How chronic physical aggression in boys passes down the generations

The best way to prevent chronic physical aggression in boys is intensive and long-term interventions initiated early in life targeting disadvantaged mothers.

Following research into aggression in children dating back to the 1980s, two leading researchers from Canada, Richard E Tremblay and Sylvana M Côté, have presented a wide range of factors that predict aggression and antisocial behavior in children, boys in particular. These include both genetic and social influences. Male aggression exceeds female aggression by a large margin and has done so for a long time. The ratio of female to male homicides in France 200 years ago was almost exactly the same as in the USA in 2014: 11.7:100 and 11.6:100 respectively.

The intergenerational nature of male aggression leads the authors to recommend that the best way to prevent chronic physical aggression and other antisocial behavior problems is intensive and long-term intervention initiated early in life and targeting disadvantaged mothers. This breaks the intergenerational transmission of violent behaviors.

Genetic influence on aggression

Large studies of twins have revealed the extent of genetic influence on physical aggression. Genetic factors explain 50% to 63% of the variance in frequency of physical aggression in children at 20 months. The link between genetics and physical aggression is stronger than the link between genetics and language development. However, this influence substantially diminishes as time goes on, while new influences come into play.

Parents’ life experience

When mothers report antisocial behavior during their own adolescence, their children are considerably more likely to display chronic physical aggression between the ages of 17 and 42 months. Chronic physical aggression in children is more frequent if the mother is young, poor, separated from the father, has not completed high school, has smoked during pregnancy, or suffers from depression.

This intergenerational phenomenon is exacerbated by assortative mating: mothers with poor childhood experiences are more likely to partner and have children with fathers who have had similar experiences.

A key mechanism of the intergenerational transmission of disadvantage is epigenetic change. The DNA methylation profiles of boys displaying chronic physical aggression are different from those of other boys (in 448 places on the genome in one study). Children whose mothers show more mental illness, more criminal behavior or more substance abuse have higher methylation of the oxytocin receptor gene at birth and later show higher callous-unemotional traits at 13 years of age. Similarly, boys displaying higher aggression have lower serotonin synthesis in the brain. This is linked to higher methylation of genes in the serotonin pathway.

Aggression begins early

In the 1990s, Tremblay found that physical aggression often starts in the second part of the first year after birth, substantially increasing in frequency up to the third year, then declining slowly. Aggression emerges between six and 42 months of age, after which a child would normally learn to control it. There is a substantial difference between boys and girls: 5% of boys between 17 and 29 months use physical aggression frequently, whereas only 1% of girls do.

Aggression in kindergarten and elementary school predicts aggression in adolescence

Other research by Tremblay in the 1990s in Canada showed that greater physical aggression among kindergarten boys predicts chronic physical aggression in adolescence. Another study in Canada, New Zealand and the USA found that boys showing high chronic physical aggression in elementary school were more likely to show the same in adolescence. This link was not seen for girls.

The authors note that the danger posed by aggression in childhood increases as children grow larger from 6 to 12 years of age.

In a randomized controlled trial in the early 1980s, Tremblay and colleagues found that a programme to address aggression and hyperactivity in kindergarten boys from low socioeconomic areas had positive outcomes for the children later in life: less aggression at the ages of 7 to 9, less physical aggressions and thefts at the ages of 11 to 17, increased rate of high-school completion and fewer criminal offenses in early adulthood. The programme consisted of both home-based parent training and school-based help for social and cognitive skills.

The evidence thus points to the importance of early intervention, starting at home before kindergarten.