On Reasoning with Default Rules and Exceptions

We report empirical results on factors that influence how people reason with default rules of the form "Most x's have property P", in scenarios that specify information about exceptions to these rules and in scenarios that specify default-rule inheritance. These factors include (a) whether the individual, to which the default rule might apply, is similar to a known exception, when that similarity may explain why the exception did not follow the default, and (b) whether the problem involves classes of naturally occurring kinds or classes of artifacts. We consider how these findings might be integrated into formal approaches to default reasoning and also consider the relation of this sort of qualitative default reasoning to statistical reasoning. Introduction Default reasoning occurs whenever the evidence available to the reasoner does not guarantee the truth of the conclusion being drawn; that is, does not deductively force the reasoner to draw the conclusion under consideration. For ex..