Adolescents with higher body weight are somewhat more likely to show aggressive, rule-breaking behaviour — and the link holds as they move into young adulthood, according to a study that followed more than 1,200 twins across seven years.
The association is modest. But what makes it interesting is the mechanism: the study suggests that some of the same genetic factors that influence body size also influence what psychologists call externalizing behaviour — things like fighting, lying, breaking rules and defying authority.
Researchers at the University of Southern California analysed data from 1,260 individuals in 603 twin pairs — 254 identical, 349 fraternal — recruited for the Risk Factors for Antisocial Behavior project. They measured height and weight when the twins were 13 to 14, then tracked their behaviour at that age and again at 19 to 20.
The study, published in Frontiers in Child and Adolescent Psychiatry, found that higher body mass index and greater body bulk were associated with higher levels of externalizing behaviour both at the initial measurement and six years later. The standardised effect sizes ranged from 0.11 to 0.42 — enough to be statistically detectable across a large sample, but far from a one-to-one relationship.
What twin comparisons reveal
Twin studies work by comparing identical twins, who share all their genes, with fraternal twins, who share about half. If identical twins are more similar to each other on a trait than fraternal twins are, that points to genetic influence.
In this sample, body mass index and body bulk were highly heritable — about 73 to 81 per cent of the variation between individuals could be traced to genetic differences. Externalizing behaviour showed moderate heritability, at 31 to 41 per cent.
The researchers then tested whether the overlap between body size and behaviour was itself genetic. The bivariate models suggested it was — meaning that some genes appear to nudge both traits in the same direction. But the genetic correlation estimates were imprecise, and the authors flag that directly in their conclusion.
This is not a finding you can take to the bank. It is a finding that warrants replication in a bigger sample with tighter confidence intervals.
Why they might travel together
The study does not identify specific genes or explain how they would influence both weight and behaviour. Several biological pathways are plausible.
Dopamine systems regulate reward-seeking, impulsivity and appetite. Serotonin affects mood, aggression and satiety. Stress-response systems involving cortisol influence both eating patterns and emotional regulation. Genes that shape any of these could, in principle, affect body size and externalizing behaviour at the same time.
Another possibility is that both are downstream consequences of a third factor — early adversity, for instance, or prenatal nutrition — that is itself partly genetic in how a person responds to it.
The study measured behaviour through self-report questionnaires: the Youth Self-Report at age 13 to 14, and the Adult Self-Report at 19 to 20. These are validated tools, but they rely on teenagers accurately reporting their own rule-breaking and aggression, which they may underestimate or exaggerate.
What this does not mean
This study does not show that being heavier causes behavioural problems, or that behavioural problems cause weight gain. The design is longitudinal — it tracks people over time — but it is not experimental. You cannot randomly assign body weight.
The genetic overlap the study detected is small and uncertain. Most of the variation in externalizing behaviour, and most of the variation in body size, came from factors not shared between the two traits. They are not the same thing wearing different clothes.
Heritability estimates also do not tell you what is fixable. A trait can be highly heritable and highly responsive to environment. Height is about 80 per cent heritable, and nutrition still matters enormously. Genetic influence is not the same as genetic determinism.
The sample was drawn from the Los Angeles area and recruited through schools. Twin studies tend to underrepresent families experiencing severe adversity or instability, because those families are harder to keep in longitudinal research. The extent to which these findings apply to adolescents growing up in very different circumstances is not known.
Where it points for parents and services
The practical implication is not that weight should be treated as a behaviour problem, or that behaviour problems should be treated as a weight issue. The implication is that both may reflect overlapping vulnerabilities — biological, environmental or both — and that addressing one without considering the other may miss part of the picture.
An adolescent struggling with impulsivity, for instance, may benefit from support that takes into account how impulsivity shows up in multiple domains: peer conflict, rule-breaking and eating patterns. Interventions that target self-regulation broadly, rather than isolating one behaviour, may have wider effects.
The study also raises questions about timing. If genetic influences on body size and behaviour are already detectable at age 13, and the association persists to age 20, early adolescence may be a more useful window for intervention than late adolescence or young adulthood — though the study does not test that directly.
What this study cannot tell you is which teenagers are at risk, or what to do if your own child fits the pattern it describes. The overlap is too small and too uncertain to guide individual decisions. What it does is point researchers toward shared pathways worth investigating with better tools.
If you need support
If you are in immediate danger, contact your local emergency number.
- United States — 988 Suicide & Crisis Lifeline: call or text 988
- United Kingdom — Samaritans: 116 123 (free, 24/7)
- International — findahelpline.com
Sources
- Oskarsson S, Raine A, Andersson A, Ling S, Bertoldi B, Baker LA (2026). The association between body mass index and body bulk with externalizing behavior: a longitudinal twin study. Frontiers in Child and Adolescent Psychiatry. https://doi.org/10.3389/frcha.2026.1905375
Published for general education. This article is not medical advice, diagnosis or treatment. If you are struggling, speak to a qualified professional.
