The effects of affirmative action on the incentives to human capital accumulation are ambiguous from a theoretical perspective and the scarce empirical evidence on the matter provides mixed results. In this paper, we address this issue by investigating the impacts of Brazil's Law of Quotas on the students' performance in the college entrance exam, the ENEM. We provide causal evidence that the law fostered incentives to pre-college human capital accumulation, inducing students to attain higher ENEM scores. Moreover, the effects of the quotas were greater in quantitative-intensive subjects and these impacts increased throughout the first years after the law's implementation.