My dissertation focuses on the transnational history of intelligence testing in the twentieth century, and explores the relationship between war and international tensions, and psychometric testing. By examining major transnational actors and trends, principally from the United States, France, and Great Britain, it sheds light on the numerous connections between international conflict and the rise of population-based national psychometric testing programs. International conflicts over the course of the twentieth century helped to heighten consciousness and concern over the quality, as well as quantity, of national populations. Unprecedented opportunities to apply intelligence tests to large populations, which were in part created by the context of war, yielded mass amounts of testing data that elevated experts' concerns about national levels of intelligence at the same time that population experts vocalized anxieties about overpopulation. Experts from the fields of psychology, demography, genetics and eugenics spoke to these concerns in their research and advisory roles.