A sweeping international study led by researchers from the Evolutionary Social Psychology Co-Laboratory at Arizona State University has found that women tend to be at least as aggressive as men when it comes to their siblings—and in many cases, more so. While previous research has long indicated that men are more physically aggressive than women, especially outside the family, this new work suggests that the dynamics of sibling relationships present a different picture. The findings were published in PNAS Nexus.
It is widely accepted that human males tend to engage in more physical and direct aggression than females. This generalization is supported by global data showing that men are far more likely to commit acts of violence, from playground fights to homicide. These patterns have been attributed to a range of factors, including evolutionary pressures related to mating and status, higher testosterone levels in males, and cultural norms that encourage aggressive behavior in boys more than in girls.
But almost all of that research has focused on interactions between unrelated individuals. Far less is known about how aggression plays out within the family, particularly among siblings. Some theories would suggest that the same sex differences should apply to sibling relationships. Others propose that the family environment may dampen or reshape aggressive tendencies, especially given the shared genetic interests and long-term interdependence between siblings.
“Family is really important, yet often overlooked in social psychology. A few years ago we collected data from dozens of countries about how important various motives were to people, and we found to our surprise that on average caring for kin was the strongest motivation people reported. This was true in every society we’ve studied so far,” said co-author Michael E. W. Varnum, a professor of psychology.
“And this got us interested in a broader question. Much of what we know about social psychology has to do with how we interact or think about non-kin, but might things look very different in the context of kin relations? In other words much of what we think we know about human social psychology, might really be a psychology of stranger or non-kin interactions. And in some cases, our psychology may look very different in the context of kin interactions.”
Previous findings from the United States hinted that women may be just as likely—or even more likely—to engage in aggression toward their siblings. But the extent to which this pattern held across different cultural and economic settings had not been tested.
To investigate this question, researchers gathered data from 4,136 participants across 24 countries, including both Western and non-Western nations, wealthy and less wealthy societies, and regions from every inhabited continent.
“The reason we collected data from around the world was because we found two surprising things in a recent study of Americans’ aggression toward brothers and sisters,” explained co-author Douglas Kenrick, president’s professor in the Department of Psychology at Arizona State University.
“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. We wanted to know if those unexpected findings would be found in other countries, which would suggest something about human nature, or just found in the United States, which would suggest something unique about American culture.”
Participants ranged in age and were asked to report how often they had engaged in various types of aggression—such as hitting, yelling, gossiping, or reporting misbehavior—toward siblings, friends, and acquaintances. They were asked to reflect separately on their behavior during childhood and adulthood.
The study used self-reported frequencies rather than asking for exact counts of behavior. While this method has limitations, it allowed the researchers to capture broad patterns of aggression over time. Participants who did not have biological siblings were excluded from the study, and all surveys were translated into the appropriate languages by researchers in each country.
The types of aggression examined were divided into two broad categories: direct aggression (physical acts like hitting or yelling) and indirect aggression (reputational tactics such as gossiping or telling an authority figure about someone’s behavior). Participants reported on how frequently they had engaged in these behaviors toward brothers, sisters, male and female friends, and male and female acquaintances, both during their childhood and as adults.
Men consistently reported higher levels of direct aggression toward non-relatives, both in childhood and adulthood. This standard pattern—greater male aggression toward nonkin—was observed in nearly every country, aligning with previous studies and suggesting that the data captured real and familiar differences.
But when it came to siblings, the results flipped. For example, during childhood, females were more likely than males to report yelling at or hitting a sibling in countries like the United States, Sweden, Chile, and Pakistan. During adulthood, the pattern persisted, although the differences were slightly smaller. Men did not become more aggressive toward siblings as they aged, and in many cases, women continued to report higher levels of sibling-directed aggression into adulthood.
The results for indirect aggression were somewhat more mixed, but again, females often scored as high or higher than males. In particular, women were more likely to report a sibling’s behavior to family authorities—such as parents or other relatives—both during childhood and adulthood. These patterns held true across a wide range of societies, regardless of economic development or cultural norms around gender.
“I think the big story here is that when it comes to sibling interactions, women are at least as aggressive as men,” Varnum told PsyPost. “This is true not only in childhood, but adulthood. And we see the same thing all over the world. Although in non-kin interactions we, like many others before us, find that men engage in more direct aggression than women, this sex difference goes away or reverses when people are interacting with siblings.”
These results suggest that sex differences in aggression are not fixed traits, but instead depend heavily on who the target is. Aggression between unrelated individuals may serve different social or reproductive functions than aggression between family members. Outside the family, males may compete for status, mates, or resources in ways that reward physical dominance. Inside the family, siblings are often competing for parental attention, affection, and shared household resources—factors that could apply equally to brothers and sisters.
Because siblings share roughly half of their genes, evolutionary theories based on inclusive fitness suggest that they may avoid more extreme or damaging forms of aggression. This might level the playing field for females, who may otherwise face higher risks or social penalties for engaging in direct confrontation outside the home. Parents may also respond differently to sibling conflicts based on the sex of the child, possibly disciplining boys more harshly for aggression, which could shape behavior patterns over time.
The researchers note that their findings challenge not only evolutionary theories based on sexual selection, but also social role theories that emphasize cultural learning and gender norms. Neither framework fully explains why women would be more aggressive than men toward siblings in such a wide variety of societies, especially when those same women report lower aggression toward unrelated peers.
“We’d already found this pattern with some U.S. samples, but it’s always an open question whether you get the same results in very different cultural contexts,” Varnum said. “And were lots of reasons why one might have predicted big cultural differences. But 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. So I think that might surprise some folks.”
But there are still some limitations to consider. The study relied on retrospective self-reports, which are subject to memory biases and differences in interpretation. For example, women and men might define or recall aggression differently, particularly for ambiguous behaviors like hitting or yelling. Social desirability bias could also play a role, especially in cases where men might be reluctant to admit to hitting a sister. That said, the researchers note that they replicated well-established sex differences in aggression toward nonkin, suggesting that the measures were reasonably valid.
“All this data was self-report,” Varnum said. “So all the limitations that go along with that type of work apply here. Ideally, it would be great to test this idea with observational data or reports by others.”
“One possibility is that boys are simply unwilling to report hitting their siblings,” Kenrick added. “However, we found the exact same pattern when we asked another group not about their aggression, but about whether their sisters or brothers ever hit them. People said their sisters were just as likely to hit them or yell at them as were their brothers. This suggests that the self reports are probably not biased in a self serving manner.”
Future research could explore these patterns using observational methods or reports from family members to cross-check self-reported behavior. It would also be useful to examine how factors like family size, birth order, parental discipline, and socioeconomic status shape sibling dynamics across cultures.
“We are currently thinking about many other classic findings from social psychology that might not quite look the same in the context of kin relationships and planning some studies that we hope to launch soon,” Varnum said. “So stay tuned!”
The study, “Commonly observed sex differences in direct aggression are absent or reversed in sibling contexts,” was authored by Michael E. W. Varnum, Amanda P. Kirsch, Daniel J. Beal, Cari M. Pick, Laith Al-Shawaf, Chiara Ambrosio, Maria Teresa Barbato, Oumar Barry, Watcharaporn Boonyasiriwat, Eduard Brandstätter, Suzan Ceylan-Batur, Marco Antonio Correa Varella, Julio Eduardo Cruz, Oana David, Laina Ngom Dieng, Dimitri Dubois, Ana María Fernandez, Silvia Galdi, Oscar Javier Galindo Caballero, Sylvie Graf, Igor Grossmann, David Guzman, Peter Halama, Takeshi Hamamura, Martina Hřebíčková, Ioana Iuga, Lady Javela, Jaewuk Jung, Johannes A. Karl, Jinseok P. Kim, Michal Kohút, Anthonieta Looman Mafra, Dieynaba Gabrielle Ndiaye, Jiaqing O, Beatriz Perez Sánchez, Eric Roth Unzueta, Muhammad Rizwan, A. Timur Sevincer, Eric Skoog, Eunkook M. Suh, Daniel Sznycer, Evelina Thunell, Arnaud Tognetti, Ayse K. Uskul, Jaroslava Varella Valentova, Yunsuh Nike Wee, Anja Lundkvist Winter, Torin Peter Young, Danilo Zambrano, Anna Ziska, and Douglas T. Kenrick.
