Models of misbelief: Integrating motivational and deficit theories of delusions

Humans are agents that want and like, and the impact of our desires and preferences upon our ordinary, everyday beliefs is well-documented (Gilovich, 1991). The influence of such motivational factors on delusions, which are instances of pathological misbelief, has tended however to be neglected by certain prevailing models of delusion formation and maintenance (e.g. Ellis and Young, 1990; Stone and Young, 1997; Davies & Coltheart, 2000; Langdon & Coltheart, 2000; Davies, Coltheart, Langdon & Breen, 2001). This paper explores a distinction between two general classes of theoretical explanation for delusions; the motivational and the deficit. Motivational approaches view delusions as extreme instances of self-deception; as defensive attempts to relieve pain and distress. Deficit approaches, in contrast, view delusions as the consequence of defects in the normal functioning of belief mechanisms, underpinned by neuroanatomical or neurophysiological abnormalities. It is argued that although there are good reasons to be sceptical of motivational theories (particularly in their more floridly psychodynamic manifestations), recent experiments confirm that motives are important causal forces where delusions are concerned. It is therefore concluded that the most comprehensive account of delusions will involve a theoretical unification of both motivational and deficit approaches. An attempt is made to develop just such a rapprochement, taking as its point of departure a current cognitive neuropsychiatric model of delusion formation, the two-deficit model of Coltheart, Langdon, Davies and Breen.