How and why did the idea of human equality—that individuals are equal in their interior biological and mental constitution despite the exterior differences of race, ethnicity, or caste—become a scientific fact and diplomatic norm in twentieth century? Past scholarship has understood this as the retreat of scientific racism, treating it as an episode in the genealogical history of the human sciences in the United States and Europe. Drawing on the history of the human sciences, colonial welfare and development, self-determination and human rights, this dissertation offers an account of the emergence of equality on a wider geographical and conceptual arc. I argue that in its original and most ambitious form, the idea of human equality emerged to claim re-distributive economic justice and address inequities of colonial racial prejudice in the British Empire. In the early twentieth century, this idea of equality animated the work of American anti-caste missionaries in India, a Jewish Marxist psychologist in South Africa, and anti-colonial labour organizers in East Africa. As starting points of a conceptual and chronological account, these episodes reveal how the meaning of equality hollowed at an ascendant point in its history: the 1950s UNESCO statements on race which created a potent scientific and diplomatic consensus against scientific racism. This new meaning of human equality underpinned a legal promise of human rights, self-determination, and decolonization for those who had previously suffered from colonial inequity. However, without its original re-distributive ambitions, equality as a scientific fact and legal norm proved unable to address issues that had its animated original articulations, as the persistence of economic inequality between states and their former colonies quickly revealed. Tracing changes in the meaning of human equality, this dissertation offers a rich history of decolonization and self-determination in the twentieth century.