This study, which indicates that both age and variation in training affect children's concept formation, provides a basis for explaining the effect of age. Sixty-four 4- and 5-year-olds learned three novel concepts (animal-like, plant-like, and machine-like). Subjects were presented with either four different examples of each concept (multiple item training condition) or with one example of the concept which was repeated four times (single item training condition). Following acquisition, children identified a set of recognition, generalization, and discrimination items. Neither age nor training condition affected rate of acquisition. Age and training condition affected all other measures (recognition, generalization, and discrimination). Older children and children trained with multiple items generalized more but discriminated less than did other children; these results were obtained even when recognition differences were accounted for. The pattern of results suggests that developmental changes in concept generalization may be due to changes in the liberality of children's response criteria rather than to changes in memory or the ability to abstract information. (Author/MP)