William D. Hamilton's name is often connected to important theoretical accomplishments, from the theory of inclusive fitness and kin selection to the so-called Hamilton's rule and the haplodiploidy hypothesis. This article asks: How did Hamilton attempt to test his theory and hypothesis against the complexity of the biological world? The article reconstructs Hamilton's empirical work with social wasps between 1963 and 1968, the years before and after the publication of the groundbreaking “The Genetical Evolution of Social Behavior” in 1964. It points out the centrality of Hamilton's work on wasps and shows how the British scientist attempted to test theories and hypotheses with naturalistic, developmental, and physiological observations as well as, at times, with experimental manipulations. The article offers a new perspective on the history of the scientific understanding of the evolution of social behavior. In contrast to existing narratives, this perspective emphasizes the importance of empirical work—e.g. natural history, physiology, comparative anatomy—which is often obscured by a nearly exclusive focus on theoretical developments in this field.