A new study carried out by the University of Montreal and CHU Sainte-Justine Hospital in Montreal, Canada, suggests that physical aggression in toddlers develops more as a result of genetic rather than environmental factors.

The theory that physical aggression in children develops over time as a result of exposure to aggressive role models (both in their social environment and the media) has dominated research on the subject over the past 25 years.

Studies of early childhood physical aggression, however, suggest that this aggression begins in infancy, peaking between the ages of 2 and 4.

It is clear there are many factors at work when it comes to the development of early physical aggression. In 2013, Medical News Today reported on a study suggesting that mothers who eat lots of junk food while pregnant may have children with increased aggression and behavioral problems.

The University of Montreal’s study – which is published in the journal Psychological Medicine – involved the participation of 667 pairs of twins, both identical and non-identical. The parents of the twins were asked to rate their children’s physical aggression – hitting, biting, kicking and fighting – at the ages of 20, 32 and 50 months. The researchers then compared the behavior, environment and genetics of the pairs of twins.

“Genetic factors always explained a substantial part of individual differences in physical aggression,” claims the University of Montreal’s Eric Lacourse. He explains:

The limited role of shared environmental factors in physical aggression clashes with the results of studies of singletons in which many family or parent level factors were found to predict developmental trajectories of physical aggression during preschool. Our results suggest that the effect of those factors may not be as direct as was previously thought.”

The study found that – consistent with other studies – physical aggression peaked during early childhood, but that the frequency at onset and rate of change of physical aggression was influenced by an interplay of genetic and environmental factors.

The researchers think that genes account for about 50% of the factors contributing to childhood aggression.

“The gene-environment analyses revealed that early genetic factors were pervasive in accounting for developmental trends, explaining most of the stability and change in physical aggression, ” Lacourse says.

Individuals who are worried about their toddler’s aggressive behavior should not lose hope. This study does not suggest a child’s behavior is set in stone because of their genes.

“These genetic associations do not imply that the early trajectories of physical aggression are set and unchangeable,” Lacourse emphasizes. “Genetic factors can always interact with other factors from the environment in the causal chain explaining any behavior.”

In fact, long-term studies analyzing physical aggression show that most children, adolescents and adults grow out of this behavior, eventually learning non-aggressive alternatives.