Past research paints a mixed picture of rationales in negotiations: Some findings suggest rationales might help, whereas others suggest they may have little effect or backfire. Here, we distinguish between two kinds of rationales buyers commonly employ — constraint rationales (referring to one's own limited resources) and disparagement rationales (involving critiques of the negotiated object) — and demonstrate their divergent effects. Across four studies, we examined spontaneous rationales and manipulated rationale content, finding that constraint rationales have more positive effects on instrumental (e.g., counteroffers) and relational (e.g., trust) outcomes than disparagement rationales. Mediation analyses suggest constraint, but not disparagement, rationales are taken by sellers as signaling a buyer's limit. We also demonstrate a role for information, showing that the divergence between these rationales' effects is attenuated when the seller has little information about their object's value. Overall, our results show how and why rationales can help or hurt negotiators.