Genetic and environmental effects on aggression

This study was part of the Longitudinal Study of Health and Behavior in Twin Children (FinnTwin12) in which baseline data were collected at age 12 with five consecutive twin cohorts including about 3000 twin pairs, and follow-up data at age 14. The role of genetic and environmental factors was examined at ages 12 and 14, and in continuity and change of aggressive behavior at those ages. In addition, analyses were expanded to ask to what extent the correlation between aggression and hyperactivity-impulsivity was due to shared genetic or shared environmental influences. Sex differences in genetic and environmental effects, rater differences, and interaction between the co-twins were considered throughout the study. The results suggested significant genetic, common environmental and unique environmental effects on aggression in both boys and girls, but model-fitting revealed differences across sexes and between teacher and parental ratings. For both sets of ratings at age 12, girls' aggression yielded higher heritabilities than boys' aggression, which in turn showed greater common environmental effects. Additionally, teacher-ratings on aggressive behavior suggested the presence of either sex-specific common environmental effects or sex-specific genetic effects. Teacher ratings at age 14 revealed an increase in the relative proportion of genetic effects in boys and of common environmental effects in girls. The longitudinal model-fitting results emphasized the significant role of genes and common environmental factors for boys and common and unique environment for girls in the continuity of aggression. Genes and unique environment contributed to change in aggression in both sexes. The correlation between aggression and hyperactivity was due to extensive genetic and common environmental overlap. However, roughly half of the variance in hyperactivity-impulsivity was accounted for by genetic and environmental factors specific to this phenotype, also suggesting in part a different etiology for these behaviors.