Available online 12 March 2024

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Two well-supported generalizations from aggression research are that: a) people are less likely to commit homicide against close kin compared to non-kin, and b) females are less likely to engage in direct aggression than are males. Aggression between siblings, however, is somewhat more complicated than one might surmise from those two generalizations. Data from 3 studies collected using undergraduate and Prolific samples (N = 1640) reveal classic sex differences in direct aggression between non-relatives, but not between sisters and brothers. Whereas only a small minority of females have hit a friend or an acquaintance, the majority of females, like the majority of males, have hit a sibling. Although reputational aggression is substantially less likely between siblings than between friends or acquaintances, mild forms of direct aggression (such as hitting) are quite frequent between siblings. Discussion considers several possible limitations of the findings reported here and considers results in light of Trivers' theory of parent-offspring conflict.

Section snippetsStudy 1

In the first study, participants reported the frequency of instances in which they had either directly aggressed (e.g., hit, yelled at) or reputationally aggressed (e.g., spread gossip about, reported) siblings, friends, and acquaintances.2

Study 2

In Study 1, participants reported their own sex, but were not asked to distinguish between the sex of their siblings or non-relatives. Study 2 asked the same set of questions about participants' aggression toward siblings and non-relatives and examined (a) whether the frequency of aggression differs toward males and females (e.g., brothers vs. sisters, male friends vs. female friends, male acquaintances vs. female acquaintances).

Because the findings from Study 1 that siblings are more directly

Discussion of Study 1 and Study 2

Both Study 1 and Study 2 demonstrated that males engage in hitting and yelling with friends and acquaintances much more than females do, but both sexes engage in a lot of hitting and yelling with their siblings. Both studies also show that siblings engage in gossiping significantly less often about each other than they do about their friends and acquaintances. While children engage in more aggression overall than adults do (perhaps in part because they spend more time together), these patterns

Participants

The survey was completed by 599 undergraduate participants at Arizona State University. Participants were all at least 18 years old. Participants averaged 19 years old (SD = 1.7) and there were 66.6% females and 33.4% males; 53.5% self-described as non-Hispanic White, 18.7% as Hispanic/Latino, 4.1% as East Asian or Pacific Islander, 7.6% as South Asian, 2.1% as Middle Eastern, 3.5% as Black, 9.2% as mixed race, and 1.3% as other. Participants self-rated their social status on a scale from 1

General discussion

Results from three studies indicate that direct aggression is fairly common between siblings. Further, sex differences in aggression – which have been well-documented among unrelated individuals – replicate here, but only with regard to aggression from and toward non-relatives. That is, males are much more likely than females to report direct aggression from and toward friends and acquaintances of their own sex. However, this sex difference is not found for inter-sibling aggression. Siblings

Conclusion

The studies reported here suggest that everyday acts of direct aggression between siblings are common and do not show the typical sex differences found in aggression between non-relatives. Although direct aggression is more common between male friends and acquaintances, as compared to females, those same sex differences in aggression are absent between siblings. At the same time, siblings are not more differentially likely to engage in all forms of aggression toward one another. Reputational

CRediT authorship contribution statement

Amanda P. Kirsch: Writing – review & editing, Writing – original draft, Visualization, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Douglas T. Kenrick: Writing – review & editing, Writing – original draft, Visualization, Supervision, Project administration, Methodology, Investigation, Conceptualization. Ahra Ko: Writing – review & editing, Visualization, Formal analysis. Cari M. Pick: Writing – review & editing, Formal analysis. Michael

Declaration of competing interest

Authors declare that they have no competing interests.

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