But low rates of return also mean that government-debt burdens may prove easier to manage than thought—and perhaps that government borrowing could be used more aggressively in times of economic weakness to make up for central-bank impotence.
Nor do low rates of return on government debt imply that the world is entering a period of “secular stagnation”, or chronically weak growth.
Low rates have in the past been as much a feature of rip-roaring economies—eg, in the 1950s and 1960s—as of the more stagnant ones experienced recently.
More still are the data's implications for debates on inequality.
Karl Marx once reasoned that as capitalists piled up wealth, their investments would suffer diminishing returns and the pay-off from them would drop towards zero, eventually provoking destructive fights between industrial countries.
That seems not to be true; returns on housing and equities remain high even though the stock of assets as a share of GDP has doubled since 1970.
Gravity-defying returns might reflect new and productive uses for capital: firms deploying machines instead of people, for instance, or well-capitalised companies with relatively small numbers of employees taking over growing swathes of the economy.
High returns on equity capital may therefore be linked to a more tenuous status for workers and to a drop in the share of GDP which is paid out as labour income.