Centre-Party Strength and Major-Party Divergence in Britain, 1945–2005

British elections exhibit two patterns contrary to expectations deriving from Duverger and Downs: centrist third parties (Liberals and their successors) win a large vote share; and the two major parties often espouse highly divergent policies. This article explores relations between the Liberal vote and left–right scores of the Labour and Conservative manifestos in the light of two hypotheses: the vacated centre posits that Liberals receive more votes when major parties diverge; the occupied centre proposes a lagged effect in which major parties diverge farther after Liberals do well in the preceding election. Data from elections since 1945 confirm the vacated-centre hypothesis, with Liberals benefiting about equally when the major parties diverge to the left and right, respectively. The results also support the occupied-centre hypothesis for Conservative party positions, but not for Labour’s. After considering explanations for this asymmetry, we identify historical events associated with turning points that our data reveal in post-war British politics.