A probability may be called "default" if it is neither derived from preestablished probabilities nor based on considerations of frequency or symmetry. Default probabilities presumably arise through reasoning based on causality and similarity. This article advances a model of default probability based on a featural approach to similarity. The accuracy of the model is assessed by comparing its predictions to the probabilities provided by undergraduates asked to reason about mammals.
Osherson, Daniel N.; Stern, Joshua; Wilkie, Ormond; and Stob, Michael, "Default Probability" (1991). University Faculty Publications. 521.