Who's kinder to their siblings, brothers or sisters? Research answers an age-old question.

I once threw a butter knife at my brother. In my defense, I missed. Science would describe this as direct aggression. History suggests males are more predisposed to direct, physical aggression than females. We have centuries of documented stories and research to confirm that.

But is there a difference in the ways we act within the family as opposed to outside of it? My personal story notwithstanding, recent research suggests men might be less aggressive to siblings than they are to the rest of the world. Sisters? It's the exact opposite.

Kids play fighting.Image via Canva - Photo by Fly_Fast

Commonly observed sex differences in direct aggression are absent or reversed

A 2025 study published in Oxford Academic analyzed data taken from 24 societies around the globe. Researchers from Arizona State University (ASU) found women to be just as aggressive and, in many instances, more than their male siblings.

"The results suggest that the dynamics of aggression within the family are different from those outside of it, and ultimately that understanding the role of sex in aggressive tendencies depends on context and target," authors of the study write.

Much research has been conducted and supports that men, on average, exhibit greater aggression than women. Yet, most studies investigated the aggression between people who are not related.

Douglas Kenrick, co-author of the study and psychology professor at ASU, was quoted in PsyPost saying, "All the earlier research suggested two truisms about aggression: 1. Males are more aggressive, and 2. People are kinder to their blood relatives. Yet, we found that when it comes to hitting or yelling at another person, a) people are more likely to aggress towards brothers and sisters than toward friends or acquaintances, and b) sisters are at least as aggressive as brothers."

A 2024 study in Science Direct observed that everyday acts of direct aggression were more common amongst male friends than female, but those differences were absent between siblings.

Kids showing peace signs.Image via Canva - Photo by Ibad Badrul

Is there a difference between cultures?

The concepts behind social role theory imply that the differences between men's and women's behavior are governed by broader societal norms. It's the idea that social policies and practices dictate the behavior displayed by both men and women. Cultures that show more equality between genders might then expect similar differences. Those communities that dictate stronger, gender specific roles would expect larger differences.

Michael E. W. Varnum, study co-author and professor of psychology, was quoted in PsyPost, saying, "... we found the same pattern everywhere we looked and there were no systematic relationships with how wealthy a society was, or how broadly egalitarian, or really with any other cultural factor we looked at."

The study's findings call into question both evolutionary theories rooted in sexual selection and social role theories that focus on cultural and gender norms.

“I think the big story here is that when it comes to sibling interactions, women are at least as aggressive as men," Varnum said. "This is true not only in childhood, but adulthood. And we see the same thing all over the world."

Sibling rivalry doesn't only exist in psychology-based research. You can watch this funny YouTube video of siblings battling it out with one another below or by clicking this link:

- YouTube www.youtube.com

My brother and I are very close. I actually meant to miss, which doesn't make the butter knife incident any better in the moment, but gives a good laugh now that we are older. Sibling rivalry is something heavily studied. It's interesting to note that the aggression dynamics shift between the community and family. More research is needed, perhaps widening the scope of data acquisition and subject matter, but it's a fascinating start.