This research paper examines the extent to which high-stakes competitive tests affect gender gaps in standardized tests of Mathematics and Language. To this end, we estimate models that predict students’ results in two national standardized tests: a test that does not affect students’ educational trajectory, and a second test that determines access to the most selective universities in Chile. We used data from different gender twins who took these tests. This strategy allows us to control, through household fixed effects, the observed and unobserved household characteristics. Our results show that competitive tests negatively affect women. In Mathematics, according to both tests, there is a gender gap in favor of men, which increases in the university entrance exam, especially for high-performance students. As the literature review shows, women are negatively stereotyped in Mathematics, so this stereotype threat could penalize high-achieving women, that is, those that go against the stereotype. In Language tests, women outperform men in the standardized test taken in high school, but the situation is reversed in the university entrance exam. From our analysis of Chilean national data, we find no evidence that the gender effect observed in the competitive test depends on the students’ achievement level. Following the literature, this gender gap may be linked to women’s risk aversion, lower self-confidence, lower preference for competition, as well as the effect of answering a test under time pressure.