Abstract: Reasoning tests, as used in the measure of intelligence, are not reasoning experiments, as used in the psychology of reasoning. Tests are meant to capture individual differences in ability; experiments are meant to highlight general mental processes, which all individuals are expected to engage. Reasoning tests assume that everybody is different; reasoning experiments used to assume instead that everybody was the same. In recent years, though, things have changed in the psychology of reasoning. Theoretical evolutions that we briefly summarize have shifted the focus from the modal response in reasoning tasks (the one most people usually give) to issues of individual variability in responses to various tasks, stable individual patterns of response across different tasks, and to considerations of structural differences between reasoners, in addition to quantitative differences. These theoretical innovations must be fueled with refined modelling techniques. We show how modern psychometrics can help us solve three broad research questions pertaining to the new theoretical agenda, which we illustrate with conditional reasoning materials.

Key words: reasoning, experiments, individual differences, psychometrics, conditionals