Theory of Mind Delusions and Bizarre Delusions
in an Evolutionary Perspective: psychiatry and the social brain

Bruce G Charlton MD

Bruce G Charlton MD
Reader in Evolutionary Psychiatry
Department of Psychology
University of Newcastle upon Tyne
NE2 4HH
England

Editor-in-Chief, Medical Hypotheses

Tel: 0191 222 6247
Fax: 0191 222 5622
bruce.charlton@ncl.ac.uk

The social brain and psychiatry

The phrase 'social brain' embodies the idea that the problems of living in a complex social group have been a dominant selection pressure in recent human evolutionary history (Humphrey, 1976; Byrne & Whiten, 1988; Brothers, 1990). One consequence is that many distinctively human behaviours can be linked with adaptations for social living.

The perspective of ‘the social brain’ has particular relevance to psychiatry, since 'psychiatric symptoms (eg. hallucinations, delusions, phobias, obsessions) are frequently dominated by social content, and a disruption of social relationships is highly characteristic of psychiatric illness. Indeed, it might plausibly be argued that the distinctive nature of many psychiatric illnesses - that thing that makes them 'psychiatric' - may be the combination of emotional pathology with social impairment. Certainly, the emotional and the social are intimately related at the level of brain function, since social reasoning depends upon evolved brain systems for monitoring and modelling emotional responses to social scenarios (Damasio, 1994; Charlton, 2000).

The following chapter will demonstrate how a human social evolution has been used to clarify and refine the diagnostic category of delusions. I will argue that two distinct types of delusions may be discriminated: ‘Theory of Mind’ delusions and ‘Bizarre’ delusions.

Delusional disorder

Delusional disorder (DD) may be described as a psychiatric condition in which a delusion is the primary symptom, and patients are otherwise ‘normal’ (Charlton & McClelland, 1999). The subject matter of the delusion is variable; with persecutory, jealous, grandiose, erotomaniac and somatic subtypes being recognised (APA, 1994). Furthermore, Delusional disorder is characterised by specifically social abnormalities of behaviour - such as morbid states of jealousy, love, self-awareness or fear of other people (Charlton & McClelland, 1999).

Delusions are false beliefs. However, this is an insufficient definition, and further attempts at definition are all somewhat unsatisfactory (Garety & Hemsley, 1994). Usually a delusion is defined as a false belief that is also strongly held such that it exerts a strong influence on behaviour and is not susceptible to counter-arguments or counter-evidence (or, at least, the delusion is unshakeable over a short timescale and in the absence of systematic attempts at belief modification). Furthermore, in order to distinguish delusions from 'religious' beliefs, a delusion is also supposed to be out of context with the usual cultural beliefs for that society (Sims, 1995).

Delusional disorder is an unusual diagnosis in general psychiatric practice, and delusions are most frequently seen to occur as only one element in more complex clinical syndromes. In other words, most delusions are observed along with other 'psychotic' symptoms such as hallucinations or incoherent speech ('thought disorder') as part of one of the classic syndromes of ‘madness’ such as schizophrenia, mania, psychotic depression, and 'organic' symptoms indicative of generalised brain dysfunction such as dementia or delirium.

But in Delusional disorder false beliefs occur largely in isolation as ‘encapsulated’ delusions. Such individuals do not have other primary psychological symptoms such as hallucinations, incoherent speech or qualitatively abnormal mood states (although the delusions may lead to secondary symptoms, for instance a belief in persecution may lead to secondary emotional change, such as fear or anger specifically in relation to the imagined persecutors). It therefore seems likely that the majority of people diagnosable with Delusional disorder are never seen by psychiatrists (Charlton & McClelland, 1999; Walston et al, 2000).

Theory of Mind (ToM)

It has recently been noted that the subject matter of Delusional disorders is distinctive, since the false beliefs are ‘social’ in content, and typically concerned with the assumed dispositions, motivations and intentions (DMIs) of other people. For this reason, Delusional disorders have been labelled ‘Theory of Mind’ delusions, because they seem to involve inferences (or theories) about what is going on in the minds of other people (Charlton & McClelland, 1999).

For example the commonest types of Delusional disorder are probably those relating to jealousy over sexual infidelity and those in which there is a false belief of persecution. Both are considered to be commoner in men. Jealous delusions may involve a man believing that his wife is concealing from him that she is having an affair with another man, while persecutory delusions typically involve a man believing that he is the victim of a hostile plot to attack and probably kill him. It has been suggested that these delusions derive from errors of inference relating to the contents of other people’s minds.

Since Delusional disorder is related to reasoning about the dispositions, motivations and intentions of other people, the mechanism by which such reasoning is performed in humans requires consideration. Theory of Mind is the ability - displayed by adult humans - of making inferences about the content of other people’s minds. Beyond this bald statement - conceptualisations of Theory of Mind vary widely between published accounts, and in different branches of biology. I suggest (for reasons argued elsewhere; Charlton, 2000) that the essence of ToM, its central adaptive importance and the reason why it evolved, is that the Theory of Mind mechanism is primarily concerned with making inferences concerning the dispositions, motivations and intentions of other people. It is not, therefore, a mechanism which evolved for making theories about the 'factual' or 'knowledge' content of other people's minds - although in language-using humans the ToM mechanism may be used for this purpose.

The ToM mechanism enables other people’s behaviour to be interpreted in the light of inferred DMIs. This is necessary because many human social behaviours are ambiguous unless interpreted with knowledge of 'intent'. For example, a clenched fist can be a threat, a salute, or a gesture of encouragement, according to the motivation of the fist-wielder. Discrimination between these different meanings requires an understanding of the social context of behaviour including the individual differences between human beings, and the different ways in which these different human beings interact.

For example only by knowing the DMIs of others can we distinguish between friends and foes, know who to trust and who to avoid - and in general build those alliances that underpin human society - especially in those small scale tribal societies in which human evolved (Walston