[This is my latest FT newspaper column, drawing on suggestions from an earlier blog post. My other newspaper columns can be read here -- most require an FT.com subscription.]

Conspiracy theorists have a bad reputation. They are usually portrayed as paranoid, isolated, deluded people, best avoided.

It is true that there are many sinister and unpleasant conspiracy theories. These are usually the ones that seek to blame all the world’s ills on a single racial or social group – Jews, Catholics, Freemasons.

But there are also conspiracy theories that are delightfully dotty. A friend in Ankara tells me many Turks are convinced that, during the cold war, the Russians infested the Sea of Marmara with a sturgeon-devouring predator that sent these valuable fish fleeing into the Russian bit of the Black Sea – thus allowing the Russians to control the world’s supply of caviar. That is a theory worthy of James Bond.

The idea that conspiracy theorists are an isolated bunch, on the fringes of society, is also wide of the mark. Some theories are so widely believed that they are now almost mainstream. A recent BBC opinion poll suggested that only 43 per cent of Britons accept the official verdict that the car crash that killed Diana, Princess of Wales, was an accident. The countless “9/11” conspiracy theories also have a surprisingly wide audience – even in America. A Zogby poll last year found that 42 per cent of Americans think the US government is “covering up” facts about the terrorist attacks of September 11 2001.

Why do conspiracy theories command such a wide audience? I have my own theory about that.