Although decision making has historically been regarded as a cold and rational process, recent research has suggested that emotional factors are actually quite central to this fundamental process. By using a modified version of the Iowa gambling task, we measured how such brands impact decision making. Participants were asked firstly to rate their liking/loyalty towards numerous brands. Some of these rated brands were later superimposed onto the card decks in a subsequent Iowa gambling task. Results demonstrate that an individual's decision making is altered depending on the congruency between brand valence (liked/not) and the deck reward structure. This bias was sometimes advantageous and led to faster and more accurate decisions. From an applied perspective, the study suggests several ways in which brands can either enhance or inhibit new product success. More broadly, the research demonstrates a seemingly irrational effect of brands on behaviour. Copyright © 2012 John Wiley & Sons, Ltd.