Dimensions and disorders of adolescent adjustment: a quantitative genetic analysis of unselected samples and selected extremes.

One of the fundamental questions for developmental psychopathology concerns the etiological links between the normal and abnormal. To what extent do disorders differ quantitatively or qualitatively from variation in the normal range? Genetic research on the normal and the abnormal differs in terms of concepts, methods, statistics, and target audiences. An approach, referred to as "DF" analysis, provides a framework for integrating these two worlds of genetic analysis. We applied traditional correlational analyses as well as DF (DeFries & Fulker, 1985) analyses to mother and father ratings of adjustment of adolescent siblings in a 3-year longitudinal twin and step-family study. At wave 1, the sample included 720 sibling pairs (average age of 12.9 years for the younger sibling and 14.5 years for the older siblings) and, in wave 2, 395 pairs still living at home. Both correlational analyses of the entire sample and DF analyses of selected extremes suggested moderate genetic influence and modest shared environmental influence for internalizing and externalizing behavior problems. Similar estimates were found for unselected individual differences and selected extreme groups. A framework is proposed that focuses on quantifying the etiologies of disorders (QED) as measured on continuous dimensions.