Work in the field of cognitive neuropsychiatry over the past 20 years has made it plain that various forms of delusional belief are scientifically understandable in the sense that plausible neuropsychological explanations of their nature and genesis have been formulated. A two-factor theory of delusional belief has emerged from this work. According to this theory, explaining the presence of a delusion requires (a) the presence a neuropsychological impairment that initially prompts the delusional belief and (b) the presence of a second neuropsychological impairment that interferes with processes of belief evaluation that would otherwise cause the delusional belief to be rejected. A very similar account of delusion has recently emerged from research on hypothesis evaluation using the associative-learning paradigm with healthy control subjects and people with psychosis. Neuroimaging studies in this context suggest that the region of the brain specifically involved in hypothesis evaluation (and therefore, according to the two-factor theory, impaired in people with delusions) is the right lateral prefrontal cortex.