Are Close Elections Randomly Determined?∗

Elections with small margins of victory represent an important form of electoral competition and, increasingly, an opportunity for causal inference. Scholars using regression discontinuity designs (RDD) have interpreted the winners of close elections as randomly separated from the losers, using marginal election results as an experimental assignment of office-holding to one candidate versus the other. In this paper we suggest that marginal elections may not be as random as RDD analysts suggest. We draw upon the simple intuition that elections that are expected to be close will attract greater campaign expenditures before the election and invite legal challenges and even fraud after the election. We present theoretical models that predict systematic differences between winners and losers, even in elections with the thinnest victory margins. We test predictions of our models on a dataset of all House elections from 1946 to 1990. We demonstrate that candidates whose parties hold structural advantages in their district are systematically more likely to win close elections. Our findings call into question the use of close elections for causal inference and demonstrate that marginal elections mask structural advantages that are troubling normatively. ∗We thank Dan Lee for helpful discussant comments and participants at the Midwest Political Science Association Annual Conference. For helpful discussions we thank Daniel Butler, Gary Cox, Andy Eggers, Jens Hainmueller, Daniel Hopkins, David Lee, Holger Kern, Gary King, and Clayton Nall. All remaining errors, omissions, and interpretations remain ours. †Assistant Professor, Department of Political Science, Stanford University; Encina Hall West 616 Serra St., Palo Alto, CA, 94305 ‡Ph.D. Candidate, Department of Government, Harvard University. 1737 Cambridge St., Cambridge, MA 02138 §J.D. candidate, Harvard Law School. ¶Allie S. Freed Professor of Government. Department of Government, Harvard University. 1737 Cambridge St., Cambridge, MA 02138.