Recent findings indicate that Type As attend more to tasks defined as central, and less to tasks defined as peripheral, than do Type Bs. If this A-B difference in allocation of attention occurs naturally, as well as in response to experimental instructions, it would have implications for other phenomena, such as the process of creating a category from a series of perceptual events. It was reasoned that across a series of acquisition stimuli, Type As, compared to Type Bs, would be more responsive to the development of central tendencies in the frequency of appearance of the attributes that comprise the stimuli. That is, Type As should be more attentive to frequently occurring attributes (by virtue of their centrality in the evolving category definition) and less attentive to rarely occurring attributes. This difference in attention should lead to a difference in encoding. On a subsequent recognition-memory task, then, Type As should report with greater certainty than Type Bs that they had seen stimuli composed of frequently observed attributes and should report with greater certainty that they had not seen stimuli composed of rarely observed attributes. In a test of this reasoning, students classified as As and Bs completed a concept-formation and recognition-memory task under one of four levels of situational challenge. The predicted patterns occurred among subjects in whom moderate and high levels of challenge had been induced. The findings thus support the arguments that As and Bs process information differently and that this processing difference must be elicited by situational challenge.