Delusions across and beyond Diagnoses

This chapter considers the heterogeneity of delusions. There is some controversy about what features of delusions should be considered defining features, and this is in part due to the observation that different types of delusions manifest differently. For instance, whereas some delusions have content that is not just false but impossible, other delusions can be true. Clinically, delusions vary in terms of theme, severity, and other key features, such as the extent to which they are elaborated or widespread in the person’s belief system. Moreover, delusions can emerge in the context of a number of disorders and syndromes, being a truly transdiagnostic phenomenon. The concept of delusions has also been applied to some beliefs that people report without attracting any psychiatric diagnosis: conspiracy beliefs, unrealistically optimistic attitudes, and forms of denialism. The features of “everyday delusions” overlap only partially with the features of their clinical counterparts: beliefs are called delusional when they are implausible, they are rarely given up in the face of counterevidence, and they play an important role in the person’s mental life. The heterogeneity of the phenomenon of delusions suggests that different models of explanation and methodological approaches can be integrated to capture its complexity, including those inspired by neurobiology, cognitive psychology, psychoanalysis, and phenomenology.