Lacourse and his colleagues from the University of Montreal and its affiliated CHU Sainte-Justine Hospital came to this conclusion after testing three general patterns regarding the roles that environment and genetic factors play in the development of physical aggression.
The first scenario followed the general consensus, that both factors are ubiquitous and involved in the development and stability of physical aggression. The second involved a “genetic set point” model, in which a single genetic factor could account for the level of physical aggression over time. The third pattern was the “genetic maturation” model, in which new sources of genetic and environmental contributions came 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 told Science Daily.
Lacourse’s study used a twin cohort to help them determine the influence of both genetic and environmental factors in physical aggression. Originally conducted by Michael Boivin of Laval University and Richard Tremblay, of the University of Montreal and University College Dublin, the cohort included 667 twin pairs. Some from the same embryo, which makes them identical, while others were dozogytic, which means they came from different embryos and are not identical.
Twin studies help in determining genetic factors because, in twins, the home environment is almost always the same. Studies that use identical and non-identical twins are even more helpful in determining genetic versus non-genetic factors because identical twins, which share the same genetic code, have more in common than those that are not identical.
In this particular cohort, mothers were asked to rate their twins on physical aggression. They reported on behavior like hitting, biting, kicking and fighting at 20, 32, and 50 months. Results found that physical aggression typically started during infancy and peaked between the ages of 2 and 4. This gap in peak time during early childhood indicated that there are some substantial differences in the frequency and onset and rate of change in physical aggression over a lifespan, which could mean that genetic variances are responsible for about half of all variances in aggression across the entire population.
“The results of the gene-environment analyses provided some support for the gene set-point hypotheses, but mostly for the genetic maturation hypothesis,” Lacourse 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 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 previously thought.”
Still, despite the evidence, Lacourse doesn’t believe that parents should just ‘accept’ aggression in their children. There are ways in dealing with it, teaching a child new coping mechanisms, and hopefully curbing aggression during the early years to keep it from being a life-long issue.
“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. 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 casual chain explaining any behavior,” Lacourse said. “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. 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.”
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