American parents eventually increased their hands-on caregiving by about 12 hours a week, compared with the 1970s. Dutch, Spanish, Italian, Canadian and British parents ramped up their child care, too. (In Japan, hyper-involved mothers are known as “monster parents.”)

Not all the changes were rational. When some parents learned that talking to toddlers helps to develop their young brains, they began monologuing at them constantly.

But for the most part, the new parenting efforts seemed effective. Dr. Doepke and Dr. Zilibotti can’t prove causality (to do that, you’d have to randomly assign parenting styles to different families). But when they analyzed the 2012 PISA, an academic test of 15-year-olds around the world, along with reports from the teenagers and their parents about how they interact, they found that an “intensive parenting style” correlated with higher scores on the test. This was true even among teenagers whose parents had similar levels of education.