaintance).

Aggression towards non-siblings as children and adults

Source: This is a public domain figure from the article being reviewed here (which is published in an open access journal)

Figure 1 shows the results for four categories of aggression directed toward people outside the family (hitting/slapping and yelling, as a child, and as an adult). Blue dots indicate that, for a given country and a given type of aggression, males are significantly more aggressive. Gray dots indicate no significant difference, and red dots indicate that females are significantly more aggressive. In 48 instances out of 96, males are significantly more aggressive than females. In three instances, females are significantly more aggressive. In the other cases, the differences are not significant. The bottom line (labelled “overall”) indicates that, averaged across all the countries, males are more aggressive in all four categories.

But now look at Figure 2, which depicts results for the same four categories of aggression directed against siblings. Here we see a different pattern.

Aggression towards siblings.

Source: From open access journal, figure is public domain

In only two of 96 cases are brothers more aggressive toward siblings than are sisters, whereas in 22 cases, sisters are more aggressive toward siblings. In roughly 75% of the instances, the sex differences are not significant. Looking again at the bottom line, the average difference across countries for hitting and slapping indicates a statistically significant, but not large, tendency for sisters to hit more as children, and no average difference as adults. For yelling, though, there are larger average sex differences, with females yelling at siblings more as children and as adults.

These findings suggest that sex differences found outside the family are either absent or reversed for sibling aggression. They do not support the idea that the pattern has anything to do with American culture. From one perspective, it has been suggested that sex differences will disappear as females gain more social power. But the pattern was independent of a country’s economic development of gender equality.

Why the difference between aggression inside and outside the family? Aggression outside the family, as I noted earlier, has been found across animal species, and has been linked to males competing for status as a means to appeal to mates. Inside the family, this is not what is going on, instead siblings are competing over shared familial resources.