Summary: A twin study finds that both proactive and reactive aggression in early childhood share much of the same genetic basis at age six, but whether aggressive behaviour increases or decreases between ages 6 and 12 is driven largely by environmental influences.
Source: University of Montreal
Genetics Provide a Starting Point; Environment Shapes the Course
Recent public attention on assaults and harassment has prompted renewed interest in why people display aggressive behaviour. To investigate the roots of childhood aggression, Stéphane Paquin, a PhD candidate in sociology at Université de Montréal, led a longitudinal study of 555 twin pairs to compare proactive and reactive aggression. Supervised by Éric Lacourse and Mara Brendgen, the research tracked teacher-reported aggression at ages 6, 7, 9, 10 and 12.
The study shows that at age 6, proactive and reactive aggression share most of the same genetic influences. However, as children grow, their aggressive behaviour typically declines for many, and the trajectories—whether aggression intensifies or subsides—appear to be influenced more by environmental factors than by new genetic changes.
Understanding Proactive and Reactive Aggression
Proactive aggression is deliberate behaviour intended to dominate, gain advantage, or obtain a desired outcome at another person’s expense. Reactive aggression is an impulsive, defensive response to a perceived threat or provocation. Although conceptually distinct, the two types of aggression are often closely related and can co-occur in the same child.
“Aggression is a normal part of early social development,” Paquin notes. “Children typically show the highest levels of aggressive interactions with peers between ages 2 and 4. As they mature, many learn to regulate emotions, resolve conflicts, and communicate more effectively, which reduces overt aggression. But the way aggression changes over time—improving for some children and worsening for others—depends heavily on the child’s environment.”
Twin Design Reveals Genetic and Environmental Contributions
The study cohort included 223 monozygotic (identical) twin pairs and 332 dizygotic (fraternal) twin pairs from Quebec. Using teacher assessments at five time points between ages 6 and 12, researchers applied multivariate latent growth curve models and biometric decompositions to disentangle genetic, shared environmental, and nonshared environmental influences.
Key findings:
- Common genetic factors explained the majority of variation in proactive and reactive aggression at age 6, accounting for roughly two-thirds of baseline differences.
- Different sets of genetic influences were implicated across developmental change: some genetic factors related specifically to baseline levels, while others were linked to changes in aggression across childhood.
- Shared environmental factors (experiences common to both twins) were notably associated with proactive aggression at baseline and across time, while unique environmental experiences contributed substantially to changes in both aggressive subtypes.
These results support a view in which early genetic predispositions help determine a child’s starting point for aggressive behaviour, but environmental conditions—family context, peer interactions, school climate, victimization experiences, and interventions—play a crucial role in shaping how aggression evolves through middle childhood.
Implications for Prevention and Intervention
These findings have clear implications for prevention programs and clinical practice. Paquin and colleagues emphasize tailoring interventions to the subtype of aggression:
- Programs addressing reactive aggression should focus on reducing victimization and improving emotional regulation, conflict management, and coping skills.
- Programs targeting proactive aggression should emphasize the development of pro-social values, empathy training, and alternative, non-aggressive strategies for achieving goals.
Interventions that involve families and schools—providing parental support, teaching consistent discipline and supervision strategies, and creating positive peer and classroom environments—are likely to influence the environmental pathways that steer change in aggressive behaviour.
Funding: The research received support from the Social Sciences and Humanities Research Council, the Canadian Institutes of Health Research, the Fonds de recherche du Québec – Santé, and the Fonds de recherche du Québec – Société et culture.
Source and authorship: University of Montreal. The original study, “Heterogeneity in the development of proactive and reactive aggression in childhood: Common and specific genetic – environmental factors,” was conducted by Stéphane Paquin, Éric Lacourse, Mara Brendgen, Frank Vitaro, Ginette Dionne, Richard E. Tremblay, and Michel Boivin and published in PLOS ONE (published online December 6, 2017).
Abstract (concise)
Background: Few studies take a developmental approach when comparing proactive and reactive aggression, despite both overlapping and distinct correlates. This study examined how genetic and environmental factors jointly and specifically contribute to differences in initial levels (intercepts) and changes over time (slopes) in proactive and reactive aggression during childhood.
Methods: The sample included 223 monozygotic and 332 dizygotic twin pairs. Teacher ratings of aggression were collected at ages 6, 7, 9, 10 and 12. Researchers applied multivariate latent growth modeling and biometric decomposition to test hypotheses about genetic set points, genetic differentiation, and genetic maturation versus environmental modulation.
Results: Common genetic factors explained a large portion of the baseline variation in both aggression types. Additional, distinct genetic and shared environmental influences were associated with baseline levels and developmental slopes for each subtype. Nonshared environmental influences accounted for much of the remaining variability in developmental change.
Conclusions: The study supports a model where common genetic factors underlie initial levels of both proactive and reactive aggression, while environmental factors significantly shape their trajectories across childhood. Differentiating the two subtypes remains important for research and for designing effective prevention and intervention strategies.