This essay considers three interrelated subjects: how the American social milieu effects American international relations (IR) theorizing, why realism has been singled out for criticism within this milieu, and how particular perspectives on human nature have served as the foundational bedrock for American social thinking. It argues that, even when incorporating Darwinian insights into their scholarship, American social scientists have subscribed to a Lamarckian perspective on human nature that reflects the American version of Enlightenment liberalism. This version combines a deep and progressive faith in individualism, capitalism, applied science and a moral national purpose derived from Protestantism. The American social scientific subscription to a Lamarckian perspective on human nature effects not only how realism is theorized in this milieu but also how American scholars see and do IR theory.