If Jeremy Grantham is talking about a US ‘superbubble’, we should listen. The Boston-based fund manager has hard-to-deny evidence to back up his prediction of a ‘wild rumpus’Grantham’s thesis is that US stocks are in a “superbubble”, an upgrade on last year’s diagnosis of “an epic bubble”, and that the US has seen only three other such extreme events in the past 100 years – the Wall Street crash of 1929, the turn-of-the-millennium dotcom mania and the housing market madness of 2006.