Suboptimal outcomes in negotiation have been associated with the implicit fixed-pie bias. The ability to correct this bias might be a critical capacity in negotiation and is often at the core of negotiation training. Cognitive reflection – an individual thinking disposition enabling people to suppress and override automatic responses – predicts performance in a variety of individual heuristics and biases tasks. A study (N = 262) investigated whether cognitive reflection predicts negotiation outcomes and whether improvements associated with training are mediated by training-enhanced cognitive reflection of the participants. The results show that cognitive reflection predicts both an individual negotiator’s gain and all aspects of joint gain. Training enhances performance and is partially mediated by increased cognitive reflection. The findings support the proposition that cognitive reflection is an independent thinking disposition that underpins resistance to bias and improves outcomes in negotiation settings