What delusions can tell us about the cognitive nature of belief

Beliefs are convictions of reality that we accept as true. They provide us with the basic mental scaffolding to understand and engage meaningfully in our world. Beliefs remain fundamental to our behaviour and identity, but are not well understood.

Delusions, on the other hand, are fixed, usually false, beliefs that are strongly held, but not widely shared. In previous work, we proposed that studying delusions provides unique insights into the cognitive nature of belief and its dysfunction.

Based on evidence from delusions and other psychological disciplines, we offered a tentative five-stage cognitive model of belief formation.

When faced with an unexpected sensory input or social communication, we seek to account for this based on existing beliefs, memories, and other social information. We then evaluate our account in terms of how well this explains our experiences and how consistent it is with our prior beliefs. If it passes these criteria, the belief is accepted. It then guides what we pay attention to and what other ideas we may consider.

We propose that delusions can arise at different stages in this model. Our approach highlights the importance of the individual’s search for meaning and social context in shaping delusions. It also draws attention to the impact of a delusion, once formed, on subsequent perceptions and thinking.

This model linking delusions and beliefs differs from earlier accounts that suggested delusions were distinct from belief or arise as a largely passive response to anomalous sensory input such as a hallucination. Previous research, for example, has found that some people who believed that family members were replaced by impostors (known as Capgras delusion) had deficits in processing familiar faces, which could have generated this idea.

Based on this, some have suggested that other delusions arise in a similar way, but in combination with an as yet undiscovered deficit in the cognitive process of evaluating our beliefs.

But these accounts didn’t fully consider other contributing factors, such as the individual’s prior beliefs, social context and their personal attempts to explain their experiences.

Informative case study

The study of delusions has been informed by select informative case studies. Unlike large group studies, case studies allow researchers a more detailed exploration of the origins and course of clinical features not explained by current theories.

We recently published a paper in the international journal Cortex that describes a unique case study of a woman who temporally experienced compelling delusions during a brief hospital admission for postpartum psychosis, which can give rise to hallucinations, delusions, mood swings and confusion. This is a rare complication of pregnancy, affecting around 1-2 in 1,000 women, thought to be due to hormonal changes or immunological factors.

Natalie (a pseudonym) had no previous medical or psychiatric history. She developed postpartum psychosis while in hospital after the birth of her second child.

As part of her condition, Natalie reported several delusions, including the belief that strangers were her parents-in-law in disguise (known as Fregoli delusion). Natalie recovered quickly with treatment. The combination of interviews and observations while she was experiencing the delusions and her later retrospective account offered a unique window into the onset and experience of her delusions.

Following a full recovery, Natalie confirmed that she considered her delusions to be strongly held beliefs. She likened them to her conviction that her husband was her husband. This is contrary to some views that suggested that delusions are different from normal beliefs.

Natalie was able to identify specific features that contributed to her delusions. In the case of believing that strangers were her in-laws, Natalie identified mannerisms, behaviours and speech patterns of the strangers that reminded her of her in-laws. This suggested that the delusion could have arisen from inappropriate activation of memory representations of familiar people based on these cues and other factors.

Natalie also recalled other beliefs, including that she was dead (known as Cotard delusion), which she did not share with clinicians at the time. She noted that she entertained this idea due to the failure of other explanations to account for her strange experiences and an idea from a television show.

Natalie said she eventually dismissed this idea as implausible, while still holding other delusional ideas. This suggests that belief evaluation may involve different thresholds for different delusions. It also highlights the private nature of some delusions.

Across all of her delusions, Natalie described her active involvement in trying to explain and manage her experiences. She reported considering different explanations and testing these by seeking further information. For example, she asked questions of the people she thought were her in-laws. This suggests a surprisingly similar approach to how we typically form beliefs.

Natalie recalled the influence of television and movies on her ideas. She also recalled how she elaborated on her delusions, once formed, based on information in her surroundings.

These features challenge theories that delusions simply arise from anomalous sensory data. They instead highlight the role of the individual’s search for meaning and social context, as well as the subsequent impact of delusions on perception and thinking.

Implications

As a case study, Natalie’s experiences are not necessarily representative of all people who experience delusions or postpartum psychosis. However, Natalie’s case presents informative features that theories of delusions need to account for.

In particular, Natalie’s personalised insights highlight the critical role of the individual in actively trying to understand their experiences and bestow meaning. This is opposed to just passively accepting beliefs in response to anomalous sensory data or neuropsychological deficits. This suggests psychological therapies may be useful in treating psychosis, in combination with other treatments, in some cases.

More generally, Natalie’s account reveals commonalities between delusions and ordinary beliefs and supports the view that delusions can be understood in terms of cognitive processes across the stages of normal belief formation that we identified.

While there remain challenges in investigating delusions, further study may offer insights into the underpinnings of everyday belief and, in turn, of ourselves.