Rethinking psychiatric symptoms: the role of measurement heterogeneity in diagnostic validity

The current research environment in psychiatry is marked by the discredit of the main psychiatric classifications. The common narrative about the DSM holds that the current diagnostic categories lack diagnostic validity. This claim is supported by the high degrees of diagnostic heterogeneity and comorbidity among diagnosed patients. Current attempts to overcome these problems emphasize the need to develop alternative ways of investigating psychopathology that no longer rely on the DSM categories. In this line, transdiagnostic research initiatives such as RDoC promote the abandonment of the DSM categories while still relying on traditional psychiatric symptoms. This reliance assumes that symptoms do not pose similar problems to the ones commonly ascribed to the DSM categories. In this article, I challenge what I call the "received view of symptoms" and argue that a closer look at symptom measurement reveals that different measurements of purportedly the same symptom differ from each other in ways that have an impact on both psychiatric research and clinical practice. Furthermore, I show that psychiatric symptoms are not "neutral" vis-à-vis the DSM categories. To illustrate my points, I use a case study from the history of the measurement of anhedonia. Based on this case study, I argue that in addition to diagnostic heterogeneity, there is also symptom measurement heterogeneity. Finally, I suggest a reassessment of the common narrative about the DSM's lack of diagnostic validity in light of my case study and argue that closer attention should be given to symptoms.