Abstract: Stephen Yeazell’s pathbreaking study of the history of group litigation revealed how disparate societies have shaped the rules of group litigation to meet their own needs. Professor Yeazell thereby demonstrated that procedural rules are socially contingent rather than universal in nature. In this Essay honoring Steve, I transform that lesson into a new approach to joinder rules. Specifically, I argue that if joinder rules arise out of specific social situations, then the simplest approach to joinder is to adopt a default rule calling for the shape of litigation to reflect the shape of the social activity that gave rise to the litigation. Labeling this concept “social loyalty,” I argue that it provides a new way of identifying what cases ought to be adjudicated in the aggregate and a new defense of their aggregation.