MONTREAL — Are toddlers’ tantrums and aggression the result of nature or nurture? A study out of Canada says genes might have more to do with it than previously thought.

Researchers from University of Montreal and CHU Sainte-Justine Hospital looked at the physical aggression of twins between the ages of 20 months and about 4 years old. They found that genetic factors account for about 50 percent of the variance in a population’s aggression.

Their findings were published in the journal “Psychological Medicine.”

By studying twins, researchers minimized the difference in social factors (parenting philosophies and practices, demographic, activities, etc.) for each child. All parents of twins – identical or fraternal – born between April 1995 and December 1998 in the Greater Montreal area were invited to participate in the study.

Parents of the 667 twin participants reported aggressive behavior like biting kicking, hitting and fighting when the twins were aged 20, 32 and 50 months. Physical aggression peaks between the ages of 2 and 4, typically.

In the last 25 years, social learning theories have influenced research on childrens' physical aggression, suggesting aggression is the result of environmental factors like parenting styles and media.

The researchers tested the idea that genetic factors could account for the level of physical aggression a child expresses. They also looked at how genes mature over time and with environmental influences.

“The results of the gene-environment analyses provided some support for the genetic set-point hypotheses, but mostly for the genetic maturation hypotheses,” lead study author Eric Lacourse, PhD said. “Genetic factors always explained a substantial part of individual differences in physical aggression. 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.”

The authors cautioned parents to tread lightly when discouraging 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.”