Using shared and unique parental views to study the etiology of 7-year-old twins' internalizing and externalizing problems.

In a sample of 1,940 Dutch 7-year-old twin pairs we studied the etiology of individual differences in Internalizing and Externalizing behavioral problems. For the majority of twins in the sample, both maternal and paternal ratings of behavioral problems were obtained from the Child Behavior Checklist. This made it possible to take into account processes underlying agreement and disagreement between maternal and paternal ratings. For both problem behaviors, a Psychometric model fitted the data better than a Rater Bias model, implying that parents, in addition to the behaviors they similarly observed, also assessed unique aspects of their children's behaviors. Relatively large genetic influences were found for Externalizing problems, explaining over 50% of the variance in both boys and girls. For internalizing problems, the heritability was over 30% in both sexes. Shared environmental factors were nearly as important as genetic influences in explaining the variation in behavioral problems. For both Externalizing and Internalizing problems, around 30% of the variance was accounted for by the shared environmental factors.