In the 18th century, Eastern Japan's population was shrinking. At the time, many villagers raised only two or three children, believing that parents could and sometimes should kill their newborns. This view, so alien to us today, rested on particular understandings of humanity, obligations to ancestors, and responsible parenting. Domain states and local elites would ultimately come to view infanticide as a demographic and moral crisis, investing prodigious resources in welfare, surveillance, and reeducation to curb the practice. Population growth eventually resumed, but the eradication of infanticide remained incomplete: Japanese parents still killed tens of thousands of newborns in every year of the early 20th century. This book pieces together this astonishing story of continuity and change from the records of a thousand Tokugawa-period villages, the statistics of Imperial Japan, and the texts and images that contested the meaning of infanticide. It offers a new perspective on Japan's regional diversity and the relationship between its states and their subjects. By highlighting the contingency of demographic history, it challenges paradigms of unidirectional change and suggests that the future may hold many surprises yet.