Recent research published in the journal Evolution and Human Behavior has highlighted an intriguing facet of family dynamics: girls are just as likely as boys to engage in physical and verbal aggression towards their siblings. The study, which examined into minor forms of violence like hitting, kicking, and yelling, reveals that such behaviors are surprisingly common among siblings and do not exhibit the gender differences typically seen in aggressive interactions outside the family.
The primary aim of the new study was to explore the nature of sibling aggression and to determine whether the well-documented gender differences in aggression apply within the familial context. Previous research has found that males generally exhibit higher levels of physical aggression due to evolutionary factors like sexual selection and competition for resources. However, researchers sought to understand whether these patterns hold true within the family, where individuals share significant genetic bonds and resources.
“Research on homicide has generally found that, although unrelated people living together are at an increased risk of being murdered, siblings are very unlikely to kill one another,” said study author Amanda Kirsch, a PhD candidate in social psychology at Arizona State University. “From a gene’s eye perspective, this makes sense — because human siblings are very likely to cooperate with, and help, one another throughout their lifespans. However, we found that people consistently were more likely to have experienced aggression between siblings than between friends or acquaintances, and sisters were just as likely to be hitting and yelling as the brothers were.”
The researchers conducted a series of three separate studies to examine the dynamics of sibling aggression in comparison to aggression within non-familial relationships, and how these interactions vary by gender.
In Study 1, the researchers used a survey to collect data from 619 undergraduate students at Arizona State University who all had at least one biological sibling. The participants, whose average age was 19.5 years, were diverse in terms of gender and ethnic background.
The survey asked participants to report instances of aggression — both received and perpetrated — involving siblings, friends, and acquaintances. Aggression was categorized into direct forms, such as hitting or yelling, and reputational forms, such as spreading gossip or reporting someone to an authority. The frequency of these behaviors was measured on a scale ranging from “never” to “many times.”
Study 1 revealed that the common assumption of males being more physically aggressive than females does not hold within the context of sibling relationships. Both male and female participants reported similar frequencies of direct aggression (hitting and yelling) towards their siblings. This contrasts sharply with their interactions with friends and acquaintances, where males were significantly more likely to engage in physical aggression.
Interestingly, the study also explored reputational aggression (such as spreading gossip or reporting someone to authorities) and found no significant gender differences in these behaviors towards siblings. Furthermore, reputational aggression was less common among siblings than among friends and acquaintances.
Building on this, Study 2 further examined these dynamics but included additional variables and a slightly different participant group. This study involved 451 participants recruited through the Prolific survey platform, ensuring a broader demographic range outside the university setting. The participants reported past aggressive incidents involving both childhood (under 16 years old) and adulthood (18 years old and older).
The results confirmed the persistence of sibling aggression into adult life, despite potential decreases in daily contact. Similar to Study 1, no significant gender differences were found in sibling aggression both in direct and reputational forms. The study also highlighted that aggression towards siblings was not only prevalent but remained consistent over time, suggesting that the patterns established in childhood continue into later stages of life.
Study 3 sought to delve deeper into the nature and context of aggressive interactions by examining the intentions behind and perceptions of these acts. This study used a more narrative approach, asking participants to describe specific incidents of aggression.
Conducted with 599 undergraduates from Arizona State University, this study required participants to recall and describe the most severe incident of being harmed by a sibling, friend, or acquaintance. The participants provided detailed accounts of these incidents, which were then analyzed to determine whether the aggression was intentional, accidental, or playful.
The detailed accounts revealed that the majority of sibling aggression was perceived as intentional rather than accidental or playful, which contrasts with aggression in non-familial relationships where playful and accidental aggression was more common. Furthermore, when describing the most severe incidents, participants recalled intentional aggression more frequently with siblings than with non-relatives.
Together, these findings challenge traditional views on gender and aggression by showing that within the family context, particularly among siblings, the expected male dominance in physical aggression does not apply. Instead, siblings exhibit a unique pattern of interaction where direct aggression is commonly shared between genders without the typical male bias seen in other social contexts.
“We know from decades of research on aggressive behavior that men are more aggressive than women,” Kirsch told PsyPost. “The sex difference in aggression begins early, beginning with children on the playground and teenagers fighting in the high school gym, and continuing into adult criminal assault and homicidal violence. It’s true in every human society that’s ever been investigated, and it’s even true for most other animal species. However, when it comes to minor forms of violence within the family – hitting, kicking, or yelling at your brothers and sisters – girls are every bit as aggressive as boys.”
“Was the aggression inside the family just playful or accidental, perhaps? No, an examination of the details of the incidents reported revealed no differences in the degree of violence. Was it just because people have more interactions with brothers and sisters than outside the family? This might be true to some extent, but the exact opposite was found when people were asked not about direct aggression, but about reputational aggression. Siblings were relatively exempt from this type of aggression. Although they could be mean to one another’s faces, brothers and sisters were kinder to one another behind their backs.”
The findings highlight an interesting paradox. The principle of inclusive fitness posits that it is beneficial for individuals to allocate resources to their siblings rather than to non-relatives. This is because siblings share a significant proportion of their genetic makeup — approximately 50%. As a result, promoting the survival and reproductive success of siblings indirectly supports the propagation of one’s own genes. So why is aggression towards one’s siblings so prevalent? It might be the result of the inherent durability of familial connections.
“Paradoxically, we believe the fact that siblings are relatively comfortable fighting with one another might be because of the a built-in lid on sibling rivalry (while your sibling isn’t likely to escalate to homicidal violence, the acquaintance from another family you can’t be so sure about),” Kirsch explained. “Sibling relationships are relatively immune to being broken off after a minor squabble. If you yell at or hit a friend or a casual acquaintance, they might well replace you with a new friend, but a brother or sister will still be your brother or sister tomorrow, next week, and next year.”
The research adds a critical dimension to our understanding of aggression and gender, suggesting that the dynamics within families are distinct from those in other social contexts. But the study, like all research, includes some caveats. For example, the reliance on self-reported data can introduce biases that may affect the accuracy of the findings. Participants might not accurately remember past events or could portray their behavior more favorably due to social desirability biases.
For future research, it would be beneficial to use a more diverse sample that includes participants from various backgrounds and age groups to enhance the external validity of the findings. Employing methodologies that reduce reliance on self-report, such as observational techniques or longitudinal designs, could provide more objective and detailed data on sibling interactions over time. Expanding the research to include non-biological siblings could also offer a broader understanding of how familial relationships influence aggressive behaviors.
The study, “Sibling aggression is surprisingly common and sexually egalitarian,” was authored by Amanda P. Kirsch, Douglas T. Kenrick, Ahra Ko, Cari M. Pick, and Michael E. W. Varnum.
