Reality – Your personal guidebook

Further evidence that this is the case has come from studying how and why belief goes wrong. “When you consider brain damage or psychiatric disorders that produce delusions, you can begin to understand where belief starts,” says Halligan. Such delusions include beliefs that seem bizarre to outsiders but completely natural to the person concerned. For example, people sometimes believe that they are dead, that loved ones have been replaced by imposters, or that their thoughts and actions are being controlled by aliens. And, tellingly, such delusions are often accompanied by disorders of perception, emotional processing or “internal monitoring” – knowing, for example, whether you initiated a specific thought or action.

These deficits are where the delusions start, suggests Robyn Langdon of Macquarie University in Sydney, Australia. People with delusions of alien control, for example, often have faulty motor monitoring, so fail to register actions they have initiated as their own. Likewise, people with the delusion known as “mirror-self misidentification” fail to recognise their own reflection. They often also have a sensory deficit called mirror agnosia: they don’t “get” reflective surfaces. A mirror looks like a window and if asked to retrieve an object reflected in one they will try to reach into the mirror or around it. Their senses are telling them that the person in the mirror isn’t them, and so they believe this to be true. Again, we trust the evidence of our senses, and if they tell us that black is white, we generally do well to believe them.

You may think that you would never be taken in like that but, says Langdon, “we all default to such believing, at least initially”. Consider the experience of watching a magic show. Even though you know it’s all an illusion, your instinctive reaction is that the magician has altered the laws of physics.

Misperceptions are not delusions, of course. Witnessing someone being sawn in half and put back together doesn’t mean we then believe that people can be safely sawn in half. What’s more, sensory deficits do not always lead to delusional beliefs. So what else is required? Harris’s brain imaging studies delivered an important clue: belief involves both reasoning and emotion.

The feeling of rightness

The formation of delusional belief probably also requires the emotional weighing-up process to be disrupted in some way. It may be that brain injury destroys it altogether, causing people to simply accept the evidence of their senses. Or perhaps it just weakens it, lowering the evidence threshold required to accept a delusional belief.

For example, somebody with a brain injury that disrupts their emotional processing of faces may think “the person who came to see me yesterday looked like my wife but didn’t feel like her, maybe it was an impostor. I will reserve judgement until she comes back.” The next meeting feels similarly disconnected, and so the hypothesis is confirmed and the delusion starts to grow.

According to Langdon and others, this is similar to what goes on in the normal process of belief formation. Both involve incoming information together with unconscious reflection on that information until a “feeling of rightness” arrives, and a belief is formed.

This two-stage process could help explain why people without brain damage are also surprisingly susceptible to strange beliefs. Our natural credulity is one thing, and is particularly likely to lead us astray when we are faced with claims based on ideas that are difficult to verify with our senses – “9/11 was an inside job”, for example. The second problem is with the “feeling of rightness”, which would appear to be extremely fallible (see “What’s your delusion?“).

So where does the feeling of rightness come from? The evidence suggests that it has three main sources – our evolved psychology, personal biological differences and the society we keep.

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