The Evolution of Party Systems between Elections Michael Laver Trinity College Kenneth Benoit Trinity College Most existing theoretical work on party competition pays little attention to the evolution of party systems between elections as a result of defections between parties. In this article, we treat individual legislators as utility-maximizing agents tempted to defect to other parties if this would increase their expected payoffs. We model the evolution of party systems between elections in these terms and discuss this analytically, exploring unanswered questions using computational methods. Under office-seeking motivational assumptions, our results strikingly highlight the role of the largest party, especially when it is “dominant” in the technical sense, as a pole of attraction in interelectoral evolution. he existing political science account of party com- (Heller and Mershon 2001; de Dios 2001), or problems of petitionpayslittleattentiontotheevolutionof leg- party consolidation (Agh´ 1999; Verzichelli 1999). Every Tislatures between elections, despite the fact that, defection from some party, however, implies that some in all real legislatures, there is a great deal of politics other party is willing to accept the defector. Existing work between elections. In particular, legislators may defect on party switching focuses exclusively on the rationale of from one party and join another, parties may split and the switchers, while ignoring the incentives of the parties fuse, and the party system may thereby evolve into one that the switchers are attempting to join. Finally, existing quite different from that produced by the election result. models have nothing to say as to which party switchers This carries obvious analytical implications for model- will wish to join when there are several possibilities. ing party competition and important normative impli- Absent from the literature on party switching is any cations for our appreciation of representative democ- consideration of the payoffs that politicians expect from racy. In supplying the link between the popular mandate affiliation to a given legislative party. Yet, in addition to and public policy in representative democracy, the forces the electoral benefits of a party label, membership of a that shape legislatures between elections are clearly very legislative party is the key to many of the payoffs of “win- important. ning” the political game by getting into office and being While there is a literature on party switching in the associated with the incumbent government, as Cox (1987) U.S. Congress (e.g., Nokken 2000), exploration of this in pointed out in his analysis of the evolution of coherent po- other contexts is very limited. Yet it is precisely in multi- litical parties in Britain. Even in countries such as Britain party systems that the phenomenon is most prevalent and or Ireland where the electoral system makes it possible to important. In the Italian legislature between 1996 and be returned to parliament as an “independent” without spring 2000, for instance, more than one in four deputies party affiliation, it is effectively impossible for an inde- changed parties at least once (Heller and Mershon 2001, pendent legislator to enter government. 2). Similar patterns of party switching can be seen in Here we model the party affiliation of legislators as Japan (Laver and Kato, 2001), Poland (Benoit and Hayden a matter of choice that is continuously under review. We 2001), and elsewhere (see Bowler, Farrell, and Katz 1999). explore ways in which some parties attract potential de- At present there is very little work modeling this process in fectors, some are willing to accept defectors, and some a multiparty context. Existing studies focus on party disci- are both attractive and willing, allowing defections actu- pline (Heller and Mershon 2001), the electoral connection ally to take place. Rather than seeing the legislative party Michael Laver is Professor of Political Science at Trinity College, University of Dublin, Dept. of Political Science, Dublin 2, Ireland ([email protected]). Kenneth Benoit is Lecturer in Political Science at Trinity College, University of Dublin, Dept. of Political Science, Dublin 2, Ireland ([email protected]). We thank Kenneth Shepsle, Patrick Dunleavy, Steven Reed, Thomas Brauninger, and three anonymous referees for comments. Work on this article was carried out while Michael Laver was a Government of Ireland Senior Research Fellow. American Journal of Political Science, Vol. 47, No. 2, April 2003, Pp. 215–233 C 2003 by the Midwest Political Science Association ISSN 0092-5853 215 216 MICHAEL LAVER AND KENNETH BENOIT system as remaining static between elections, we treat the trate on the legislative arena, accepting that this is only composition of legislative parties as the output of one cy- part of the story. cle in the process of legislative politics, as well as an input into the next cycle. In this way, we explore the evolution Motivational Assumptions of legislative party systems between elections and pro- vide insights into the structure and stability of different Many types of payoff motivate politicians to join legisla- types of party system. We describe our approach to ana- tive parties. In the existing literature, payoffs are divided lyzing the evolution of legislative party systems between into two classes, one focused on the benefits of office elections in the second section. In the third section we and the other on policy. Office-seeking models assume explore analytically some of the implications of this ap- that the paramount objective of legislators is to get into proach for party competition. In the fourth section we office (e.g., Downs 1957), and that features such as pol- continue this exploration using computational methods icy positions are essentially instrumental. Policy-seeking to investigate the comparative statics of our model. In the models, in contrast, are grounded in the assumption that fifth section we characterize what makes certain parties policy outputs rather than the perquisites of office pro- “attractive” to, and “willing” to accept, defectors. We con- vide the fundamental political payoffs (e.g., McKelvey and clude in the sixth section with a review of the substantive Schofield 1986, 1987). Most authors recognize that a more implications of these results for multiparty systems, with realistic model would combine both office- and policy- special reference to the bases of competition between the seeking motivations, but there is no basic agreement on two largest parties. the rather arbitrary trade-offs between office and policy payoffs that such models require. Nor it is clear that such models could be solved or even clearly specified. Analyzing the Evolution of Party Our approach is deliberately simple. We focus on the Systems between Elections office-seeking payoffs to members of a party that goes into government, defined as a party that nominates members Most models of government formation and party com- of the cabinet. Nonetheless, when we discuss the analytical petition describe a system in which nothing happens in a implications of our model, we give some consideration to legislature between elections, except perhaps reactions to the effects on our conclusions of assuming alternative, a stream of exogenous shocks.1 Our focus, in contrast, is policy-seeking motivations. precisely upon moves made by legislators between elec- tions. We envisage a process in which legislators contin- Operationalizing Office-Seeking uously keep their party affiliation under review, being Expectations attracted to defect to other parties if these offer higher payoffs. Within the government formation literature and follow- Members of existing parties are willing to accept new ing Riker (1962), office-seeking motivations are con- members if this enhances their own expectations. Each ceived in terms of competition for a fixed set of rewards time a legislator changes party affiliation there is a new of office. The expected collective payoff, or expectation, party configuration. All legislators then reconsider their of a set of legislators forming a Party P in a given party position since their expected payoffs will have changed. configuration, D,isthusEpd, defined on the interval Thus each change in affiliation potentially provokes an- [0,1] as their expected collective share of the total fixed other, unless an equilibrium is reached in which no legis- payoff. lator who wants to change parties can find a party he both Going beyond the unitary actor approach, we must wants to join and that is willing to accept a new member. make an assumption about how much a legislator expects What we construct below is a model of legislative rather to receive from any party to which he belongs. The sparest than electoral politics. In an ideal world we would con- assumptionusesnoinformationaboutintrapartyfactions struct a model that took account of the interaction be- or decision making. All legislators are seen as autonomous tween legislative, executive, and electoral politics. We feel actors. In the intraparty politics that distributes the spoils we would set ourselves an impossible analytical task if we of victory, all legislators are assumed to be equivalent— tried to do this at the outset, however, and thus concen- by symmetry, all have equal expectations. The expectation of a generic legislator L in Party P with a seat share sp in configuration D is thus E /s . This measure of the ex- 1For work modeling reactions in a legislature to exogenous shocks, pd p see Lupia and Strom (1995); Laver and Shepsle (1998); Diermeier pectation of a generic party legislator is the “expectation- and Stevenson (1999, 2000); Diermeier and Merlo (2000). to-weight” ratio (EWR) of the party in question. THE EVOLUTION OF PARTY SYSTEMS BETWEEN ELECTIONS 217 Note that if we had information about factional struc- tion of the incentive structure of an office-seeking party tures, the substantive roles of individual legislators or system.3 other aspects of intraparty politics, we might want to model intraparty resource allocation in a different way and would then need more complex indicators of the ex- Modeling the Evolutionary Dynamics pectations of legislators.