The everyday irrationality of monothematic delusion

Irrationality comes in hot and cold varieties. The hot involves the influence of motivation, or emotion more generally, the cold does not. Under which category does delusion lie, or can it come in both varieties? Empiricist accounts of monothematic delusion (those delusions concerning a single theme) claim that anomalous experiences are explanatorily relevant to the formation of the distinctive delusional beliefs. So far, no irrationality has been identified. But many philosophers and psychologists have argued that, in addition, there is a special clinically significant irrationality involved in delusion formation, understood as the manifestation of some cognitive deficit, bias, or performance error. All of these are cold varieties of irrationality with the possible exception of some kinds of performance error. Those who propose such irrationality note that not all people with anomalous experiences develop delusions, and so, the thought goes, we should explain why some subjects develop delusions off the back of anomalous experiences by identifying some clinically significant irrationality at work.

We have argued that it is a mistake to argue from variation in response to experience to the presence of some clinical irrationality in folk who form delusions (Noordhof and Sullivan-Bissett 2021). Individual differences of various kinds explain why some subjects have delusional beliefs and others do not. Actually, we do not think that all cases of delusion involve irrationality but we set aside that possibility here. Where delusions are the result of irrationality, the irrationality involved is quite everyday. In this paper we argue for this claim in two ways. First, an examination of non-clinical paranormal beliefs shows that it is a mistake to think that monothematic delusional belief usually involves a kind of irrationality distinct from everyday kinds. Crucially, in these non-clinical cases, we have beliefs with bizarre contents, often based on strange experiences that are also had by subjects who do not form paranormal beliefs. Nevertheless, researchers interested in their genesis have not sought to identify clinically significant irrationality. By reflecting on the structure of non-clinical cases of paranormal belief, we show that the appeal to clinically significant irrationality to explain delusion formation is unmotivated. These everyday irrationalities may be ‘cold’ – that is, not the result of motivation – but they are still mundane.

Second, there are various kinds of motivated irrationality in everyday life with which we are familiar, for example, wishful thinking and self-deception. It wouldn’t be surprising if some cases of delusion involved these forms of irrationality and, as we shall see, there is evidence that they do. But, here again, the situation is rather different from what might reasonably be called the orthodoxy. Delusions are often seen as the most serious form of irrationality, at the extreme end of self-deception or involving a further, more substantial form of irrationality. This is a mistake. Motivated irrationalities are more substantially irrational than the cold cases of delusion and when delusions run hot, they typically involve fewer motivational features than everyday motivated irrationality. Overall then, we conclude that not only do we need not appeal to any clinical irrationality to explain monothematic delusion formation, but that in addition, the reputation of delusions as the most serious form of irrationality is unmerited. Throughout the paper, we remain neutral on the question of whether monothematic delusions more often come in cold or hot varieties.