When Alexandria Ocasio-Cortez, the influential new Democratic congresswoman, was heckled recently by parents during a debate about New York’s public schools, she responded by talking about the shortage of decent opportunities for the city’s children. “This right here, where we’re fighting each other, is exactly what happens under a scarcity mindset,” she said.

Ms Ocasio-Cortez put her finger on a phenomenon that is showing up in many different guises as economic growth has slowed in the developed world. When the pie stops growing, the fights become fiercer and dirtier over how to divide it. One of the areas where this is playing out most emotively is education — an issue critical to the life chances of our children.

As developed countries grew steadily richer over much of the 20th century and educational opportunities expanded, absolute social mobility — the likelihood that children would do better than their parents — was commonplace.