Considerable ambiguity exists regarding the effect of government/opposition status on party platform change. Existing theories predict that (1) it has no effect, (2) opposition parties change more, (3) opposition parties change more after several spells in opposition, and (4) parties’ responses vary because of different goal orientations. We propose that a party's aspiration to office, measured by its historical success or failure in entering office, determines a party's reaction to being in opposition or government. We hypothesize that, because of loss aversion, parties with low office aspiration change more when they are in government than when they are in opposition. Conversely, parties with high office aspiration change more as opposition party than as government party. We find evidence for these hypotheses through a pooled time-series cross-sectional analysis of 1,686 platform changes in 21 democracies, using the Comparative Manifesto Data and an innovative measure of party platform change.