Coalition formation and polarisation

Party ideology plays an important role in determining which government coalitions form. Research on coalition formation tends to focus on the ideological distance between coalition parties. However, the distribution of preferences within the coalition, and the legislature, also has implications for which government coalition forms – that is, a party's willingness to join a coalition depends not only on its prospective coalition partners, but also on the alternative coalitions it could form. Several hypotheses about the effects of legislative polarisation are offered and tested using data on coalition formation in 17 parliamentary democracies in the postwar period. This article also demonstrates how the traditional measure of ideological divisions within coalitions fails to capture certain aspects of ideological heterogeneity within the cabinet (and the opposition) and how Esteban and Ray's polarisation index helps in addressing these deficiencies.