Studies have shown there is a genetic basis for physical aggression in toddlers, but it does not mean a person will remain aggressive forever.

Researchers on a new study have 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,” Eric Lacourse from the University of Montreal 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.”

The most common thinking still says that physical aggression is mainly built by exposure to aggressive role models in the media and broader social environment.

New studies have challenged this stance, showing that physical aggression can start during infancy and peak between the ages of 2 and 4.

The latest Candian study was conducted with 667 monozygotic and dizygotic twin pairs.

Monozygotic means the twins originated from the same embryo and are genetically identical. Dizogytic twins develop in separate embryos and are not identical.

Mothers were asked to rate their twins’ physical aggression, reporting behaviour such as hitting, biting, kicking and fighting, at the ages of 20, 32 and 50 months.

“Genetic factors always explained a substantial part of individual differences in physical aggression,” Lacourse said.

“More generally, 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,” he said.

Long-term studies of physical aggression clearly show that most children, adolescent and adults eventually learn to use alternatives to physical aggression.

“Because early childhood propensities may evoke negative responses from parents and peers, and consequently create contexts where the use of physical aggression is maintained and reinforced, early physical aggression needs to be dealt with care,” Lacourse said.

“These cycles of aggression between children and siblings or parents, as well as between children and their peers, could support the development of chronic physical aggression.”

More details will be available in the full report titled; “A longitudinal twin study of physical aggression during early childhood: Evidence for a developmentally dynamic genome”, to be published in the journal Psychological Medicine.