Education truly unites parents across the world, although the level of challenge differs, with the United States and other wealthy nations’ results stagnating at relatively high levels, whereas children in the world’s poorer half struggle with even reading a simple sentence or doing basic math.

But after years of experience, it has become clear which policies don’t work at all—even if they have loud backers.

Increasing per-pupil spending sounds like a no-brainer, but it can deliver little or no learning at all if the money isn’t used wisely. India showed this when it increased spending per primary school pupil by 71 percent over seven years, but reading and math test scores still declined sharply.

The go-to policy for many teachers unions and politicians is making classes smaller. This sounds like it should make a big difference: teachers can devote more time to individual needs. But analysis finds that reducing class sizes is one of the least cost-effective ways to improve student learning.

A 2018 review of 148 reports from 41 countries found smaller classes had “at best a small” effect on reading proficiency and no effect on math.

Paying teachers more is another favored approach. But even dramatically increasing salaries can have little effect on learning. Indonesia embraced all these popular policies at once: it doubled education spending to achieve one of the lowest class sizes in the world while doubling teachers’ pay, yet a landmark randomized controlled study showed “no improvement in student learning”.