If you grew up with siblings, you probably remember the fights. Maybe they were about toys, who got the bigger slice of cake, or whose turn it was to sit in the front seat. You might also remember how shocking it was when a sister — who you’d been told was “meek” — threw the first punch.
Now, science says this is more universal than one might think.
A new study published in PNAS Nexus surveyed more than 4,000 people in 24 countries. The results show that women are at least as aggressive as men when it comes to their brothers and sisters, and often more so.
The Family Battlefield
Research clearly shows that men are more aggressive than women. Men commit more homicides, start more fights in school, and generally escalate conflicts outside the family. Women are also significantly more likely to experience repeated and severe forms of domestic abuse, including sexual violence.
But that pattern flips when the conflict happens at home among siblings.
“We find that across 24 diverse societies, females and males tend to be equally likely to engage in direct (as well as indirect) forms of aggression toward siblings,” the researchers write.
In this context, aggression wasn’t just about shoving, slapping or other forms of physical aggression. The study also looked at yelling, spreading gossip, and reporting bad behavior to authority — classic kid moves like “telling on” your sibling. Across nearly every society studied, girls and women were slightly more likely to hit, slap, or yell at siblings than boys and men, though the overall sex differences were not particularly significant.
That trend showed up in childhood and persisted into adulthood. In Germany, men edged women slightly in sibling yelling matches, but that was an exception.
When the researchers looked at non-sibling relationships — friends, spouses, and acquaintances — the old pattern re-emerged. Men were more physically aggressive than women, confirming decades of earlier studies.
Why Sisters Fight
So, why do women unleash their fury at home, even when society tells us they’re the gentler sex? The study suggests that context matters. Outside the family, aggression is tied to status and competition for mates. Inside the family, siblings are battling over shared resources, such as parental attention, money, food, space.
This makes sibling aggression less risky for women than aggression against strangers. “Inclusive fitness considerations may reduce the relative danger of aggression in conflicts among siblings, making severe retaliation less likely,” the authors explain. In other words, a brother might hold back from hurting his sister because harming her harms the family as a whole.
There’s also a cultural angle. Parents are more likely to punish boys for being aggressive than girls, research shows. That difference in discipline could embolden girls to lash out more often at brothers and sisters.
The study also demolished the idea that sibling aggression is shaped by gender equality in society. Patterns were the same in wealthy countries and poor ones, in the West and in the Global South, in progressive and traditional societies. No clear link appeared between gender equality indexes and sibling aggression.
What It Means
Aggression toward strangers seems to be a male specialty. Aggression toward siblings is an equal-opportunity sport.
That has implications for how we understand family dynamics. Previous research shows that 74 to 90 percent of children engage in some form of sibling violence. These clashes might be irritating for parents, and in some cases even frightening, but they’re part of a universal human pattern of competition within families.
Psychologists call this “sibling rivalry.” And the new study suggests it may be one of the few spaces where girls are given permission — or even evolutionary incentives — to fight back.
The authors leave us with a reminder: “Understanding the role of sex in aggressive tendencies depends on context and target”.
