It's actually quite difficult to find out just how common these kinds of delusions are. You can't just ask people a straight question, because there's a good chance that they won't give you a straight answer (nobody wants to seem to be a lunatic).
So Rachel Pechey and Peter Halligan, at Cardiff University in Wales, created a new questionnaire specifically to try to find out how common bizarre delusions actually are. They did this by asking about symptoms without framing them in terms of mental illness, and by asking about them as part of a larger questionnaire covering all kinds of beliefs - including religious and political beliefs.
They interviewed 1,000 people from around Britain, and found that a staggering 78% of them said that they currently experienced one or more bizarre delusions to some degree. Some 26% reported a 'strong' experience of a bizarre delusion.
So, for example, when asked "Do you believe that people you know disguise themselves as others to manipulate or influence you?", 4.4% said that they strongly believe' this to be true.
They also asked about a range of paranormal and religious beliefs - and you can see the results in the figure below.
Just over 25% were atheists, but of course some of them might have held one of the other kinds of paranormal beliefs. Hopefully there were no atheists among the 5% of the population who believe in werewolves!
Then they looked at how these paranormal and religious beliefs correlated with the delusional beliefs (to do this, they excluded the werewolf and astrology questions - because they didn't form part of a common statistical factor along with the other paranormal/religious beliefs).
Well, you probably know where this is heading. There was a good correlation between paranormal/religious beliefs and delusional beliefs. In contrast, there was no correlation between either of these and a third basket of questions relating to political and social beliefs.
This fits with other research, showing that delusional beliefs are more common among the religious and among 'New-Age believers' (see When people stop believing in God... they go mental?). Halligan points that delusional beliefs don't seem to result in as much distress among these populations, and that fits with other evidence that psychotic patients with religious beliefs are less distressed (see Why psychotic patients with religious delusions are harder to cure).
Just why there should be this correlation is harder to say. Certainly, if you've had some freaky experiences, then it stands to reason that you're going to be more open to unorthodox ideas about how the world works.
But Halligan suggests that there may be a deeper connection:
One potential explanation is that holding a belief may impact upon an individual’s wider belief system so that the endorsement of similar (e.g. irrational/unscientific) beliefs becomes more likely. This is in line with the web-of-belief hypothesis advocated by Quine and Ullian, which suggests that a belief coheres with other beliefs held by an individual. In addition, other cognitive factors such as the reasoning biases associated with delusions may also play a role in the development of other abnormal beliefs.
In other words, religion and delusional beliefs may form part of a reinforcing worldview, and both may also be prompted by failures of rational thought. Other research has pointed in a similar direction (see You either believe it all... or you don't).
Pechey R, & Halligan P (2011). The Prevalence of Delusion-Like Beliefs Relative to Sociocultural Beliefs in the General Population. Psychopathology, 44 (2), 106-115 PMID: 21196811
This article by Tom Rees was first published on Epiphenom. It is licensed under Creative Commons.