This paper compares fuzzy and classical decision rules. The hypothesis of this paper is that whether one of these rules is superior depends on the situation. For that comparison the paper uses system dynamics (SD), which models the behavior of systems including human beings. This comparison was made for a simple heating system that is controlled by a human operator. Under various changes of external and internal parameters, the results are that the major differences between fuzzy and crisp systems emerge at extreme values of these parameters. In conclusion, the superiority of crisp rules or fuzzy rules in a decision-making environment depends on the situation. (C) 2002 Elsevier Science B.V. All rights reserved.