The goal of this paper is to analyze the role that non-cognitive skills and, in particular, regional differences in those skills, play on the observed differences in 15-year-old student’s academic performance, across Spanish regions, on PISA 2009. Previous research has shown the relevance of differences in student’s personal, family and school characteristics in accounting for academic differences across Spanish regions but it has also found that a sizeable part of the observed differences remained unexplained. We have found that differences in the distribution of certain non-cognitive skills associated to academic performance like focus, perseverance and resilience play a prominent role in accounting for differences in student performance in PISA 2009. We observe these skills by developing new measures of student effort on standardized tests. In particular, our estimates suggest that a standard deviation reduction in the dispersion of non-cognitive skills across Spanish regions would lead to a 25% reduction in the magnitude of the observed differences in student performance across regions. This is a relevant effect as, for example, a one standard deviation reduction in the regional dispersion of parent’s educational levels or occupational status would only lead to at most a 2% reduction in the magnitude of observed differences in performance on PISA across Spanish regions. Put plainly, a substantial portion of the regional variation in test scores appears attributable to effort on the PISA test, and not necessarily just differences in actual knowledge.