Today’s fall in fertility is both very large and very fast. Poor countries are racing through the same demographic transition as rich ones, starting at an earlier stage of development and moving more quickly. The transition from a rate of five to that of two, which took 130 years to happen in Britain—from 1800 to 1930—took just 20 years—from 1965 to 1985—in South Korea. Mothers in developing countries today can expect to have three children. Their mothers had six. In some countries the speed of decline in the fertility rate has been astonishing. In Iran, it dropped from seven in 1984 to 1.9 in 2006—and to just 1.5 in Tehran. That is about as fast as social change can happen.