A striking peculiarity of American demographic history is its compara tively early decline in fertility. Although some of the decrease before the Civil War may be attributed to later and less universal marriage, marital fertility also fell Past scholar ship on the subject has focused on alternative explanations and statistical approaches to cross-sectional variations in the child-woman ratio. After showing the inherent limita tions of this approach, particularly at the level of states, this article draws on the findings of historians of political behavior and women's activities to suggest that the understand ing of the early decline in marital fertility in the United States may be most profitably pursued through the study of the fertility behavior and values of the principal action- groups of antebellum America: religious, ethnic, and economic. Both the motivations for, and possibly the techniques of, family limitation were transmitted and sustained, this hypothesis contends, through membership and participation in these groups.