In this paper we use new data on Danish monozygotic (MZ) twins to analyze the effect of cultural capital on educational success. We report three main findings. First, cultural capital has a positive direct effect on the likelihood of completing the college-bound track in Danish secondary education. Second, cultural capital leads teachers to form upwardly biased perceptions of children's academic ability, but only when their exposure to children's cultural capital is brief (as in oral and written exams) rather than long (as in grades awarded at the end of the school year). Third, we find that the positive direct effect of cultural capital on educational success is higher for children from high-socioeconomic status (SES) backgrounds than for those from low-SES backgrounds. This result suggests that high-SES children are more likely to be in schooling contexts that enable them to convert cultural capital into educational success.