The powerful effects of genes on physical aggression can be detected even in children under the age of two, according to a new study.
Ginette Dionne and colleagues studied a group of 562 19- month-old children, including 107 identical (monozygotic) twin pairs and 174 fraternal (dizygotic) twin pairs, evaluating the children's levels of aggression and their verbal skills. The researchers report that a modest correlation was detected between aggression and expressive vocabulary. They theorize that poor verbal abilities may play a role both in accelerating development of aggressive behavior and in forestalling the normal drop in this behavior that occurs as children reach school age.
In addition, the researchers found a "substantial heritability" (58%) for physical aggression. The second finding, Dionne and colleagues say, "means that individual differences in the frequency of physical aggression at 19 months may be substantially driven by genetically based factors." Their study, they say, is the first to demonstrate the heritability of physical aggression at such a young age. The researchers say their findings highlight the importance of identifying children with language delays early in life, and offering intervention.
"Physical aggression and expressive vocabulary in 19- month-old twins," Ginette Dionne, Richard Tremblay, Michel Boivin, David Laplante, and Daniel Perusse, Developmental Psychology, Vol. 39, No. 2, 2003, 261-73. Address: Ginette Dionne, Ecole de psychologie, Universite Laval, Quebec, Quebec G1K 7P4, Canada. firstname.lastname@example.org.