Amicus curiae participation is a staple of interest group activity in the U.S. Supreme Court. While a reasonably large body of scholarship has accumulated regarding the effectiveness of this method of participation, little attention has been paid to examining the reasons why amicus participation might increase litigation success. In this article, I test two separate, but not mutually exclusive, theories as to why amicus briefs may be effective. The first, the affected groups hypothesis, suggests amicus briefs are influential because they signal to the Court how many groups and individuals will be potentially affected by the decision. The second, the information hypothesis, proposes that amicus briefs are effective because they provide the Court with added information that buttresses the arguments of the direct parties. When subjected to empirical verification, the results indicate that not only does amicus participation increase litigation success, but also that this influence may be best explained by the information hypothesis.
Interest groups pursue their goals in a wide array of venues, including the courts. Indeed, solely within the realm of the U.S. Supreme Court, groups have numerous methods for participation, from setting up test cases to sponsoring cases that others bring to testifying at judicial confirmation hearings; some groups even hold vigils outside the marble pillars awaiting the Court's final decisions on cases touching on group interests. However, the most common method of interest group involvement in the Supreme Court is the amicus curiae brief (Caldeira & Wright 1988; Epstein 1991). In fact, amicus briefs are filed in almost every case the Court accepts for review (Epstein & Knight 1999:221; Kearney & Merrill 2000). Far from their literal translation, however, amicus curiae ("friend of the court") briefs are not neutral sources of information. Instead, these briefs are advocates for the parties (Krislov 1963). Amicus curiae briefs are most commonly submitted to the Court urging the justices to rule in favor of one litigant over another (Kearney & Merrill 2000:841-2; Spriggs & Wahlbeck 1997:371).
Past research on the impact of amicus briefs in the Supreme Court has indicated that the presence of amicus briefs increases the likelihood of a grant of certiorari (Caldeira & Wright 1988; Perry 1991) and influences litigant success on the merits (Kearney & Merrill 2000; McGuire 1990, 1995; Puro 1971; compare with Songer & Sheehan 1993).1 However, virtually none of this research has explicitly examined why amicus briefs influence litigation success. "One of the shortcomings of previous ... studies of amicus briefs is that they often fail to articulate a clear hypothesis about how information contained in amicus filings influences the decision making of the Supreme Court" (Kearney & Merrill 2000:774).
In this study, I examine two theories regarding why amicus briefs might increase litigation success. The first, the affected groups hypothesis, holds that amicus briefs are efficacious because they signal to the Court that a wide variety of outsiders to the suit will be affected by the Court's decision.2 Thus, regardless of the social scientific, legal, or political information that the briefs contain, this hypothesis asserts that it is simply the number of organizations present on a brief that should influence the Court's decision. The second, the information hypothesis, asserts that amicus briefs are effective, not because they signal how many affected groups will be impacted by the decision, but because they provide litigants with additional social scientific, legal, or political information supporting their arguments.
This article proceeds as follows. First, I offer a brief discussion of the rules and norms regarding amicus participation. Next, I discuss two sources of information that allow the justices to take full advantage of their policy preferences and create effective law. …