Over the past years, a new demographic crisis has emerged as a subject of intense debate: the most affluent, most advanced, freest societies of the world are not having enough children to sustain themselves. Recent books—including Phillip Longman’s The Empty Cradle (2004) and Ben J. Wattenberg’s Fewer (2004)—have described the potentially tragic consequences of this decline. Lamenting the collapse of modern birthrates, world leaders as diverse as Vladimir Putin and Pope Benedict XVI have advocated pro-natalist state policies. Popular magazines and newspapers that once worried about the horrors of a “population explosion”—mass starvation in developing countries, environmental catastrophe, the subjugation...