IF YOU Google the phrase “Middle East rumours”, the first link that pops up is not, as you might expect, a website propagating conspiracy theories. It is Coca-Cola's website. For several years now the company has struggled to rebut ridiculous rumours about its products.

For example, some people believe that if you read Coke's Arabic logo backwards, it says: “No Muhammad, No Mecca”. Others insist that the company is owned by Jews, or that it bankrolls Israel. These rumours are one reason why Coke does worse than Pepsi in Arab countries. Yet they are all false, as Coke's website explains in painstaking detail.

Such rebuttals are unwise, argue Derek Rucker and David Dubois, of the Kellogg School of Management, and Zakary Tormala, of Stanford business school, three psychologists. By restating the rumours, Coke helps to propagate them. Its web page is a magnet for search engines. And people who read rebuttals tend to forget the denial and remember only the rumour, says Mr Rucker.

As information is passed around, important qualifiers are lost. A rumour may start as “I'm not sure if this is true, but I heard that…” Then it evolves into: “I heard that…” Finally it becomes: “Did you know that…?” Even when no one intends to spread falsehoods, they spread.

In several experiments, Mr Rucker and Mr Dubois planted rumours among undergraduates. They found that with each repetition, scepticism diminished. The rumours themselves did not change; only the likelihood that the students would believe them. These findings were published in a report called “The Failure to Transmit Certainty”.

Instead of denying false rumours, a company should put out a stream of positive messages about itself, reckon Mr Rucker and Mr Dubois. This deprives myths of oxygen and also nudges people to doubt nasty things they may hear about the company in question.

Other companies could learn from this. McDonald's hamburgers have been said to contain worm meat, Procter and Gamble is reputed to have Satanic links and Facebook is rumoured to be shutting down so that its founder, Mark Zuckerberg, can have his life back. All these rumours are utterly false, but the firms in question would be well advised not to bother denying them.