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4George Tsebelis, Veto Players: How Political Institutions Work ( Princeton, NJ : Princeton University Press, 2002).
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6Boris Shor and Nolan McCarty, "The Ideological Mapping of A- merican Legislatures," American Political Science Review, Vol. 105, No. 3 ( Aug. ,2011 ) , pp. 530-551.
7Tom S. Clark, "Measuring Ideological Polarization on the United States Supreme Court," Political Research Quarterly, Vol. 62, No. 1 (Mar. 2009), pp. 146-157.
8David Brady and Hahrie Han, "Polarization Then and Now: A HistoricalPerspective," 2006, pp. 119 - 151.
9Michael Barber and Nolan McCarty, "Causes and Consequences of Polarization," in Jane Mansbridge and Cathie Jo Martin eds. , Negotiating Agreement in Politics (American Political Science Association, 2013 ), p. 20.
10Alan I. Abramowitz, The Disappearing Center: Engaged Citizens, Polarization, and American Democracy ( New Ha- ven, CT: Yale University Press, 2010) , p. 141.
