We next turned to examining the specific genetic and environmental influences on each of the internalizing syndromes to determine which etiological factors contribute to differences between the observed phenotypes. We constructed a residual score from a regression equation predicting each of the individual syndromes from the total internalizing factor score (e.g., the residual of a regression equation predicting depression from internalizing). We then examined the MZ and DZ twin correlations for each of these four residual scores (see Table 1). Of note, almost all twin correlations were small and non-significant (MZ correlations were .11, −.06, and −.05 for depression, generalized anxiety, and panic attack, respectively; corresponding DZ correlations were .05, .03, −.15). These findings suggest strong etiologic overlap among the symptoms of these disorders, and argue against any familial (genetic or shared environmental) influences specific to each of the individual syndromes. The unique variance in the symptoms of each disorder were largely due to non-shared environmental effects or error in the psychometric sense (i.e., unique idiosyncratic variation in a variable that does not correlate with other variables). The only significant
