Carl Hirschie Johnson

everywhere, as it has done across Europe. Even in France, which competes with Ireland for the title of the EU’s most fertile member state, the total fertility rate dropped below reproduction level last year. Other European countries, including Germany, are most worried about how to raise it to get closer to a steady state and save the provisions for the elderly from collapse. The threat of an exponential ‘explosion’ of world population appears to have subsided. New demographic challenges have replaced it, from the age distribution and gender balance through to migration between countries and also within countries. For instance, the rapid growth of China’s economy has depended on an unprecedented migration of around 200 million people from rural areas to the cities, as Xin Meng explains in a recent book review (Science (2014) 343, 138–139). The next challenge for governments in China, India, and other populous countries is now to fine-tune the development and mobility of their populations in ways that are both sustainable and acceptable to the people.