The overjustification hypothesis suggests that extrinsic rewards undermine intrinsic motivation. Extrinsic rewards are common in strengthening behavior in persons with intellectual and developmental disabilities; we examined overjustification effects in this context. A literature search yielded 65 data sets permitting comparison of responding during an initial no‐reinforcement phase to a subsequent no‐reinforcement phase, separated by a reinforcement phase. We used effect sizes to compare response levels in these two no‐reinforcement phases. Overall, the mean effect size did not differ from zero; levels in the second no‐reinforcement phase were equally likely to be higher or lower than in the first. However, in contrast to the overjustification hypothesis, levels were higher in the second no‐reinforcement phase when comparing the single no‐reinforcement sessions immediately before and after reinforcement. Outcomes consistent with the overjustification hypothesis were somewhat more likely when the target behavior occurred at relatively higher levels prior to reinforcement.