ABSTRACT: The delta rule of associative learning has recently been used in several models of human category learning, and applied to categories with different relative frequencies, or base rates. Previous research has emphasized predictions of the delta rule after extensive learning. Our first experiment measures the relative acquisition rates of categories with different base rates, and the delta rule significantly and systematically deviates from the human data. We suggest that two additional mechanisms are involved, namely, short-term memory and strategic guessing. Two additional experiments highlight the effects of these mechanisms. The mechanisms are formalized and combined with the delta rule, and provide good fits to the data from all three experiments.

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