Using hypnosis to model Fregoli delusion and the impact of challenges on belief revision.

Fregoli delusion involves the belief that strangers are known people in disguise. We aimed to model aspects of this delusion for the first time using hypnosis. We informed hypnotised subjects that someone would enter the room (a confederate) and they would believe this person was someone they knew in disguise. After testing their reaction to the confederate, we challenged their delusion by directly contradicting their belief and then asking them to focus on the confederate's voice and gait. Finally, we indexed whether they could identify photographs of the confederate. We found that just over half of our high hypnotisable subjects identified the confederate as someone they knew in disguise. Although many highs abandoned their belief in response to challenges, some maintained strong, unwavering conviction that the confederate was a known person. We discuss these findings in terms of how evidence might be evaluated during both hypnotic and clinical delusions.