Change in Parliamentary Party Systems and Policy Outcomes: Hunting the Core Carol Mershon Department of Politics University of Virginia Charlottesville VA 22904 mershon@virginia.edu Olga Shvetsova Department of Political Science Binghamton University Binghamton NY 13902 shvetsova@gmail.com June 2012 Accepted June 2012, Journal of Theoretical Politics Acknowledgements . We thank Mikhail Filippov, Daniela Giannetti, William Heller, Brian Humes, Christian Jensen, Michael McDonald, Norman Schofield, Brian Schaffner, and Francesco Zucchini for comments, and Nina Barzachka, Drew Kurlowski, Nick Merkulov, and Adriana Stojan for research assistance.