DEMOGRAPHY is back. Not that its subject matter—the size and structure of populations—ever went away. But from the 1980s to the late 2000s demography retreated to the wings of public debate, a concern mostly of geeks and obsessives. Over the past few years, though, that has started to change. Population science is once more centre-stage, pushed by climate change, which raises worries about the impact so many billions have on the environment of the earth, and food-price spikes, which imply doubts about whether it will be possible to feed them all.