This study examines the effects of initial response training and criterion training on the discrimination shift performance of preschool children; results are discussed in terms of differing theoretical orientation. After an initial task involving either criterion training or response training, 109 subjects were presented with either intradimensional shift (ID) or extradimensional shift (ED) problems. Subjects who failed to learn the criterion training task (nonlearners) were found to learn the ED shift more quickly than the ID shift. These results are best explained in terms of mediation theory, since an explanation based on the single stage process would require that preferences or dimensional dominance increased the initial habit strength for the irrelevant cue or dimension. Attention theory could also explain these results, because attention to relevant and irrelevant dimensions determines the ease of shift learning. The response-trained group, after 10 trials of reinforced motor training, learned a reversal (ID) shift more rapidly than the ED shift. This finding can be explained in terms of attention theory, because alteration of orienting probabilities would result in shift performance demonstrated by children who went through all the work of attaining criterion. Since such results are not well explained by mediation theory, the attentional model of children's learning is supported. (GO)