Mate preference research has focused on traits people desire in partners (i.e., dealmakers) rather than what traits they avoid (i.e., dealbreakers), but mate preferences calibrate to both maximize benefits and minimize costs. Across six studies (N > 6,500), we identified and examined relationship dealbreakers, and how they function across relationship contexts. Dealbreakers were associated with undesirable personality traits; unhealthy lifestyles in sexual, romantic, and friendship contexts; and divergent mating strategies in sexual and romantic contexts. Dealbreakers were stronger in long-term (vs. short-term) relationship contexts, and stronger in women (vs. men) in short-term contexts. People with higher mate value reported more dealbreakers; people with less-restricted mating strategies reported fewer dealbreakers. Consistent with prospect and error management theories, people weighed dealbreakers more negatively than they weighed dealmakers positively; this effect was stronger for women (vs. men) and people in committed relationships. These findings support adaptive attentional biases in human social cognition.