The development of physical aggression in toddlers is strongly associated with genetic factors and to a lesser degree with the environment, according to a new study led by Eric Lacourse of the University of Montreal and its affiliated CHU Sainte-Justine Hospital. Lacourse’s team worked with the parents of identical and non-identical twins to evaluate and compare their behaviour, environment and genetics.

“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 said. “However, it should be emphasized that these genetic associations do not imply that the early trajectories of physical aggression are set and unchangeable. Genetic factors can always interact with other factors from the environment in the causal chain explaining any behaviour.”

Over the past 25 years, research on early development of physical aggression has been highly influenced by social learning theories that suggest the onset and development of physical aggression is mainly determined by accumulated exposure to aggressive role models in the social environment and the media. However, the results of studies on early childhood physical aggression, initiated by the University of Montreal team, indicate that physical aggression starts during infancy and peaks between the ages of 2 and 4. Although for most children the use of physical aggression peaks during early childhood, these studies also show that there are substantial differences in both frequency at onset and rate of change of physical aggression due to the interplay of genetic and environmental factors over time. Genetically informed studies of disruptive behavior and different forms of aggression across the lifespan generally conclude that genetic factors account for approximately 50% of the variance in the population.

Lacourse and his colleagues posited and tested three general patterns regarding the developmental roles of genetic and environmental factors in physical aggression. First, the most consensual and general point of view is that both sources of influence are ubiquitous and involved in the stability of physical aggression. Second, a “genetic set point” model suggests a single set of genetic factors could account for the level of physical aggression across time. A third pattern labeled ‘genetic maturation’ postulates new sources of genetic and environmental influences with age.

“According to the genetic maturation hypothesis, new environmental contributions to physical aggression could be of short duration in contrast to genetic factors,” Lacourse explained.

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