My dissertation examines the role of eugenics and evolutionism in U.S.-Mexican relations from 1900-1940 by documenting, narrating and analyzing three historical lineages. Firstly, it reconstructs a binational network of eugenicists, anthropologists, and other elites who were actively involved in creating new social-scientific formations and discourses, paying specific attention to the complex and often asymmetrical ways in which theories of medicine, individual, and society moved across borders. Secondly, it explores the ways in which eugenic ideas and practices in the United States helped shape early twentieth-century U.S. nativism and nationalism as well as the delimitation of the body politic. By focusing on the role of Mexico, Mexicans, and the invention of the U.S.-Mexican border, I examine a previously unexplored aspect of U.S. state formation and a dimension of racializaton which challenges historical formulations of racial differentiation and exclusion based on dichotomies of black/white or South/North. Thirdly, over half of my dissertation examines the role of eugenics and evolutionism in Mexico following the revolution of 1910. During the postrevolutionary period of the 1920s and 1930s Mexican 'nation-builders' often found legitimacy for their cultural project in theories and practices of anthropology, eugenics, and public hygiene. As a historical exploration into questions of morality, science, and nationalism my dissertation seeks to illustrate the different ways in which the biological and physical sciences critically shaped nation-building in two contiguous yet culturally-distinct countries.