Sibling rivalry is quite often dismissed as harmless bickering or just part of growing up. However, recent research published in PNAS Nexus paints a far more complex picture. 

Contrary to long-held assumptions that males are universally more aggressive, the results from the study highlight that females are just as likely, if not more so in some contexts, to display both direct and indirect aggression toward siblings. These patterns persist from childhood into adulthood and appear across cultures. This challenges traditional gender norms and social role theories.

I recently spoke with the lead author of the study, Michael E. W. Varnum — Professor at the Department of Psychology at Arizona State University — about why sibling aggression has remained largely underexplored in psychological research and what these findings reveal about the hidden dynamics of family life. Here’s a summary of our conversation.

What inspired you to focus specifically on sibling aggression, given that much of the literature on aggression looks at peers or romantic partners?

We had a hunch that this might be an interesting space to explore precisely because so little research in evolutionary or social psychology has looked at this question. We also had some suspicions that the dynamics of aggression in sibling contexts might be quite different from how aggressive behavior looks with non-kin.

Did you expect to find that females engage in as much sibling aggression as males, or was that a surprising result?

Well, we’d found this in a set of prior studies with American samples, which surprised us initially. But for this paper, we suspected we would likely do the same thing. Although it’s always an open question whether when you observe something in one culture, you also see the same thing in others. There were sensible reasons to expect that perhaps there would be substantial variability across cultures, although from an evolutionary lens we might expect this particular pattern to be fairly universal. And that’s, in fact, what we found.

Your paper describes sibling aggression as a “conceptual hole” in relationship research. Why do you think sibling relationships have been overlooked compared to friendships and romantic partnerships, and what makes them so important to study?

I think the study of psychology and behavior within families has largely been the province of developmental psychology or family dynamics. Social and evolutionary psychologists have traditionally focused on other types of social interaction. Maybe, in part, because it’s hard to set up kin interactions in experimental settings with adults, and maybe because of a tacit set of theoretical assumptions that how we think about and behave with friends, co-workers, strangers or even romantic partners might generalize.

Could sibling aggression be considered a “functional universal,” and what does that mean in practical terms?

I think the answer is yes, although it can sometimes be a little tricky to decide what “level” of universality a psychological or behavioral tendency is. In practical terms, our findings — from countries rich and poor, Western and Non-Western — suggest that we might expect to see the same kind of thing across space and historical time. If the reasons for this universality have to do with our evolved psychology, then this is likely a deep part of human nature.

One of the puzzling findings is that females sometimes report more direct aggression toward siblings than males. What do you think might explain this reversal?

We think there are at least a couple non-mutually exclusive reasons why we observed these findings, which are indeed somewhat puzzling if one uses a superficial view of kin selection or adopts a more social constructivist framework. 

First, because of shared genes, there may be “bumpers” or limits on how much a conflict will escalate among siblings. We may engage in small or moderate acts of hostility toward them more frequently, but statistics show we rarely maim or kill them. So, in that case, it may be “safer,” so to speak, to try to gain advantages by being a bit mean to your brother or sister. 

This may help us to understand why the typical restraint we see from women and girls in terms of direct aggression toward non-kin is absent in sibling relationships. This likely reflects things like the fact that females tend to be smaller and less formidable than males on average, and that women tend to have higher obligate parental investment — factors which should make engaging in risky behavior, like punching people, less likely. The risks of it getting out of hand are much smaller. 

Further, if we yell at or hit a stranger, a friend or an acquaintance, it’s much more likely that that cooperative relationship will dissolve. But even if they’ve been mean to us, we’re likely to be there for our siblings when they really need us. And we are far less likely to cut them off or stop interacting with them.

Do you see sibling aggression as a potential developmental training ground for conflict management, or more as a risk factor for later relational difficulties?

That’s an interesting idea! And it may well have some merit. But it may also be the case that the calculus for aggression is different in relationships with non-kin. So, we wouldn’t necessarily expect that how people deal with conflict with brothers or sisters necessarily tells us much about how aggressive they are with other kinds of people. It’s an empirical question, though, one that would be interesting to dive deeper into in the future.

Do you react aggressively toward family more than you realize? Take this research-backed test to find out: Anger Management Scale