This dissertation describes how innovations in the science of mind -- laboratory measurements, psychoanalysis, and mental testing -- changed the ideas and institutions of British imperialism. Psychology did not function as a tool of empire in any straightforward way: in many cases, the knowledge it generated called racial stereotypes into question, uncovered the traumatic effects of British rule, and drew unflattering contrasts between the hierarchical values of imperialism and an idealized vision of meritocracy. Psychology did, however, strengthen the authority of Western experts to intervene in other cultures. While they kept their distance from the political culture of officials and settlers, psychologists embraced a modernizing mission, arguing that knowledge of abilities and emotions could make colonized societies fairer and more efficient. The development projects which defined the postwar and postcolonial periods -- usually seen as the golden age of abstract, impersonal, "high modernist" planning -- relied in significant ways on the measurement and management of minds.