A total of 141 children, aged 6 to 10 years, were read eight illustrated attribution problems involving play and non-play behaviors and were asked to infer information concerning the story character's intrinsic or extrinsic motivation for the behaviors. Children were grouped into three age levels (mean ages 5.9, 8.8 and 10.3 years) for analysis of developmental effects. In each age group, half the children were given information about the story character's intrinsic motivation and were asked to infer extrinsic motivation, and half were given the reverse. The intensity of the depicted behaviors was also varied. It was hypothesized that, contrary to the overjustification hypothesis in attribution theory, young children would perceive that extrinsic rewards add to, rather than undermine, the contribution of intrinsic motivation in play behavior, and that this effect would be a simple additive function. Results confirmed the hypothesis: when the story character was given a reward, 6-year-olds inferred greater intrinsic motivation whereas 10-year-olds inferred less intrinsic motivation. A similar pattern occurred in the reverse condition. Attribution of the non-play behaviors did not change with age. Functional measurement analysis revealed that simple addition and subtraction operations characterized the children's judgments. (Author/SS)