Genetic and environmental influences on aggression in 4- to 7-year-old twins

Abstract

Aggressive behaviors of 38 twin pairs (21 monozygotic [MZ], 17 dizygotic [DZ]) between the ages of 4 and 7 years were assessed through mothers' ratings and observations in their homes. An observation checklist was developed in which mothers were asked to observe and report each child's specific aggressive behaviors for a total of 6 hours spread over 3 days. In particular the observations focused on the following behaviors: rejection, destructiveness, negativism, noncompliance, teasing, physical negative, insult, verbal threat, and yelling. Biometrical genetic analysis evidenced significant heritable influences on composite aggression scores in both the observation checklist and the mothers' ratings; heritability estimates varied from.24 to.94 depending on the aggression nieasnre used, with scores from the mothers' observations demonstrating smaller genetic effect than those of the parental ratings of the environmental sources of variance, within-family factors were far more important than the between-family ones. The results also yielded significant sex and zygosity differences for means of some of the specific aggressive behaviors, where males and DZ twins scored higher than females and MZ twins. It was suggested that future studies should include more detailed investigations of the context in which specific aggressive behaviors occur and the sources toward which they are directed in order to determine the extent to which different types of aggression may have different etiologies.