The development and promotion of the "expanding communities" curriculum design model for teaching elementary school social studies was a crucial episode in the history of social studies. This article profiles how the model developed in the mind of its most effective promoter, Paul Robert Hanna. Paul Hanna understood early in his career the profound power of schooling as a tool of acculturation. The influences of events and individuals in Hanna's life persuaded him that young citizens needed a solid base of knowledge in the social sciences, but they also needed encouragement to develop analytical skills and independent thought processes to use that knowledge effectively. The result was the expanding communities curriculum model, which dwarfed alternative approaches to elementary social education and shaped the way that generations of schoolchildren were taught to view their world. (Contains 48 endnotes.)