Demography is not destiny. As Giacomo Casanova explained over two centuries ago: 'There is no such thing as destiny. We ourselves shape our own lives.' Today we are shaping them and our societies more than ever before. Globally, we have never had fewer children per adult: our population is about to stabilize, though we do not know when or at what number, or what will happen after that. It will be the result of billions of very private decisions influenced...