Consistent contrast and correlation in free sorting.

Two experiments investigated free sorting, a type of unsupervised learning, with multiattribute drawings of alien animals. In previous research on concept formation, with simpler stimulus structure than ours, participants were insensitive to correlational structure in the stimuli, producing primarily "ID sorts," based on the values ofjust 1 dimension or attribute. Our experiments showed that participants used many strategies in categorizing but preferred to generate groupings that reflected the correlations in input when this did not violate consistent contrast. The second experiment used hierarchically structured stimuli to show that participants' sort strategies favor consistency within a set of contrasting categories, distinct from any preference for 1D sorting. Finally, both experiments show that correlational sorts are much more likely when the correlation-based sort contrasts consistently. Our data show complexity at work in free sort tasks: People are sensitive to multiple and sometimes conflicting biases for consistency and correlational structure in the category systems they create.