For much of the last decade, the bubble in equity markets was sustained by what was known on Wall Street as the 'Greenspan put'. Put simply, the rule stated that it was perfectly safe to invest in equities, since if they fell, former Fed chairman Alan Greenspan would always wade into the markets with a series of interest-rate cuts to bail them out. Now we have something that looks like a QE put – when markets collapse, central bankers will print more and more money until they get them moving again.