Electors' propensities to vote tactically is the main component of what Duverger calls the "psychological effect" of electoral systems. Electoral research has therefore given considerable attention to the amount of tactical voting and its consequences for electoral results. In much of this literature, tactical voting has been analysed in terms of the influence of the performance of parties' candidates in prior elections, taking for granted that voters form expectations about candidates' chances accordingly. The formation of such expectations and whether they actually guide voting decisions has rarely been empirically investigated. We fill this gap by analysing the formation of expectations and of vote intentions during the campaign to the 2010 election to the British House of Commons, within the framework of an explicit expected utility model. We find that voters rely on both past constituency results and contemporaneous polling information to form expectations and, while developing their vote intentions, use them to weight party utilities that are based on leader evaluation and positional distance.
