Network Analysis and the Law: Measuring the Legal Importance of Precedents at the U.S. Supreme Court

Timothy R. Johnson Department of Political Science, University of Minnesota, Twin Cities, 1414 Social Sciences Building, 267 19th Ave. South, Minneapolis, MN 55455 e-mail: trj{at}umn.edu James F. Spriggs, II Department of Political Science, Washington University in St. Louis, Campus Box 1063, One Brookings Drive, St Louis, MO 63130 e-mail: jspriggs{at}artsci.wustl.edu Sangick Jeon Department of Political Science, Stanford University, 616 Serra Street, Encina Hall West, Room 100, Stanford, CA 94305-6044 e-mail: sjeon{at}stanford.edu Paul J. Wahlbeck Department of Political Science, George Washington University, 1922 F Street, N.W. Suite 401, Washington, DC 20052 e-mail: wahlbeck{at}gwu.edu e-mail: jhfowler{at}ucsd.edu (corresponding author) We construct the complete network of 26,681 majority opinionswritten by the U.S. Supreme Court and the cases that cite themfrom 1791 to 2005. We describe a method for using the patternsin citations within and across cases to create importance scoresthat identify the most legally relevant precedents in the networkof Supreme Court law at any given point in time. Our measuresare superior to existing network-based alternatives and, forexample, offer information regarding case importance not evidentin simple citation counts. We also demonstrate the validityof our measures by showing that they are strongly correlatedwith the future citation behavior of state courts, the U.S.Courts of Appeals, and the U.S. Supreme Court. In so doing,we show that network analysis is a viable way of measuring howcentral a case is to law at the Court and suggest that it canbe used to measure other legal concepts. Authors' note: We appreciate the suggestions of Randy Calvert,Frank Cross, Pauline Kim, Andrew Martin, Richard Pacelle, JimRogers, Margo Schlanger, Amy Steigerwalt, and participants inthe Workshop on Empirical Research in the Law at WashingtonUniversity in St Louis School of Law. We presented former versionsof this article at the 2006 meeting of the Midwest PoliticalScience Association, Chicago, April 20–23; the 2006 meetingof the Southern Political Science Association, Atlanta, GA,January 5–7; and the 2006 Empirical Legal Studies Conference,Austin, TX, October 27–28.