|Tests measuring and comparing educational achievement are an important policy tool. We experimentally show that offering students extrinsic incentives to put forth effort on such achievement tests has differential effects across cultures. Offering incentives to U.S. students, who generally perform poorly on assessments, improved performance substantially. In contrast, Shanghai students, who are top performers on assessments, were not affected by incentives. Our findings suggest that in the absence of extrinsic incentives, ranking countries based on low-stakes assessments is problematic because test scores reflect differences in intrinsic motivation to perform well on the test itself, and not just differences in ability.