Abstract This study investigates the influence of a decision aid on decision makers' model‐based choices, emotions during the use of the model, and attitudes towards the model. A time allocation decision model was biased to purposefully provide optimistic or pessimistic criterion levels, on which subjects based their allocations. The results of our experiment indicate that the degree of “optimism” and “pessimism” inherent in the decision model had a significant impact on the decision maker's choices of criterion values, with optimism leading to higher criterion level choices and pessimism to lower levels. Furthermore, compared to pessimistic models, optimistic models significantly improved the decision makers' emotional states and, to some degree, their attitudes towards the decision aid. The implications of these conscious and sub‐conscious influences on decision makers' choices, emotions, and attitudes are discussed and the need for model‐builders and users to be aware of them is highlighted. Copyright © 2007 John Wiley & Sons, Ltd.
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