Traditionally, thematic relatedness (chicken and egg) and similarity (chicken and turkey) have been thought of as distinct phenomena, the former the result of associative processes, and the latter reflecting comparison processes. However, recent studies (Bassok & Medin, 1996; Wisniewski & Bassok, 1996) suggest that similarity is a result of both association and comparison. This could call for a radical redefinition of similarity as inherently fused with association. We term this view the integration account. We consider an alternative, the confusability account, under which thematic influences intrude upon assessments of similarity but are not an essential part of the similarity process. W e present two experiments supporting the confusability account. The first indicates that comparison and association are independent processes. The second shows that thematic influences rise with increased cognitive load. We believe that while a redefinition of similarity is not warranted, similarity is more vulnerable to error and intrusion than is generally thought.

planation and evidence play important and non?interchangeable roles in argument. However, previous research has shown that subjects often confuse explanation and evidence (Kuhn, 1991). This study investigates the circumstances under which this confusion occurs. In Experiment 1, subjects generated arguments about issues of popular interest such as problems in schools and drug abuse. In Experiments 2 and 3, subjects rated the strength of evidence presented to them. The results of the protocol analyses and ratings tasks suggest that subjects tend to overestimate the strength of explanations when they lack sufficient knowledge of the domain or when they are unable to generate alternatives to the hypotheses presented to them. W e consider reasons why relying on explanations in these circumstances might be a valuable heuristic