The paradox of delusions: are deluded individuals resistant to evidence?

Delusions are pathological beliefs, often with bizarre content. They feature in neurological disorders and in psychoses associated with psychiatric disorders (e.g., schizophrenia). This chapter reviews two apparently opposing ways that the research literature characterizes delusions. While delusions are often defined as beliefs that are rigid and unaffected by evidence, they are also frequently conceptualized as beliefs that are unduly swayed by minimal, insufficient evidence. The authors review empirical studies and theory from psychology, neuroscience, and computational psychiatry which embody this contrast. Prevailing perspectives, they show, frame belief formation as a process of integrating new evidence into existing beliefs. This view tends to sculpt hypotheses about delusions (and other psychotic symptoms such as hallucinations) into two types. First, relevant evidence fails to affect prior beliefs, which are too entrenched. Second, evidence overly dominates prior beliefs, which have too weak an influence. This paradox calls for a profound rethinking of these perplexing and distressing symptoms.