What can specificity designs say about causality in psychopathology research?

Specificity designs in psychopathology research are used to examine whether a variable is unique to a given disorder. When nonspecificity is observed, it typically is assumed that the variable in question is not a cause of the disorder, but is instead a nonspecific consequence of general psychopathology. Such reasoning is flawed, however, because it fails to distinguish between testing a particular causal model containing the variable versus testing the causal status of the particular variable within the model. A variable may be nonspecific, but still causal, if it is one of several multiple interacting causes or if a nosological error has been made. A recognition of these complexities has implications for the nature of the psychopathology control group selected.