Human reasoning with negative defaults

Abstract

This paper examines psychological data on human reasoning with sets of negative defaults. A negative default is a statement of the form: Xs are typically not Ys. While there is pragmatic motivation for chaining positive defaults, chaining negative defaults (concluding from, As are typically not Bs and Bs are typically Cs that As are typically not Cs) is far less reasonable. Default inheritance reasoners universally prohibit ‘negative chaining’. However, examination of the psychological plausibility of various conflicting proof theories for default inheritance has demonstrated that some fundamental assumptions of the inheritance literature do not actually hold. This work has also revealed reasoning strategies which do describe human behaviors. In an effort to define inheritance reasoners that are more predictive of human reasoning with defaults, it is important to attend to these findings. This paper focuses on the fact that many people do in fact chain negative defaults. The paper identifies a group of subjects who consistently do so, and evaluates reasoning strategies which are predictive of the behavior of those subjects with respect to other ‘benchmark’ problems that have been addressed in the literature. Corroboration is found for ‘most-path’ reasoning.

Thanks to Robin Cooper, Claire Hewson, Jon Oberlander and Jeff Pelletier for encouragement and feedback. I am grateful to the Marshall Aid Commemoration Commission for funding my Ph.D. research at the Centre for Cognitive Science at the University of Edinburgh, and to the SFB-340 B-9 for taking me to Stuttgart.