In the past half century, humanity has made extraordinary progress. This is unquestionable. This consists of more than higher incomes. It consists also of longer and better lives. We know this has happened. We also know why. But achievements bring new challenges. These are no exception.

In the early 1970s, the average woman produced just under five children. Many prophets of doom warned of unmanageable population explosion. Today, the global fertility rate is down to 2.4. In China, it is well below replacement level. In Brazil, too, where the Catholic Church was deemed an overwhelming obstacle to birth control, it is also below replacement level. Sub-Saharan Africa is the only big region where fertility rates remain high. (See charts.)

Why are families so much smaller? It is partly because wealthier people want fewer and better-educated children. But perhaps still more it is because their children survive. In 1960, 246 out of every 1,000 Indian children died before the age of five. By 2016, this was 43. In Brazil, the child mortality rate has fallen from 171 in 1960 to 15 in 2016. Even in rich Japan, it has fallen from 40 in 1960 (today’s world average) to three. We all love our children. So think what this transformation means to happiness.