The instant book is grounded on a framework for studying the content of law in a society that is democratically governed and structurally complex, but the framework applies only to law that has a bearing on activities that are (1) social in character and (2) important to the society. This chapter begins by explicating both (1) and (2) so that social activities are empirically distinguishable from nonsocial activities and societally important activities are empirically distinguishable from societally unimportant activities. Findings from quantitative research are then used to formulate theorems that identify the impact of particular society-level agents on doctrines of law and that can thereby account for between-society differences, as well as within-society changes, in the content of law on activities covered by the framework. Theorems are developed for five agents: culture, knowledge, social disorder, societal fragmentation, and population structure. The chapter concludes by underscoring the powerful effect that macrosociological agents have on the content of law: In a set of seven studies, an average of just four explanatory variables correctly anticipated state law doctrines in four-fifths of U.S. states.