The primacy order effect in complex decision making

The goal of the present study was to assess the role of information order in situations of complex decision making in which participants have to process a large amount of information (e.g., Dijksterhuis et al. Science 311(5763): 1005-1007, 2006). In two experiments, participants were presented with information about four cars, each characterized by 12 attributes. Immediately following the presentation of the 48 sentences describing these four cars, participants had to choose the one they would prefer to purchase. Two cars shared exactly the same positive and negative attributes, but they were displayed in a different order for each car. For one car, positive attributes were systematically displayed at the beginning while it was the reverse for the other car. The two remaining cars were used as fillers and had a lower number of positive attributes than the target cars in Experiment 1 and a higher number of positive attributes in Experiment 2. Results revealed a massive effect of information order with a clear preference for the car with positive information presented at the beginning. The second experiment further showed that this order effect was maintained and still strong even if the target cars did not have more positive attributes than the filler cars. Interestingly, in both experiments, participants never noticed that two cars were exactly characterized by the same list of attributes. These data clearly demonstrate that information order is a critical factor in complex decision-making situations involving a large amount of information.