WHEN PARTIES ARE NOT TEAMS: PARTY POSITIONS IN SINGLE MEMBER DISTRICT AND PROPORTIONAL REPRESENTATION SYSTEMS 1 Stephen Ansolabehere Department of Political Science William Leblanc Department of Political Science James M. Snyder, Jr. Departments of Political Science and Economics Massachusetts Institute of Technology 1We wish to thank Kristen Kanthak, Kiyoon Kim, Rebecca Morton, and participants at work- shops at Harvard University and Princeton University for their comments. Stephen Ansolabehere is Professor of Political Science at MIT,
[email protected]; Wlliam Leblanc is a candidate for PhD in the Department of Political Science at MIT,
[email protected]; James M. Snyder, Jr., is Professor of Economics and Political Science at MIT,
[email protected]. Please direct all communications to Stephen Ansolabehere, Department of Political Science, Building E53-Room 449, MIT, Cambridge, MA 02139, 617-253-5236. Abstract Theoretical analyses of party positions commonly assume that parties act as teams to maximize their legislative representation. This assumption runs counter to another line of theorizing in which individual legislators maximize their own chances of winning reelection. To resolve this tension, the paper presents a model of party platform choice that relaxes only the assumption that parties are teams in the classical two-party spatial model. Platforms are chosen by majority rule among all legislators within a party. Politicians seek to win their own seats in the legislature, but they must run under a common party label. In both single- member district and proportional representation systems, equilibrium platforms are shown to diverge substantially, with one party located near the 25th percentile of the voter distribution and the other near the 75th percentile, rather than converge to the median.