A national experiment shows that a short, online growth mindset intervention improves academic achievement, particularly for lower-achieving students. The study, involving 6,320 U.S. high school students, found that the intervention, which teaches that intelligence can be developed, increased grades and advanced math course enrollment. The intervention was most effective in schools where peer norms supported growth mindset beliefs. The study used a nationally representative sample and rigorous methods, including pre-registration and independent data analysis, to ensure validity. Results showed a 0.11 standardized mean difference effect size on core course GPAs, with a 11% relative risk reduction in students at risk of not graduating on time. The intervention also increased the likelihood of taking advanced math courses by 3 percentage points. The study highlights the importance of school context in sustaining intervention effects and underscores the potential of growth mindset interventions to improve educational outcomes for disadvantaged students. The findings suggest that such interventions, when implemented in supportive environments, can have meaningful, scalable impacts on academic achievement.A national experiment shows that a short, online growth mindset intervention improves academic achievement, particularly for lower-achieving students. The study, involving 6,320 U.S. high school students, found that the intervention, which teaches that intelligence can be developed, increased grades and advanced math course enrollment. The intervention was most effective in schools where peer norms supported growth mindset beliefs. The study used a nationally representative sample and rigorous methods, including pre-registration and independent data analysis, to ensure validity. Results showed a 0.11 standardized mean difference effect size on core course GPAs, with a 11% relative risk reduction in students at risk of not graduating on time. The intervention also increased the likelihood of taking advanced math courses by 3 percentage points. The study highlights the importance of school context in sustaining intervention effects and underscores the potential of growth mindset interventions to improve educational outcomes for disadvantaged students. The findings suggest that such interventions, when implemented in supportive environments, can have meaningful, scalable impacts on academic achievement.