We present a new event-level predictor of comparative optimism: comparative optimism is larger for more socially undesirable events. A meta-analysis shows that event social undesirability predicts comparative optimism effect sizes reported in the literature, over and above the effects of other known predictors. Four experiments corroborate this finding and demonstrate the key role played by respondents’ impression management motives. The effect of social undesirability decreases with stronger than usual anonymity assurances, increases with greater impression management tendencies, and reverses when people want to make a negative impression. Because social undesirability is correlated to other known predictors of comparative optimism (e.g., controllability, severity), it is important to take its effects into account when assessing the effect of other event characteristics. The current research adds to, and bridges, the literatures on event-level predictors and impression management in comparative optimism.