Negotiating about a larger number of issues is often argued to enhance the potential for integrative bargaining. However, the enhanced complexity may also make negotiators more susceptible to bias, making it less likely for them to reach win-win agreements. We argue that epistemic motivation, the motivation to hold accurate perceptions of the world, provides a key to solve this paradox. In a negotiation experiment we manipulated complexity by having participants negotiate about 6 or 18 issues and we manipulated epistemic motivation by making participants process-accountable or not. Under low complexity, there was no effect of epistemic motivation on created value. Under high complexity, however, negotiators with high epistemic motivation created more value than negotiators with low epistemic motivation. Thus, negotiating about larger numbers of issues was only beneficial for negotiators if they were motivated to think deeply and thoroughly.