Many corporate executives spend their working lives railing that the market doesn't understand them, and that their shares ae woefully undervalued. So when one of them breaks with tradition and tells Wall Street that his stock, along with the rest of his sector, is absurdly overvalued, investors are bound to take fright.

When Steve Ballmer, president of Microsoft, said last week: "There is such an overvaluation of technology stocks it is absurd," the tech-laden Nasdaq Composite index dropped 3.8 per cent and the Dow Jones Industrial Average suffered its second 200-point fall of the week.

Mr Ballmer is something of a special case among market Cassandras. The latest Forbes list of the US's richest American put him in fourth place, with an estimated wealth of $23 billion (€22 billion), so he more than most can afford to make such comments.