Specializing the operation of an explicit rule

The effect of practice on the operative form of a rule was investigated by giving subjects an easy, perfectly predictive classification rule, followed by training in applying that rule to a set of practice items. On a subsequent transfer test, the accuracy and speed of classifying new items was strongly affected by similarity to previously seen items, suggesting that the effect of practice was not simply to automatize the rule. The effect occurred with pictorial, easily integrated stimuli, but not with lists of verbally stated features. Subjects generally did not have insight into the role of previous items in their performance. This dependence on prior episodes may be frequent in ecologically common conditions and is of special interest when the categorization rule becomes uncertain, as when a rule has only heuristic value.