The Signals of Unlikely Allies: Ideologically Diverse Amicus Coalitions and Judicial Behavior

Interest groups use amicus curiae briefs to supply courts with essential information while advocating for their political and policy preferences. Through these briefs, they form alliances with various interests, supporting either the liberal or conservative side of a case. The composition of these alliances – considering the status, prestige, and ideology of the involved interest groups – acts as a heuristic for justices, helping them prioritize relevant and credible information, learn about policy realignments, and the impact of their decision on public policy. This paper posits that the ideological structure of amicus coalitions sends influential signals that affect justices’ voting behavior. Using a merged dataset of the amicus curiae network, the ACNet ideal scores, and the Supreme Court database (1955–2012), the findings suggest that support from a diverse amicus coalition increases the likelihood of a justice’s vote for that litigant. Moreover, these signals are moderated by a justice’s ideology and the issue area of the case.