Theoretical and methodological issues in psychiatric comorbidity

The occurrence of multiple diagnoses in one patient is a phenomenon of major clinical and theoretical importance. This paper reviews the various factors involved in real and artifactual comorbidity. Important causes of spurious comorbidity are discussed, including invalidity of the individual diagnoses, use of inappropriate diagnostic paradigms, descriptive overlap of diagnostic criteria, ascertainment bias, and diagnostic bias. To illustrate some of the concepts discussed, two examples are presented: the comorbidity of schizophrenia and substance use disorders, and the comorbidity of posttraumatic stress disorder and major depression. The study of comorbidity can advance 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.