This year has been a good one for the West. The alliance has surprised observers with its united front against Russian aggression. As authoritarian China suffers one of its weakest periods of growth since Chairman Mao, the American economy roars along. A wave of populism across rich countries, which began in 2016 with Brexit and the election of Donald Trump, looks as if it may have crested. Yet away from the world’s attention, rich democracies face a profound, slow-burning problem: weak economic growth. In the year before covid-19, advanced economies’ gdp grew by less than 2%. High-frequency measures suggest that...