Individual differences in same-aged children's reasoning about effort and ability, as well as the consequences of different forms of reasoning in actual achievement situations, were investigated. It was hypothesized that different forms of children's reasoning would be related to different (helpless versus mastery-oriented) motivational patterns. Research documented the existence of two distinct and essentially opposite forms of reasoning in young adolescents. One form, typically linked to developmental maturity, assumes that effort and ability are inversely related and asserts that effort discredits ability. The other form, typically ascribed to younger children, assumes a positive relationship between effort and ability and asserts that effort utilizes abililty. In two studies, 13- and 14-year-old adolescents' form of reasoning was a significant predictor of their motivational pattern, but "mature" reasoning led to the helpless pattern, whereas "immature" reasoning led to the mastery-oriented pattern. Results suggest that effort-ability reasoning may best be understood by considering both developmental and individual differences. (Author/RH)