The process by which scientists adopt and use socially constructed categories to classify their human subjects is complex. Scientists have used a wide variety of seemingly incongruous racial, ethnic, geographic, linguistic and national categories in their studies of human biological variation. This article details the epistemic rationale behind the system of classification found in mid-twentieth century human biology. The populationist rationale, I argue, entails agnosticism towards the reality of categories and supported the use of flexible standards around sampling and labeling. Looking closely at the categories used in projects in Latin America illustrates how the distinction between “primitive” and “industrialized” structured the classification system used by researchers to render populations productive.