MIDWAY THROUGH HIS CAREER, Christophe Guilmoto stopped counting babies and started counting boys. A French demographer with a mathematician’s love of numbers and an anthropologist’s obsession with detail, he had attended graduate school in Paris in the 1980s, when babies had been the thing. By the time Guilmoto started his PhD, birth rates had started falling around the world, but the populations of many developing nations were still growing, and it was hard to shake the idea that overpopulation was a grave threat. Like many of his contemporaries he concentrated on studying the drop in fertility, searching for clues to what factors proved decisive in lowering a country’s birth rate. He did his dissertation research in Tamil Nadu, where the birth rate had fallen to European levels even as income levels remained low, and as he graduated and started working as a scholar he returned there many times. By 1998 he headed up the South India Fertility Project, a formal effort to catalogue the successes of Tamil Nadu and surrounding states. But over the course of working in the region, he realised demography’s big story had changed. People in India were not simply having fewer children. They were having fewer girls. Population growth had been slowed, in part, by reducing the number of daughters.