Human beings have two separate parts of the brain for dealing with suspicion - and the second one only 'lights up' when dealing with people we don't trust.

To begin with, suspicion is emotional - and volunteers wired to magnetic resonance imaging (MRI) scanners - had activity in a region of the brain dealing with fear.

But after that, ANOTHER region lights up - dealing with memory and recognition.

It seems human beings have evolved to be very suspicious creatures indeed.

The sheer amount of brain circuitry we have devoted to dealing with suspicion and distrust might have been crucial to early man.
Scientists at the Virginia Tech Carilion Research Institute have found that suspicion resides in two distinct regions of the brain:
the amygdala, which plays a central role in processing fear and emotional memories, and the parahippocampal gyrus, which is associated with memory and the recognition of scenes.

‘We wondered how individuals assess the credibility of other people in simple social interactions,’ said Read Montague, director of the Human Neuroimaging Laboratory and the Computational Psychiatry Unit at the Virginia Tech Carilion Research Institute, who led the study.

‘We found a strong correlation between the amygdala and a baseline level of distrust, which may be based on a person’s beliefs about the trustworthiness of other people in general, his or her emotional state, and the situation at hand.

'What surprised us, though, is that when other people’s behavior aroused suspicion, the parahippocampal gyrus lit up, acting like an inborn lie detector.’

But the tests - which involved buying and selling - show that not trusting people isn't always a good starting point.

‘People with a high baseline suspicion were often interacting with fairly trustworthy buyers, so in ignoring the information those buyers provided, they were giving up potential profits,’ said Meghana Bhatt, the first author on the research paper.

‘The ability to recognize credible information in a competitive environment can be just as important as detecting untrustworthy behavior.’

The findings may also have implications for such psychiatric conditions as paranoia and anxiety disorders, said Montague.

‘The fact that increased amygdala activation corresponds to an inability to detect trustworthy behavior may provide insight into the social interactions of people with anxiety disorders, who often have increased activity in this area of the brain,’ he said.
The scientists used functional magnetic resonance imaging, or fMRI, to study the neural basis of suspicion.

Seventy-six pairs of players, each with a buyer and a seller, competed in 60 rounds of a simple bargaining game while having their brains scanned.

At the beginning of each round, the buyer would learn the value of a hypothetical widget and suggest a price to the seller. The seller would then set the price.

If the seller’s price fell below the widget’s given value, the trade would go through, with the seller receiving the selling price and the buyer receiving any difference between the selling price and the actual value.

If the seller’s price exceeded the value, though, the trade would not execute, and neither party would receive cash.