Hunting the Core: Change in Parliamentary Parties and Policy

Abstract

In this paper we focus on the partisan mobility of individual incumbent MPs as a possible cause of major change in policy outcomes. Our theoretical argument proceeds as follows. We treat political parties in legislatures as coalitions of individual incumbents, even though parties typically behave so as to approximate unitary actors for the purposes of passing legislation and (in parliamentary democracies) forming governments. We show that individual MPs, as strategic actors, can execute moves between parties to effect change in the location of the parliamentary median and of the core. The agenda-setting powers of the ruling coalition ensure that substantial policy change ensues. We assess and illustrate our theoretical findings with cross-national evidence providing variation on the key parameter of the number of parties gaining legislative representation at election time.