BACKGROUND: Research has documented high levels of co-morbidity among childhood externalizing disorders, but its etiology remains in dispute. Specifically, although all behavior genetic studies of the etiology of the co-occurrence of @entity71 ( @entity71 ), @entity952 ( @entity952 ), and @entity9054 ( @entity9054 ) agree that genetic factors are important, differences exist across studies in the relative weight assigned to genetic, shared environmental factors (i.e. factors that increase similarity among family members), and non-shared environmental factors (i.e. factors that decrease similarity among family members). Because heritability estimates can vary across informants, we used a biometric informant-effects model to determine whether these discrepancies were a function of systematic differences in maternal and @entity1 informant reports of @entity71 , @entity9054 , and @entity952 . METHOD: We studied 1782 11-year-old twins from the Minnesota Twin Family Study. Symptom counts for each disorder were obtained from interviews administered to twins and their mothers. We fit a model that allowed us to examine, both across and within informants, the genetic and environmental contributions to the co-occurrence among @entity71 , @entity9054 , and @entity952 . RESULTS: The results revealed that the co-occurrence among the disorders common to maternal and @entity1 informant reports was influenced largely by shared environmental forces. Genetic factors also contributed, though their impact was only marginally significant. In contrast, the co-occurrence unique to each informant was influenced exclusively by either genetic or non-shared environmental factors. CONCLUSIONS: Such findings offer additional evidence that shared environmental factors are important to the co-morbidity among @entity71 , @entity9054 , and @entity952 , and highlight the necessity of considering informant effects when drawing conclusions about the origins of co-morbidity from analyses of genetically informative data.

Sources of covariation among the XXXX -externalizing disorders: informant effects and the shared environment.

@entity1 :: ('9606', 'Species') :: ['child'] @entity71 :: ('MESH:D001289', 'Disease') :: ['ADHD', 'attention deficit-hyperactivity disorder'] @entity9054 :: ('MESH:D019955', 'Disease') :: ['CD', 'conduct disorder'] @entity952 :: ('MESH:D019958', 'Disease') :: ['ODD', 'oppositional defiant disorder']
@entity1 :: (9606,Species) :: ['child']
