On what makes delusions pathological

Dr Kengo Miyazono – Research Fellow – University of Birmingham

Delusional beliefs are typ­ic­ally patho­lo­gic­al. Being patho­lo­gic­al is not the same as being false or being irra­tion­al. A woman might falsely believe that Istanbul is the cap­it­al of Turkey, but it might just be a simple mis­take. A man might believe without good evid­ence that he is smarter than his col­leagues, but it might just be a healthy self-deceptive belief. On the oth­er hand, when a patient with brain dam­age caused by a car acci­dent believes that his fath­er was replaced by an imposter, or when anoth­er patient with schizo­phrenia believes that ‘The Organization’ painted the doors of the houses on a street as a mes­sage to him, these beliefs are not merely false or irra­tion­al. They are pathological.

What makes delu­sion­al beliefs patho­lo­gic­al? One might think, for example, that delu­sions are patho­lo­gic­al because of their extreme irra­tion­al­ity. The prob­lem with this view, how­ever, is that it is not obvi­ous that delu­sion­al beliefs are extremely irra­tion­al. Maher (1974), for example, argues that delu­sions are reas­on­able explan­a­tions of abnor­mal experience.

“[T]he explan­a­tions (i.e. the delu­sions) of the patient are derived by cog­nit­ive activ­ity that is essen­tially indis­tin­guish­able from that employed by non-patients, by sci­ent­ists, and by people gen­er­ally. The struc­tur­al coher­ence and intern­al con­sist­ency of the explan­a­tion will be a reflec­tion of the intel­li­gence of the indi­vidu­al patient.” (Maher 1974, 103)

Again,  Coltheart and col­leagues (2010) argue that it is ration­al, from the Bayesian point of view, for a per­son with the Capgras delu­sion to adopt the delu­sion­al hypo­thes­is giv­en his neuro­psy­cho­lo­gic­al defi­cits. Bayes’s the­or­em pre­scribes a math­em­at­ic­al pro­ced­ure of updat­ing the prob­ab­il­ity of a hypo­thes­is on the basis of pri­or beliefs and new obser­va­tions. Coltheart and col­leagues claim that the delu­sion­al hypo­theses get high­er prob­ab­il­it­ies than com­pet­ing non-delusional hypo­theses giv­en rel­ev­ant pri­or beliefs and the obser­va­tions of the neuro­psy­cho­lo­gic­al deficits.

“The delu­sion­al hypo­thes­is provides a much more con­vin­cing explan­a­tion of the highly unusu­al data than the nondelu­sion­al hypo­thes­is; and this fact swamps the gen­er­al implaus­ib­il­ity of the delu­sion­al hypo­thes­is. So if the sub­ject with Capgras delu­sion uncon­sciously reas­ons in this way, he has up to this point com­mit­ted no mis­take of ration­al­ity on the Bayesian mod­el.” (Coltheart, Menzies, & Sutton 2010, 278)

The claim by Coltheart and col­leagues is, how­ever, con­tro­ver­sial. In response, McKay (2012) argues that adopt­ing delu­sion­al hypo­theses is due to the irra­tion­al bias of dis­count­ing the ratio of pri­or prob­ab­il­it­ies. Even if McKay is cor­rect, how­ever, it is not clear that delu­sion­al beliefs are extremely irra­tion­al since sim­il­ar biases might be found among nor­mal people as well.

For instance, in the fam­ous exper­i­ment by Kahneman and Tversky (1973), nor­mal sub­jects, first, received the base-rate inform­a­tion about a hypo­thet­ic­al group of people (e.g., “30 engin­eers and 70 law­yers”). Then, the per­son­al­ity descrip­tion of a par­tic­u­lar per­son in the group was provided and the sub­jects were asked to pre­dict the occu­pa­tion (e.g., an engin­eer or a law­yer) of the per­son. The cru­cial find­ing was that the manip­u­la­tion of the base-rate inform­a­tion, which provides the pri­or prob­ab­il­ity of the hypo­theses at issue (e.g., the hypo­thes­is that this per­son is a law­yer), had almost no effect on the pre­dic­tion of the sub­jects (“base-rate neg­lect”). The find­ing sug­gests that the bias of dis­count­ing pri­or prob­ab­il­it­ies can be seen among nor­mal people. As Bortolotti poin­ted out (2009), the irra­tion­al­ity that we find in people with delu­sions might not be very dif­fer­ent from the irra­tion­al­ity we find in nor­mal people.

It is even con­ceiv­able that people with delu­sions are more ration­al than nor­mal people. In the well-known exper­i­ment by Huq and col­leagues, the sub­jects were asked to determ­ine wheth­er a giv­en jar is the jar A, which con­tains 85 pink beads and 15 green beads, or the jar B, which con­tains 15 pink beads and 85 green beads, on the basis of the obser­va­tion of the beads drawn from it. It was found that the sub­jects with delu­sions need less evid­ence (i.e., less beads drawn from the jar) before com­ing to the con­clu­sion than the sub­jects in con­trol groups (“jumping-to-conclusion bias”). Interestingly, Huq and col­leagues do not take this to show that the sub­jects with delu­sions are irra­tion­al. Rather, they note; “it may be argued that the deluded sample reached a decision at an object­ively “ration­al” point. It may fur­ther be argued that the two con­trol groups were some­what over­cau­tious” (Huq et al. 1988, 809) (but see Van Der Leer et al. 2015).

In my paper, Delusions as Harmful Malfunctioning Beliefs (http://www.sciencedirect.com/science/article/pii/S1053810014002001), I also exam­ine the views accord­ing to which delu­sion­al beliefs are patho­lo­gic­al because of (1) their strange con­tent, (2) their res­ist­ance to folk psy­cho­lo­gic­al explan­a­tions and (3) the impaired responsibility-grounding capa­cit­ies. I provide some counter­examples as well as dif­fi­culties for these proposals.

I argue, fol­low­ing Wakefield’s (1992a, 1992b) harm­ful dys­func­tion ana­lys­is of dis­order, that delu­sion­al beliefs are patho­lo­gic­al because they involve some kinds of harm­ful mal­func­tions. In oth­er words, they have a sig­ni­fic­ant neg­at­ive impact on well­being (harm­ful) and, in addi­tion, some psy­cho­lo­gic­al mech­an­isms, dir­ectly or indir­ectly related to them, fail to per­form the func­tions for which they were selec­ted (mal­func­tion­ing).

There can be two types of objec­tions to the pro­pos­al. The first type of objec­tion is that delu­sion­al beliefs might not involve any harm­ful mal­func­tions. For example, delu­sion­al beliefs might be play­ing psy­cho­lo­gic­al defence func­tions. The second type of objec­tion is that involving harm­ful mal­func­tion­ings is not suf­fi­cient for a men­tal state to be patho­lo­gic­al. For example, false beliefs might involve some mal­func­tions accord­ing to tele­ose­mantics (Dretske 1991; Millikan 1989). But, there could be harm­ful false beliefs that are not patho­lo­gic­al. The paper defends the pro­pos­al from these objections.



Bortolotti, L. 2010. Delusions and oth­er irra­tion­al beliefs. Oxford: Oxford University Press.

Coltheart, M., Menzies, P. and Sutton, J. 2010. Abductive infer­ence and delu­sion­al belief. Cognitive Neuropsychiatry 15(1–3), pp. 261–287.

Dretske, F. I. 1991. Explaining beha­vi­or: Reasons in a world of causes. Cambridge, MA: The MIT Press.

Huq, S., Garety, P. and Hemsley, D. 1988. Probabilistic judge­ments in deluded and non-deluded sub­jects. The Quarterly Journal of Experimental Psychology 40(4), pp. 801–812.

Kahneman, D. and Tversky, A. 1973. On the psy­cho­logy of pre­dic­tion. Psychological Review 80(4), pp 237- 251.

Maher, B. A. 1974. Delusional think­ing and per­cep­tu­al dis­order. Journal of Individual Psychology 30, pp. 98–113.

McKay, R. 2012. Delusional infer­ence. Mind & Language 27(3), pp. 330–355.

Millikan, R. G. 1989. Biosemantics. The Journal of Philosophy 86, pp. 281–297.

Van Der Leer, L., Hartig, B., Goldmanis, M. and McKay, R. 2015. Delusion-­proneness and ‘Jumping to Conclusions’: Relative and abso­lute effects. Psychological Medicine 19(3), pp. 257–67.

Wakefield, J. C. 1992a. The Concept of Mental Disorder: On the bound­ary between bio­lo­gic­al facts and social val­ues. American Psychologist 47(3), pp. 373–388.

Wakefield, J. C. 1992b. Disorder as harm­ful dys­func­tion: A con­cep­tu­al cri­tique of DSM-III‑R’s defin­i­tion of men­tal dis­order. Psychological Review 99(2), pp. 232–247.