ASU study identifies sex-based differences in physical aggression

Within siblings, girls found to be just as aggressive as boys

Young girl glaring at a woman next to her

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Males are often more physically aggressive than females, whether it involves a school playground fight or assaulting another adult.

When it comes to sibling dynamics, though, research recently published in the journal Evolution and Human Behavior shows that females are sometimes just as aggressive as males. The study was conducted by researchers in Arizona State University's Department of Psychology.

ASU Professor Douglas Kenrick
Douglas Kenrick 

Douglas Kenrick, President’s Professor of psychology, spoke with ASU News about the study and its implications. Additional contributors to the research include Michael Varnum, associate professor of psychology; Amanda Kirsch, psychology graduate student and first author; and ASU psychology alumnae Ahra Ko and Cari Pick.

Question: Why did you study differences in physical aggression between the sexes?

Answer: Family relationships are understudied by evolutionary and social psychologists, and we suspected that some of the knowledge about aggression that applies to relationships between friends or strangers or romantic partners might not hold within the family.

We know that, in general, men are more aggressive than women. Data on homicides and assaults across societies shows this, and this sex difference is even found on kindergarten playgrounds and in fights after high school. It’s also found in other animal species and is typically explained as due to the fact that females pick males who outcompete the other males. However, close family relatives are pretty much immune from extreme violence. Homicides are high for unrelated individuals living together, including spouses, but not for brothers and sisters or parents and children, who spend much more time with one another and have more opportunities to get on one another’s nerves. Similarly, stepparents are much more likely to hurt or assault stepchildren than their own biological children.

This study was set up to see whether brothers were more aggressive than sisters. Based on evolutionary theory and social role theory, which assumes boys practice their social roles in the family context, and on prior findings, we expected brothers would be more aggressive than sisters. We also compared aggression between brothers and sisters to aggression outside the family.

Q: What did you find?

A: We asked the study participants whether they had ever hit, yelled at or kicked brothers, male friends, male acquaintances, sisters, female friends or female acquaintances, and about aggression they had experienced from others. Participants were also asked about aggression after they reached age 18, when family contacts begin to be outnumbered by contacts with people outside the family, and during childhood. 

Outside the family, there was substantially more aggression by males towards other males, including friends and acquaintances. This was true whether we asked about aggression towards the study participant or that the participant experienced from others.

Inside the family, all the rules were broken: Girls were just as aggressive as boys, and siblings in general were much more aggressive towards one another than towards friends and acquaintances.

This finding was not just due to greater opportunity. Saying bad things about someone behind their back was very low between siblings even though there was as much opportunity to learn harmful things about siblings as about friends or relatives. The findings were the same for adults and children, not just when kids were living at home.

Q: Were any of the findings unexpected?

A: We were surprised by the levels of aggression by sisters because they violate both the social role theory and the theory of sexual selection. Though they do fit with a theory of intra-family conflict advanced by Robert Trivers, who pointed out that siblings are competing for parental attention and resources.

Q: What do the findings suggest about understanding or improving family dynamics?

A: I think the prevalence of sisters showing aggression to siblings in our findings show that even though sibling conflict does not fit other stereotypes, it is quite common.

We are currently analyzing data from over 20 other countries to assess whether this finding is something peculiar about American family life or something more general about human nature. Our initial analyses suggest that all around the world, within the family, girls are as aggressive as boys, but outside the family, it's typically the boys who are more aggressive.

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