Why conspiracy theories live on despite the facts

Britain's most popular conspiracy theory is that the food supply is being poisoned and the Government is covering up the evidence.

The theory is more widely believed than suggestions that Diana, Princess of Wales was murdered, that the Aids virus was created in a laboratory or that the European Union is trying to take over the country, researchers say.

The popularity of such theories has puzzled psychologists because they tend to persist in the face of the facts. Claims that Princess Diana was murdered are still made despite extensive police and media inquiries.

Now psychologists believe they are closer to understanding why. Patrick Leman of the Royal Holloway University of London tested six theories on 64 students. He found everyone was prone to forge conspiracy theories to make the explanation seem proportionate to the magnitude of the event.

"Everyone ... tends to believe a major event has a major cause," he said. "So in a scenario in which a President is attacked and dies, they are more likely to invoke a conspiracy theory in place of a 'lone gunman' explanation than if he survives." But the readiness to believe such theories varied. Asked to rate six scenarios on whether they were the product of conspiracies, the students gave the highest vote to the poisoning of the food supply. The results showed that those most prone to believe such theories were less likely to accept facts that undermined their theory.

Earlier research has suggested people from ethnic minorities are prone to believe conspiracy theories. Mr Leman said: "Ethnic minorities tend to feel less connected to the authority. If people become distanced from institutions of power they are more likely to distrust official accounts."