Our central question is whether comparison of co-presented instances promotes category learning. We report results of four experiments testing acquisition of relational categories under conditions of Comparison learning versus traditional Single item learning. In order to control for frequency of exposure, the Single group received twice as many learning trials. Experiment 1 showed more accurate single-item classification at test for both old and new items by the Comparison group relative to the Single group. Experiment 2 used only withincategory pairs in the Comparison condition (rather than both types of pairs), but no accuracy advantage was found. Experiment 3 repeated this design using a reduced training set and showed a learning effect of comparison and a marginal advantage in transfer to new items. In Experiment 4, a novel paradigm revealed further evidence of a facilitative effect for within-category comparison. The power of comparison to promote learning and transfer is discussed in terms of mechanisms of encoding and knowledge change
