Bubbles in productive assets - roads, Bridges, telecom lines - are also tolerable; capital has been put in place that can be exploited by somebody.

Investment banking is notoriously volatile, but there is reason to worry that it won't bounce back as quickly as it did after the downturn that followed the bursting of the dotcom bubble in 2000.

IT and telecoms firms never recovered from the dotcom bust in 2000, though IT companies posted strong gains in 2009.