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Our principal hypothesis is that reasoning and decision making are alike in that they both depend on the construction of mental models, and so they should both give rise to similar phenomena. In this paper, we consider one such phenomenon, which we refer to as "focussing": individuals are likely to restrict their thoughts to what is explicitly represented in their models. We show that focussing occurs in four domains. First, individuals fail to draw inferences in the modus tollens form: if p then q, not-q, therefore not-p, because they focus on their initial models of the conditional, which make explicit only the case where the antecedent (p) and consequent (q) occur. Second, in Wason's selection task, they similarly tend to select only those cards that are explicitly represented in their initial models of the conditional rule. Third, their requests for information in order to enable them to make a decision about whether or not to carry out a certain action are focussed on the action to the exclusion of alternatives to it. In each of these cases, we show how the focussing bias can be reduced by certain experimental manipulations. Finally, in counterfactual reasoning, focussing underlies individuals' attempts to imagine an alternative scenario that avoids an unfortunate ending to a story.