A new study from the University of Montreal explored the question of whether aggression in children is a result of nature or nurture. To look for an answer, Stéphane Paquin, a PhD candidate in sociology, led a study on 555 sets of twins to compare incidences of proactive and reactive aggressive behaviour. His results demonstrate that, at age 6, both types of aggression have most of the same genetic factors, but the behaviour diminishes in most children as they age. Increases or decreases in aggression between the ages of 6 and 12 appear to be influenced by various environmental factors rather than genetics.

Proactive aggression refers to physical or verbal behaviour meant to dominate or obtain a personal advantage at the expense of others, whereas reactive aggression is a defensive response to a perceived threat. While some children only exhibit reactive aggressive behaviours, proactive and reactive aggression are generally closely related.

“Too often we forget that aggression is a fundamental part of a young child’s social development,” said Paquin. “Human beings show the highest levels of aggressive behaviour towards their peers between the ages of 2 and 4. As children grow, they learn how to manage their emotions, communicate with others and deal with conflict. They are able to channel their aggressive impulses, whether proactive or reactive.”

Because the study included almost equal sets of identical twins – with identical genetic code – and fraternal twins, researchers were able to tell the difference between behaviors that were genetically determined versus influenced by environment. The children’s aggressive behaviours were assessed and documented in a report by their teachers at ages 6, 7, 9, 10 and 12. The results of the study also demonstrated that genetic factors influencing aggression at age 6 are different than those associated with changes in behaviour up to age 12. This suggests a common genetic maturation process is taking place, including the maturation of cognitive functions such as planning, decision-making, control and concentration.

With these results, researchers can now move on to studying the specific social factors associated with changes in proactive and reactive aggression in kids. “This work will also have a direct impact on clinical practices and prevention programs,” said Paquin. “Our results have revealed the importance of developing different prevention methods for reactive and proactive aggression, specifically by offering support to families and providing interventions in schools.”

The findings may be helpful in the design of programs meant to head off aggression in kids; if those children who exhibit proactive aggression get more specific attention on the development of pro-social behaviors, it may help prevent some of the behaviors that get kids labelled as problem kids before age 12.