ONE of the biggest political issues in recent years has been that Wall Street has done better than Main Street. That is not just a populist slogan. A new study from the Bank for International Settlements (the central bankers’ central bank, as it is dubbed) shows exactly why rapid finance sector growth is bad for the rest of the economy.

The study, by Stephen Cecchetti and Enisse Kharroubi, is a follow-up to a 2012 paper which outlined the negative link between the finance sector and growth, after a certain point. When an economy is immature and the financial sector is small, then growth of the sector is helpful. Enterprising businessmen can get the capital they need to expand their companies; savers have a secure home for their money, making them more willing to provide finance to the business sector; and so on.

But you can have too much of a good thing. The 2012 paper suggests that when private sector debt passes 100% of GDP, that point is reached. Another way of looking at the same topic is the proportion of workers employed by the finance sector. Once that proportion passes 3.9%, the effect on productivity growth turns negative. Ireland and Spain are cases in point. During the five years beginning 2005, Irish and Spanish financial sector employment grew at an average annual rate of 4.1% and 1.4% respectively; output per worker fell by 2.7% and 1.4% a year over the same period.