Brothers vs. Sisters: Who is actually kinder to their siblings? Science finally has an answer.

I once threw a butter knife at my brother. In my defense, I missed.

Science would describe this as direct aggression. History suggests that males are generally more predisposed to direct, physical aggression than females. We have centuries of documented stories and research to confirm that men are usually the ones starting the fights.

But is there a difference in the way we act inside the house versus how we act outside of it? My personal story notwithstanding, recent research suggests that while men might be aggressive to the world, they are often softer with their siblings. Sisters? It turns out the dynamic is the exact opposite.

Siblings engaging in a pillow fightCanva

The Findings

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

"The results suggest that the dynamics of aggression within the family are different from those outside of it," the authors of the study wrote.

Essentially, context is everything. Much of the previous research on this topic focused on how men and women treat strangers or peers. When you look at how they treat their own blood relatives, the script flips.

The "Safe Zone" for Fighting

Douglas Kenrick, a psychology professor at ASU and co-author of the study, noted that these findings challenge two major assumptions we have held for years.

"All the earlier research suggested two truisms about aggression: 1. Males are more aggressive, and 2. People are kinder to their blood relatives," Kenrick told PsyPost.

However, their data showed that people are actually more likely to yell at or hit a brother or sister than a friend. Furthermore, they found that "sisters are at least as aggressive as brothers."

This backs up a 2024 study in Science Direct which observed that while male friends fought more than female friends, those differences vanished completely when looking at siblings.

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It Is Not Just Cultural

One might assume this is a result of Western culture or specific societal norms. Social role theory usually implies that gender behavior is governed by how we are raised.

But the researchers found this pattern everywhere.

"We found the same pattern everywhere we looked and there were no systematic relationships with how wealthy a society was, or how broadly egalitarian," said Michael E. W. Varnum, a study co-author.

Whether the society was rich or poor, strict or loose, sisters were consistently just as aggressive as brothers.

"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."

The Takeaway

My brother and I are very close today. And for the record, I actually meant to miss with that butter knife, which doesn't make the incident much better, but it gives us a good laugh now that we are older.

It is interesting to note that the aggression dynamics shift so heavily between the community and the living room. It seems that for sisters especially, the family unit is a space where the gloves can truly come off.

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This article originally appeared last year.