Theoretical and methodological issues in psychiatric comorbidity

The occurrence of multiple diagnoses in one patient is a phenomenon of majo r clinical and theoretical importance. This paper reviews the various facto rs involved in real and artifactual comorbidity. Important causes of spurio us comorbidity are discussed, Including invalidity of the individual diagno ses, use of inappropriate diagnostic paradigms, descriptive overlap of diag nostic criteria, ascertainment bias, and diagnostic bias. To illustrate som e of the concepts discussed, two examples are presented: the comorbidity of schizophrenia and substance use disorders, and the comorbidity of posttrau matic stress disorder and major depression. The study of comorbidity can ad vance psychiatry by helping us to clarify our thinking about categories of illness and the boundaries between them, as well as the relationships among these categories.