Fifth is an over-reliance on the output gap - the difference between the level of output and an estimate of what is sustainable - in forecasting. That allowed policymakers to believe everything was fine in the economy, because inflation was under control and growth was not excessive.

Sixth is the natural tendency to seek rationales for events as they unfold, rather than question whether they are sustainable. Kenneth Rogoff, a Harvard professor who is also a former IMF chief economist, thinks the tendency to look on the bright side is particularly prevalent on Wall Street, where "it is difficult to make a living as a mega-bear", he says.

Academics and the Fed also fell into the trap of rationalising unsustainable features of the global economy. In 2005 a paper by Ricardo Hausmann and Federico Sturzenegger of Harvard caused excitement about the possibility that financial "dark matter" would prevent a big bang in the world economy. The failure to believe in this dark stuff, the authors concluded, made "analysts predict crises that, for good reason, remain elusive".