Personality Predicts Aggressive Behavior Far Better Than Body Size, Study Finds

Height, Weight, and Strength Reveal Little About Aggressive Behavior

Being the biggest person in the room does not automatically make someone the most likely to cause trouble. A new study of nearly 1,200 young adults found that personality was a far better clue than height, weight, or strength to who reports getting into fights, breaking rules, and acting out.

Published in the Journal of Research in Personality, the work comes from researchers at the University of Georgia, Emory University School of Medicine, and Purdue University. They set out to test a long-standing assumption: that a person who could win a physical fight is more willing to start one. Evolutionary biologists have made that argument about animals for decades, and earlier studies of male college students and of professional hockey and lacrosse players found modest links between size and aggression.

One model even proposed that aggressive tendencies get tuned to a person’s physical power, so a strong, disagreeable person might lean on aggression more often. Few studies, though, had measured personality and body size in the same people, which left the idea largely untested.

Personality Traits Tracked Closely With Aggression and Rule-Breaking

Data came from the Human Connectome Project, a federally funded study of healthy young adults run through Washington University in St. Louis. Of 1,206 original participants, 1,196 had complete data and were included. Their average age was about 29, and 54% were women. Roughly 74% were White and 16% were Black or African American, with smaller shares from other racial groups.

Each person completed a 60-question survey measuring five broad personality traits, including agreeableness (kindness and cooperation), conscientiousness (self-discipline and reliability), and neuroticism (a tendency toward anxiety and emotional ups and downs). A separate 123-question checklist covered problem behavior, such as getting into many fights, damaging or destroying things, and seeking lots of attention. Answers were scored as aggression, rule-breaking, and intrusive behavior, a category that includes attention-seeking. Participants reported their own behavior, rather than having researchers observe their actions directly.

Results matched earlier personality research. Lower agreeableness and higher neuroticism went with more aggression, while lower agreeableness and lower conscientiousness went with more rule-breaking, in men and women alike. Outgoing participants were also more likely to report pushy, attention-seeking behavior. Men and women were examined separately, since men are on average taller, heavier, and stronger and also tend to report more of these behaviors, which could muddy the results.

Body Size Added Almost Nothing Beyond Personality

Height and weight were self-reported, while grip strength was measured with a hand-held squeezing device, a common stand-in for physical strength because it tracks closely with overall upper-body strength. Across the full group, bigger bodies did appear to go with more problem behavior. Once men and women were compared on their own, that link mostly vanished, and a combined size score showed no reliable connection to problem behavior in either group after personality was taken into account. The team had expected size to matter on its own, and called the result “perhaps most surprising for the aggression outcome,” given how many theories lean on physical power.

Researchers also tested the movie-villain idea: that being both physically imposing and disagreeable would make someone especially likely to act out. Out of 160 comparisons, each pairing a personality trait with a size measure, only four cleared the usual statistical bar. None involved agreeableness or neuroticism, the two traits the researchers had expected to pair with size, and the authors said chance likely explains the four, none of which would hold up under even mild statistical corrections.

Weight Showed a Small but Consistent Link to Problem Behavior

When height, weight, and grip strength were tested one at a time, weight stood out as the measure that kept showing up. Heavier participants scored slightly higher on problem behavior. That pattern showed up in both men and women, though more consistently in women, and ran roughly one-quarter to one-fifth the strength of the personality links. The data cannot reveal why. In the authors’ words, “it is entirely possible that both weight and aggression are caused, in part, by a third variable,” such as trouble managing emotions or being bullied.

Caveats apply. Participants were healthy young adults, so the picture could look different in settings where aggression is more common, such as professional hockey. Grip strength captures only part of physical power, and broader strength measures might change the picture. Because personality was also self-reported, shared reporting style could partly explain why it looked so strong. It is also possible that how strong someone believes they are, or how intimidating others find them, matters more than tape-measure size, a question the authors flagged for future work. The researchers publicly registered their analysis plan before analyzing the data, then changed part of it during peer review to account for gender more fully.

Size is the easy suspect, but in these 1,196 young adults it was a poor one. How someone handles emotions, impulses, and other people said far more about aggression than a tape measure or a scale.

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Disclaimer: This article is for informational purposes only and summarizes findings from one published research study. It is not medical, psychological, or professional advice, and its results describe group-level patterns in healthy young adults, not any individual’s behavior. Anyone with concerns about aggression or mental health should consult a qualified professional.

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