This book argues that a unique late marriage pattern, discovered in the 1960s but originating in the Middle Ages, explains the continuing puzzle of why western Europe was the site of changes that, from about 1500, gave rise to the modern world. Contrary to views that credit upheavals from the late eighteenth century were reponsible for ushering in the contemporary global era, it contends that the roots of modern developments themselves are located in an event more than a millennium earlier, when the peasants in northwestern Europe began to marry their daughters almost as late as their sons. The appearance of this late marriage system, with its unstable nuclear household form, will also be shown to have exposed for the first time the common ingredients whose presence has perpetuated beliefs in the importance of gender difference and of a sexual hierarchy favoring males.