Ask an Expert: The Science of Delusion


Words || Jarred Noulton

Have you ever wondered what it would be like to look at someone you love and feel nothing? Your boyfriend? Your girlfriend? Your father or mother? For people with Capgras delusion, this is exactly what they experience – and it leads them to think their family members are imposters.

To find out more about this bizarre delusion, I spoke with Emeritus Professor Max Coltheart, who is a cognitive scientist currently working at Macquarie University. He describes Capgras succinctly.

“The deluded person believes that someone very familiar to them [often a spouse] has been replaced by a complete stranger who looks just like the spouse. Nothing can shake that belief, and indeed it is risky for family members to challenge it: the result is often aggression and sometimes homicide.” Though it sounds very unusual, clearly Capgras is a very serious delusion to hold.

One famous case of Capgras resulted in the patient decapitating their father to try and find the batteries in his head. I wondered if the relationship between violence and delusion was exaggerated, as it can be with mental illness. Coltheart agreed that this is an issue, but confirmed that Capgras can lead to violence.

“One paper reports a 30% incidence of violence in Capgras patients.” Thankfully, he’s positive about it. “My speculation is that this won’t occur unless the belief is tactlessly challenged by the family.”

So how does Capgras delusion develop? How could someone end up distrusting someone they love? According to Coltheart, who has been working on delusional beliefs for twenty years, Capgras first develops when a person loses a natural emotional response to familiar faces. This is caused by brain trauma. “There are 15 different types of brain pathology that have been reported in different Capgras patients: dementia is one, but so is stroke, head injury… So the presence of brain damage is important but what caused that damage is irrelevant.”

If the person is healthy, they’ll reject the idea that their loved ones have been replaced by imposters. But sometimes, the patient has damage to just the right part of the brain: the right dorsolateral prefrontal cortex. This controls belief evaluation, and without it, the person can’t reject a belief that would normally be dismissed as ridiculous.

Delusions are still not very well understood, so how do we know that this theory is accurate? In fact, I was excited to discover that Coltheart and his colleagues have just published another study affirming the reliability of the two factor theory.

“When you apply electrodes to the scalp over a particular region of the brain and stimulate with a current, you temporarily stop that region of the brain from functioning. This is TMS (transcranial magnetic stimulation). We have just published a study in which TMS was applied to the right dorsolateral prefrontal cortex – in healthy subjects. We predicted that this would impair people’s ability to evaluate beliefs. It did.” They tested it against a control, where the left prefrontal cortex was disabled, and belief formations remained normal.

And this theory helps explain other strange delusions too, from somatoparaphrenia, where someone believes their limb is owned and controlled by someone else, to Cotard delusion, where someone believes that they are dead.

“These sound bizarre; but they are scientifically understandable.”

Somatoparaphrenia stems from paralysis of the particular limb, and Cotard delusion from a lack of autonomic reaction to any stimuli. And there are more! One delusion is the belief that their reflection is a stranger, another that every stranger is an enemy in disguise – and there’s a scientific explanation for all of them.

Now that causes have been established, the next challenge for cognitive psychologists is treatment. Coltheart explains that some cases of Capgras have fully recovered, but it hasn’t been conclusively proven why – or what treatments consistently increase the chance of recovery. One way that treatments are being studied is through hypnosis, surprisingly. One of these delusions are simulated on a healthy, hypnotised person, and therapy can be tested without fear of entrenching the false belief. However, Coltheart says there’s only so much we can learn through this method until we understand both hypnosis and delusions better. Capgras and other delusions still mystify us, but the more people like Coltheart study them, the better we’ll be able to help people with delusions – and learn about ourselves in the process.