This chapter shows how human sciences researchers in Puerto Rico faced pressure to abandon earlier traditions and embrace the methods and biomedical enterprise of the United States empire’s scientific modernity. Drawing on the history of mental testing and inmate assessment as well as designs for a new penitentiary, the chapter contends that while mid twentieth-century US-American social science engaged in intense processes of othering that aligned with imperial expansion, Puerto Rican social scientists combined US-American psychometrics with older Spanish ethnographic traditions that powerfully resurfaced in the 1940s. These diagnostic and descriptive tools revealed that incarcerated people required discipline, tutelage, and treatment, but that they also had redemptive potential regardless of social difference. Social scientists put mental test results into dialogue with ethnographic narratives of convicts to forge what to them were forward-looking treatment programs, illustrating how racialized racelessness and intersubjective exchanges transformed Puerto Rican corrections for a time. The result was a blended, “creole” nationalist science with decolonial aspirations, although one that was colonial-populist in practice.