Markets are, in a sense, a giant collective action problem. As much as judging the fundamentals of the economy, investors also have to work out how their peers will behave – and the challenge for regulators is to work out how to get the behaviour, in aggregate, that they want.

And if anyone ever had a collective action problem, it is the holders of money market funds. The most recent figures from the Federal Reserve show that in November, the amount of money held in US money market funds exceeded the amount in equity funds, for the first time in 15 years.