J Abnorm Psychol 97: 346-353

Distinguishing between depression and anxiety has been a matter of concern and controversy for some time. Studies in normal samples have suggested, however, that assessment of two broad mood factors—Negative Affect (NA) and Positive Affect (PA)—may improve their differentiation. The present study extends these findings to a clinical sample. As part of an ongoing twin study, 90 inpatient probands and 60 cotwins were interviewed with the anxiety and depression sections of the Diagnostic Interview Schedule (DIS; Robins, Helzer, Croughan, & Ratcliff, 1981). Respondents also completed trait NA and PA scales. Consistent with previous research, NA was broadly correlated with symptoms and diagnoses of both anxiety and depression, and acted as a general predictor of psychiatric disorder. In contrast, PA was consistently related (negatively) only to symptoms and diagnoses of depression, indicating that the loss of pleasurable engagement is a distinctive feature of depression. The results suggest that strenghtening the PA component in depression measures may enhance their discriminative power.