“…Advocates of the dead centre hypothesis have offered a variety of arguments to explain the alleged increase in foreign policy polarization. The most prominent amongst them attribute polarization to the impact of the Vietnam War on the policy preferences of members of the two parties (Kupchan and Trubowitz, 2007;Nincic and Datta, 2007;Shapiro and Bloch-Elkon, 2005), the effects of the end of the Cold War (Kupchan and Trubowitz, 2007;Marshall and Prins, 2002), changes in the foreign policy issue agenda to include more economic and 'intermestic' issues (Prins and Marshall, 2001;McCormick and Wittkkopf, 1992), and the broad changes in partisan ideology and institutional procedures which are held to be responsible for the general increase in partisan polarization in Congress since the 1970s (Fleisher and Bond, 2004;Jacobson, 2000;McCarty et al, 2006;Theriault, 2008). Whichever individual explanation or combination of these explanations is preferred, however, all are premised on an ongoing and/or permanent change in the independent variable(s) which, in turn, produces a similarly permanent and ongoing change in the dependent polarization variable as a result.…”