A total of 540 first, fourth, and eighth graders, equally divided by sex, were run on a two-choice discrimination task under one of three reinforcement conditions: reward (R) for correct responses, punishment (P) for incorrect responses, or reward and punishment (RP) for correct and incorrect responses. Half the Ss were tested by a male E; half, by a female E. Across all developmental levels, learning was superior in the P group. Complex interactions involving sex of S and E underline the importance of organismic variables in discrimination learning and in specifying parameters of reinforcer effectiveness.