Coalition formation on the U.S. Supreme Court: 1969-2009

We apply a fallback model of coalition formation to decisions of the U.S. Supreme Court, focusing on the seven natural courts, which had the same members for at least two terms, between 1969 and 2009. The predictions of majority coalitions on each of the courts are generally borne out by the 5-4 decisions, whereas the predictions of the Martin-Quinn (Political Analysis 10(2):134-153, 2002) model, which assumes a single underlying dimension along which the justices can be ordered, are not. The present model also provides insight into the dynamic process by which subcoalitions build up into majority coalitions and, in addition, identifies "kingmakers" and "leaders" on the natural courts. Furthermore, it provides evidence, from coalitional memberships, that a few justices shifted over time from one ideological camp to another.