This has been no happy new year for the world's equity markets. US stocks have had their worst January in more than a century - and this sell-off has prompted both a wave of selling across the world and a truly extraordinary response by the Federal Reserve, with America's central bank making an emergency cut in target interest rates of 0.75 percentage points.

By the time news of a historic trading loss at France's Société Générale became public yesterday, the market could no longer even show much surprise.