Monothematic Delusions and the Limits of Rationality

Today's post is by Quinn Hiroshi Gibson and Adam Bradley, on how to understand monothematic delusions.

Quinn Hiroshi Gibson

Subjects with Capgras delusion form the delusion that a loved one has been replaced by an imposter:

The day after her arrival at home, [her] father could not open the front door because YY had locked it from the inside. He rang the bell and YY called the police because ‘there was an impostor outside the house who was picking the lock and pretending to be her father’. (Brighetti at al. 2007, p. 191)

 

Capgras is a monothematic delusion, a delusion whose content is restricted to a single topic, in this case the identity of YY's father.

In ‘Monothematic Delusions and the Limits of Rationality’ (published in the British Journal for the Philosophy of Science in 2021), we put forward a new account of such delusions. Our view is a version of the two-factor model according to which two factors are responsible for monothematic delusions (Davies et al. 2001). The first is a disruption in experience, e.g., a missing affective response to loved ones. The second is a cognitive impairment which inclines the subject towards delusional belief. This second factor distinguishes subjects with disordered experience but no delusion from subjects with disordered experience who form the delusion (Tranel et al. 1995).

Adam Bradley

We are motivated to develop a new view because we believe that existing theories do not satisfactorily explain why subjects initially entertain the delusional thought, e.g. "This person who looks like my father is an imposter" (Parrott 2016). The contents of most monothematic delusions are bizarre, and one wonders why subjects would even consider them, let alone believe them. A satisfactory account of monothematic delusions should help us understand this.

Existing two-factor views cannot explain the subject’s initial entertainment of the delusional thought because of their commitment to Maherian Rationality:

Maherian Rationality: The cognitive processes involved in delusion entertainment, adoption, and maintenance are of a generally rational kind whose operation is impaired in the domain of the delusion.

Traditional endorsement and explanationist views both seek to explain delusion formation on the model of rational belief-forming processes that have gone off the rails: either endorsement of the content of experience as a perceptual belief, or else an attempt to explain one’s disordered experience. But both endorsement and explanation are, in general, rational processes. So these views seek to explain delusion formation on the model of rational cognitive processes.

But the contents of monothematic delusions are typically so bizarre and that we must look beyond the rational faculties of the mind in order to explain them. When a subject with the Cotard delusion forms the belief that she is dead, she is not endorsing the content of experience or trying to explain anything. Instead, we argue, she is trying to express what her experience is like. Similarly, YY is not trying to explain bizarre experiential ‘data’, or taking on the content of her experience at face value. Instead, she is trying to articulate what her experience of her father is like.

The contents of monothematic delusions arise, then, not out of any rational process (not even a malfunctioning one), but from the sorts of processes involved in figurative thought and metaphorical language. There we have a rich repository of associations to draw on, associations which have already laid down ‘tracks’ in the mind along which thought naturally flows. 

 On our view, the delusional subject entertains thoughts like "I am dead" or "My wife has been replaced by an impostor" for the same reason that one might say "I am going to explode" as a way of describing their anger. We have a latent supply of figurative and metaphorical associations to draw on in thought and speech. These connections are already intelligible to us--we track them to understand poetry, for instance--but are not rational.

What is distinctive about the delusional subject is that she does not merely entertain such thoughts, she adopts them as beliefs. This calls for a second factor. Cotard delusion is typically thought to result from severe depression. But subjects who are not delusional express feelings of severe depression using terms such as ‘dead’ or ‘inanimate’ or ‘nonexistent’. A subject suffering from depersonalization may say: "I feel as if I am dead". The delusional subject, however, just thinks "I’m dead". They lose the ‘as if’ operator. 

We posit, therefore, that the second factor that causes the thought to become fixed as a delusion is a selective deficit in the subject’s capacity to understand figurative language as figurative in the domain of the delusion. As a result, delusional subjects come to confuse their figurative expressions of their experience for the literal truth.