It is well known that those with higher SES characteristics tend to vote at higher rates in U.S. elections. Over the past several decades many proponents of eased voter registration requirements have predicted that liberalizing voter registration laws will significantly improve turnout, especially among the least well- educated and the poor. In this article we offer a rational choice model of turnout that leads us to expect the greatest turnout gains from virtually eliminating voter registration costs will instead accrue to those with medium income and education. We test this prediction longitudinally over the period 1972–1992 using a vast survey and a natural experiment comparing voters in states that adopted election day registration (EDR) with those residing in states maintaining more traditional closing dates. Contrary to much of the literature, citizens with medium education and medium income voted more under EDR, as the model predicts. We conclude that the methods used here better capture and empirically identify the curvilinear relationship between voter registration laws and the turnout probabilities at various SES levels.