In this chapter I hope to demonstrate that answering the question “who does what in reasoning experiments?” can also help us answer fundamental questions about the nature of thought. Because it is nearly always the case that some experimental participants display phenomena of interest and others don't, almost every experiment run by cognitive psychologists produces data on individual differences. But, as most psychologists do not wish to take differences between their participants as the starting point of their investigations, individual differences tend to be ignored. Thus, the means that experimental psychologists report when they describe their data abstract across differences between individuals, with only the standard deviation or standard error indicating the extent to which participants varied in their responses. In this chapter I will address what the study of differences between individuals might tell us about a range of issues in inductive reasoning, including what constitutes a good inference, what processes underlie induction, and how induction differs from deduction.

My primary focus will be on the general question of how individual differences data support dual-process theories of thinking (Evans & Over, 1996; Stanovich, 1999; Sloman, 1996). Dual-process theories have been applied to deduction, decision making, and induction. I will outline the dual process approach and describe evidence in its favour, paying particular attention to individual differences. I will also describe how individual-differences methodology has been used to arbitrate between different normative accounts of specific thinking tasks (see Stanovich & West, 1998b, c). With this background covered I will consider what individual differences might tell us about category-based induction.