Policy debates in education are greatly influenced by international differences in test scores. The presumption is that differences in test scores reflect differences in cognitive skills and content knowledge. We challenge this presumption by studying how much of the variation in the Program for International Student Assessment (PISA) scores is associated with student effort. We build a number of measures of student effort on the basis of both the PISA test and the student survey. Together, our measures of student effort explain between 32 and 38 percent of the variation in test scores across countries.