How people differ in syllogistic reasoning

Psychologists have studied syllogistic inferences for more than a century, but no extant theory gives an adequate account of them. Reasoners appear to reason using different strategies. A complete account of syllogisms must therefore explain these strategies and the resulting differences from one individual to another in the patterns of conclusions that they draw. We propose a dual-process theory that solves these two problems. It is based on the manipulation of mental models, i.e., iconic simulations of possibilities. We also propose a new way in which to analyze individual differences, which depends on implementing a stochastic computer program. The program, mReasoner, generates an initial conclusion by building and scanning a mental model. It can vary four separate factors in the process: the size of a model, its contents, the propensity to consider alternative models, and the propensity to revise its heuristic conclusions. The former two parameters control intuitive processes and the latter two control deliberative processes. The theory accounts for individual differences in an early study on syllogisms (Johnson-Laird & Steedman, 1978). The computational model provides an algorithmic account of the different processes on which three subsets of participants relied (Simulation 1). It also simulates the performance of each individual participant in the study (Simulation 2). The theory and its implementation constitute the first robust account of individual differences in syllogistic reasoning.