How did an obscure academic idea pave the way to the Holocaust within just fifty years? Control is a book about eugenics, what geneticist Adam Rutherford calls “a defining idea of the twentieth century.” Inspired by Darwin’s ideas about evolution, eugenics arose in Victorian England as a theory for improving the British population, and quickly spread to America, where it was embraced by presidents, funded by Gilded Age monopolists, and enshrined into racist American laws that became the ideological cornerstone of the Third Reich. Despite this horrific legacy, eugenics looms large today as the advances in genetics in the last thirty years―from the sequencing of the human genome to modern gene editing techniques―have brought the idea of population purification back into the mainstream.