Introduction

In the standard Downsian model, voters are assumed to choose parties based on the extent of ideological proximity between the voter's own position and that of the party. Yet voters may misestimate the policy platforms of candidates or parties, either out of ignorance or in a fashion that reflects systematic bias. For example, some voters may subjectively position parties that they favor closer to the voter's own preferred policy location than do voters as a whole.

Insights taken from the psychological literature on persuasion (Sherif and Hovland 1961; Parducci and Marshall 1962) distinguish between two different types of bias/projection effects: assimilation and contrast. Assimilation effects refer to shortening the perceived ideological distance between oneself and parties (or candidates) whom one favors; contrast effects refer to exaggerating the distance between oneself and parties (or candidates) whom one does not support.

The importance of assimilation and contrast to our study of optimal party locations and equilibrium configurations is that perceptions of where the parties are located can alter the effects of party positioning on voters' decision calculus. If, for example, Republican voters in the United States perceive the Democratic candidate to be significantly more liberal than do other voters (and we present evidence that this is the case), they will be even less likely to vote for the Democrat (and less likely to abstain due to indifference) than might be predicted by our models, which use mean placement by all voters to specify party position.

Therefore, assimilation and contrast have the effect of intensifying the centrifugal effect of party ID on party strategies.