Twins' rearing environment similarity and childhood externalizing disorders: a test of the equal environments assumption.

The equal environments assumption (EEA) of the twin method posits that environmental influences that are etiologically relevant to a given phenotype are no more likely to be shared by monozygotic (MZ) than dizygotic (DZ) twin pairs. One method of testing the EEA is to evaluate whether increased rearing environment similarity in MZ twin pairs compared to DZ twin pairs is related to increased phenotypic correlation. In a sample of 885 twin pairs, we contrasted similarity in rearing environment between MZ and DZ twin pairs, examined the correlation between similarity in rearing environment and conduct disorder (CD), oppositional-defiant disorder (ODD), inattention, and hyperactivity-impulsivity symptom dimensions, and tested the effects of differential similarity in rearing environments between MZ and DZ twin pairs by testing whether rearing environment similarity moderated the correlations for the externalizing symptom dimensions. We found that MZ twins experienced substantially more similar rearing environments than DZ twins, but that there was little evidence that MZ and DZ correlations for the externalizing symptom dimensions varied by rearing environment similarity. Thus, these results constitute evidence for the validity of the EEA for childhood externalizing disorders.