Stressful situations hinder judgement. Effects of stress induced by anticipated public speaking on the Iowa Gambling Task (IGT) were examined. The Cognitive Reflection Test (CRT) was used to examine the relationship between reflective thinking and IGT performance. The stress manipulation increased blood pressure and was associated with poorer IGT and CRT performance. Stressed participants were slower to avoid the disadvantageous decks. Moreover, CRT scores correlated with optimal deck selections indicating the importance of reflective thinking for good performance on the IGT. These correlations were observed in relatively early trials, which challenges the view that analytic thinking is not important when card contingencies are being learned. Data revealed that IGT performance in healthy individuals is not always optimal; stress levels impair performance. A mediation analysis was consistent with the proposal that the stress manipulation reduced IGT performance by impeding reflective thinking. Thus, reflective processing is an important explanation of IGT performance in healthy populations. It was concluded that more reflective participants appear to learn from the outcomes of their decisions even when stressed. Copyright © 2016 John Wiley & Sons, Ltd.