The process is so wearing that after actually meeting black people they perform worse on simple tests designed to measure mental agility.

A team from Dartmouth Colege in New Hampshire identified 30 white males as racially biased by using a test that measures how easy they find it to link negative concepts such as ugliness with white people, and positive ones such as beauty with black people.

They then scanned the individual’s brains using functional magnetic resonance imaging, a technique that shows up which parts of the brain are being used. When the men were shown pictures of black faces, the dorsolateral prefrontal cortex of the brain of some of them lit up, which the team took to represent an attempt to suppress racist sentiments.

In a second experiment, the men were asked to do standard brain tests after meeting black men face to face. Those identified as racist by the brain scans did less well on the tests.

The team responsible conclude in Nature Neuroscience that harbouring racial prejudice, even unintentionally, stirred up an inner struggle that exhausted the brain.

Dr Jennifer Richeson, the lead author, said: “To my knowledge, this is the first study to use brain-imaging data in tandem with more standard behavioural data to test a social psychological theory.”

In an editiorial, Nature Neuroscience says: “The (research) raises concerns about mind-reading and social control, but in reality this prospect is remote . . . The main foreseeable benefit is the enlightenment that must surely come from a better understanding of our own mental processes.”

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