What is a'mood-congruent' delusion? History and conceptual problems.

This article investigates the history of the concept of mood-congruent delusions and the problems accompanying this concept. In the late nineteenth century, there were conflicting views regarding the relationship between the contents of an individual's delusional thought and his/her affective state. The differentiation between delusion-like ideas secondary to affective state and incomprehensible primary delusions was introduced in the early twentieth century; this differentiation is the origin of the present-day distinction between mood-congruent and -incongruent delusions. Although the themes of delusions are clearly described in the operational diagnostic criteria for mood-congruent psychotic symptoms, the concept of mood congruence inevitably involves ambiguity. This article argues that a dilemma between reliability and validity emerges when diagnosing mood-congruent (and -incongruent) psychotic symptoms.