Über die Autorin bzw. den Autor

Michael Laver is professor of politics at New York University. He is the coauthor of Multiparty Government: The Politics of Coalition in Europe. Ernest Sergenti is a consultant at the World Bank.

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"Laver and Sergenti argue that politics is best viewed as a complex dynamical system that is neither random nor in equilibrium. Their use of agent-based modeling to better understand the flux of politics will be of great interest to the next generation of modelers seeking to make sense of the wide variety of political systems in the real world."--Norman Schofield, Washington University in St. Louis

"Laver and Sergenti present a bold and rigorous new approach to interparty electoral competition. Their results are remarkable and often puzzling, offering plentiful food for thought for modelers, analytical theorists, and empiricists alike. Ignore this book at your peril, or better, read it and become an admirer."--Kaare Strøm, University of California, San Diego

"Party Competition is an ambitious and pioneering work. Laver and Sergenti present a new methodology for the study of a quite central and traditional problem in political science. This is to my knowledge the first book-length treatise on the evolutionary modeling of party competition."--Hannu Nurmi, author of Models of Political Economy

"Distinct and important. The tools that Laver and Sergenti bring to bear on computational modeling will start a debate that is long overdue in the social sciences. This is another step forward in developing the methods needed to solve real-world problems that have so far resisted our best efforts."--Scott de Marchi, Duke University

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Party Competition

PRINCETON UNIVERSITY PRESS

Contents

Preface............................................................................viiAcknowledgments....................................................................xiiiPart One: Preliminaries............................................................11. Modeling Multiparty Competition.................................................32. Spatial Dynamics of Political Competition.......................................153. A Baseline ABM of Party Competition.............................................284. Systematically Interrogating Agent-Based Models.................................56Part Two: The Basic Model..........................................................835. Benchmarking the Baseline Model.................................................856. Endogenous Parties, Interaction of Different Decision Rules.....................1067. New Decision Rules, New Rule Features...........................................132Part Three: Extensions and Empirics................................................1578. The Evolutionary Dynamics of Decision Rule Selection............................1599. Nonpolicy Factors in Party Competition..........................................18310. Party Leaders with Policy Preferences..........................................20611. Using Theoretical Models to Analyze Real Party Systems.........................22812. In Conclusion..................................................................258References.........................................................................267Index..............................................................................275

Chapter One

We hold these truths to be self-evident:

? Politics is dynamic. It evolves. It never stops; It is never at, nor en route to, some static equilibrium. Politics evolves.

? Politics is complex. Political outputs today feed back as input to the political process tomorrow.

? Politicians are diverse. In particular, different politicians attack the same problem in different ways.

? Politics is not random. Systematic patterns in political outcomes invite systemic predictions, making a political "science" possible.

Politics in modern democracies is largely the politics of representation. It concerns how the needs and desires, the hopes and fears of ordinary citizens affect national decision making at the highest level, doing this via public representatives who are chosen by citizens in free and fair elections. Representative politics is to a large extent about party competition: about how a small number of organized political parties offer options to a large number of voters, who choose at election time between alternative teams of public representatives. Party competition is therefore a core concern for everyone, be they professional political scientist or ordinary decent civilian, who cares about politics in democratic societies.

We believe that party competition is a complex and evolving dynamic process that can be analyzed in a rigorous scientific manner. More precisely, we analyze the dynamics of multiparty competition, by which we mean competition for voters' support among more than two parties, opening up the possibility that no single party wins a majority of votes cast. Figure 1.1 plots some observations of multiparty competition in the Netherlands over the period 1970–2005. The left panel shows positions of the three main Dutch parties on a left-right scale of party ideology, estimated from their party manifestos. The right panel shows support for these same parties in the Dutch electorate, estimated using Eurobarometer surveys. While some of the plotted "variation" in party sizes and policy positions is surely the result of measurement error, by no stretch of the imagination was the Dutch party system "flatlining" in steady state during the period under observation. It was clearly a dynamic system and, as a result, there were frequent changes in the partisan composition of Dutch governments. These dynamics are clearly a central concern for all political scientists analyzing Dutch politics during this period, be they theorist or country specialist. Equivalent plots can be generated for any party system in which we might be interested.

We Need a New Approach to Modeling Party Competition

Formal models of party competition have been an abiding preoccupation of political scientists since the early 1960s. A vast body of existing work has added hugely to our understanding of party competition. Our own substantive interest, however, and we believe the substantive interest of most people who want to understand party competition in democratic societies, concerns crucial features of party competition that these models typically assume away as a price to be paid for analytical tractability. We ourselves are interested in party competition among many more than two parties. We are interested in "multidimensional" political environments in which politicians and voters care about more than one type of issue. We see politics as a continuously evolving dynamic process that never settles at some static equilibrium, to be perturbed only by random shocks. Pursuing these interests poses formidable theoretical challenges. We show in chapter 2 that dynamic models of multiparty competition, especially when voters care about a diverse set of issues, are analytically intractable. They are not just "difficult" to solve, they cannot be solved using conventional analytical techniques.

The analytical intractability of the relevant theoretical models does not make us any less interested, substantively, in dynamic multiparty competition. Indeed, this very intractability gives us an important and liberating theoretical insight. If analysts cannot use tractable formal models to find optimal courses of action in this setting, then neither can real people making real decisions about real party competition. These people still need to make decisions about what to do. If no formally provable best-response strategy is available, real humans must employ informal decision rules or heuristics. To preview a decision rule we investigate extensively in this book, a party leader might decide to move party policy toward the position currently advocated by some larger rival party, on the grounds there must be more voters who prefer this rival's policy position. We find that this decision rule (which we call Predator) is sometimes very, very good and sometimes perfectly horrid. It is certainly not a "best" response in any conceivable situation but, in the analytically intractable setting of dynamic multiparty competition, it is one of many potentially good rules that politicians may use in certain circumstances when they set party policy positions.

Agent-Based Modeling

Analytical intractability of the decision-making environment, and the resulting need for real politicians to rely on informal decision rules, suggests strongly that we use agent-based modeling to study multiparty competition in an evolving dynamic party system. Agent-based models (ABMs) are "bottom-up" models that typically assume settings with a fairly large number of autonomous decision-making agents. Each agent uses some well-specified decision rule to choose actions, and there may well be...