DiLalla, L.F. (1999). Biological and environmental pathways to aggressive behavior in children. Paper presented to the Society for Research on Child Development, April, Albuquerque, NM.

Early childhood social competence has been demonstrated to relate to problem behaviors later in life. Although most of the research on early child aggression has focused on environmental etiologies, there is a small but growing body of research examining the development of aggression in children from a behavior genetics perspective. Genetic influences have been identified for violent behaviors in adults, primarily using adoption studies, but less is known about the genetic influences on aggressive behaviors in non-clinical samples of children. Equally important are the genetic influences on cooperative behaviors in children, as these represent an alternate behavior from aggression during normal peer interactions. The present project assessed cooperative and aggressive behaviors in preschoolers during uninstructed free play with same-age, same-sex peers. In order to explore influences of genotype and environment on these behaviors, identical or monozygotic (MZ) and fraternal or dizygotic (DZ) twins were compared. If genetic influences are important, then MZ twins, who share 100% of their genes, should behave more similarly than DZ twins, who share on average 50% of their genes.

Five-year-old twins were brought to a laboratory playroom where they were paired with same-sex, five-year-old children whom they had not met before. One member of the twin pair and one singleton child were allowed to play freely for 20 minutes in the playroom. In the room were a toolbench, puppets, action figures, kitchen toys, and a checker set. Afterwards, the other twin and another singleton child were allowed to play freely for 20 minutes. Play was videotaped and later rated by trained coders. Children were coded on Aggression and Agreeableness scales using 5-point rating scales. Parents also were asked to complete a series of questionnaires, including the Behavioral Style Questionnaire (BSQ; McDevitt & Carey, 1978) and the Child Behavior Checklist (CBCL; Achenbach, 1991).

In addition to testing twins, siblings of many singleton children who had been tested in the free play paradigm were brought to the lab for comparable testing. Therefore, the final sample comprised identical twins, fraternal twins, and sibling pairs (not necessarily the same sex) who were tested at different points in time but at the same ages. Intra-pair correlations were calculated for each on the Aggression and Agreeableness ratings from the behavioral ratings during free play and from the parent ratings using the BSQ and the CBCL. The identical twins were much more similar to each other on all measures than were the other two sibling types, whereas fraternal twins and sibling pairs were comparable to each other. For example, on a second-order factor of Agreeableness using the BSQ, MZ twins were correlated .96, whereas DZ twins were correlated .47 and siblings were correlated .49.

These results are the first to suggest that genetic influences are important for both aggressive and cooperative play behaviors in preschoolers during free play with unfamiliar peers. These influences must be considered in addition to environmental ones in developing a full understanding of children’s abilities to interact appropriately with peers in novel situations.