Sometimes you get out on the wrong side of bed and everything that was peachy the day before suddenly looks rotten. Nor does it help when friends point out that nothing has really changed. So it seems with financial markets this week. After two months of leaping about in rose tinted glasses, investors have turned gloomy again. World stock markets have fallen for four days in a row.

Why? Sure the latest US retail sales numbers were below consensus. But April's month-on-month drop of just 0.1 per cent in core sales – which excludes autos, gasoline and food – would recently have been interpreted as bullish. March was down 1.2 per cent, after all, a sign the slowdown might be moderating. And yes, US mortgage applications also fell last week. But they were negative the week before that, too.

Global indicators are also being viewed through newly-darkened lenses. Rising commodity prices were once taken as a positive sign that a protracted slump had been avoided. Now oil at $60 is seen as a brake on consumption. Similarly, Tuesday's data showed Chinese investment up by a third for the first four months of the year, versus 2008. But that is creating deflationary overcapacity, fear the newly converted bears. And plummeting Asian exports is now worrying, rather than a hopeful indication that economies hitherto dependent on foreign demand are rebalancing themselves.