

“When our world changes unexpectedly, we want to blame that volatility on somebody, to make sense of it, and perhaps neutralize it,” said Yale’s Philip Corlett, associate professor of psychiatry and senior author of the study. “Historically in times of upheaval, such as the great fire of ancient Rome in 64 C.E. or the 9/11 terrorist attacks, paranoia and conspiratorial thinking increased.”

Paranoia is a key symptom of serious mental illness, marked by the belief that other people have malicious intentions. But it also manifests in varying degrees in the general population. For instance, one previous survey found that 20% of the population believed people were against them at some time during the past year; 8% believed that others were actively out to harm them.


“We think of the brain as a prediction machine; unexpected change, whether social or not, may constitute a type of threat — it limits the brain’s ability to make predictions,” Reed said. “Paranoia may be a response to uncertainty in general, and social interactions can be particularly complex and difficult to predict.”

In a series of experiments, they asked subjects with different degrees of paranoia to play a card game in which the best choices for success were changed secretly. People with little or no paranoia were slow to assume that the best choice had changed. However, those with paranoia expected even more volatility in the game. They changed their choices capriciously — even after a win. The researchers then increased the levels of uncertainty by changing the chances of winning halfway through the game without telling the participants. This sudden change made even the low-paranoia participants behave like those with paranoia, learning less from the consequences of their choices.

In a related experiment, Yale collaborators Jane Taylor and Stephanie Groman trained rats, a relatively asocial species, to complete a similar task where best choices of success changed. Rats who were administered methamphetamine — known to induce paranoia in humans — behaved just like paranoid humans. They, too, anticipated high volatility and relied more on their expectations than learning from the task.


“Our hope is that this work will facilitate a mechanistic explanation of paranoia, a first step in the development of new treatments that target those underlying mechanisms,” Corlett said.



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