With the arrival of independence in 1947, India’s first generation of post-colonial leaders embraced the concept of ‘development’ as a central objective of nation-building and a raison d'être of the post-colonial state. The opportunity of freedom, it was argued, would pave the way for a dramatic process of economic, social and political transformation that would turn India from an impoverished, colonial society into a ‘modern’, prosperous and democratic one. Set against this backdrop, this thesis explores intersections and entanglements between the post-independence pursuit of development and the forms of knowledge produced by post-war American social science. Foregrounding the concept of ‘psychologized development’, the thesis focuses in particular on the ways in which Indian elites – including government officials, intellectuals, industrialists and more – drew on American psychological expertise in the hope of realizing development dreams. With its claims to understand the complex processes that shaped human action (and interaction), I argue, American psychological knowledge promised solutions to the most pressing contemporary problems, from the treatment of ‘communal’ tension to the engagement of rural communities in uplift programmes. Psychology revealed the foundations of effective economic entrepreneurship and the basis of sound industrial leadership. It even explained how new ideas and practices could be ‘diffused’ throughout society. For Indians, psychologized theories of human nature offered knowledge of great utility in the context of plans for rapid societal change. For Americans, they offered tools that would turn India into a model of democratic development in the context of a global Cold War. Using a case study approach, this thesis explores the diverse settings in which Indians and Americans came together to psychologize development. In doing so, it examines both the common themes and the recurring challenges that came with attempts to realize development through social science expertise. The resulting history offers new perspectives not just on the character and complexion of developmentalism in post-colonial India, but also on the forms of cross-border connection that shaped India’s post-1947 transition. The thesis makes novel contributions to a number of historiographical fields, including the history of American social science, the history of Indo-US relations, the history of development and global history.