Behavioral genetic research has concluded that the more important environmental influences result in differences between siblings (referred to as nonshared; e2), whereas environmental influences that create similarities between siblings (referred to as shared; c2) are indistinguishable from zero. However, there is mounting evidence that during childhood and adolescence, c2 may make important contributions to most forms of psychopathology. The aim of the meta-analysis was to empirically confirm this hypothesis. The author examined twin and adoption studies (n=490) of internalizing and externalizing psychopathology prior to adulthood. Analyses revealed that c2 accounted for 10%-19% of the variance within conduct disorder, oppositional defiant disorder, anxiety, depression, and broad internalizing and externalizing disorders, regardless of their operationalization. When age, informant, and sex effects were considered, c2 generally ranged from 10%-30% of the variance. Importantly, c2 estimates did not vary across twin and adoption studies, suggesting that these estimates reflect actual environmental influences common to siblings. The only exception was attention-deficit/hyperactivity disorder, which appeared to be largely genetic (and particularly nonadditive genetic) in origin. Conceptual, methodological, and clinical implications of these findings are discussed.
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