Are there measurable differences in general mathematics achievement, inductive reasoning, syllogistic reasoning, perceptual speed, and tendency to overgeneralize between groups learning mathematical concepts through the use of positive instances (C groups), and groups learning mathematical concepts through the use of both positive and negative instances (E groups)? 84 eighth grade mathematics students in 4 classes served as subjects for 65 days. Analysis of variance and covariance of pre- and posttest means yielded significant differences (p<.05) indicating that the E groups tended to overgeneralize less frequently than the C groups. No other differences in means were significant (p<.05). It appears that the use of negative instances may be a means for controlling the common error of overgeneralization.