For more than two hundred years, on the topic of human population, a disturbingly vicious thread has run through Western history, basing itself on biology and justifying cruelty on an almost unimaginable scale. When I began researching this book I thought of Malthusian theory, eugenics, Nazi genocide and modern population control as separate and distinct episodes in human history. I am no longer so sure. I think there is some persuasive evidence that a direct, if meandering, intellectual thread links the Poor Laws, the Irish famine, the gas chambers of Auschwitz and the one-child policies of Beijing. In all cases, cruelty as policy, based on faulty logic, sprang from a belief that those in power knew best what was good for the vulnerable and weak.