We document that women compete worse against men in field tournaments in over 150 countries and across all ages. Our field setting is the game of chess and we benefit from a large and rich dataset to investigate the robustness and heterogeneity of our uncovered gender differences in competition. We find a macro gender gap in every country: there are fewer female than male players, especially at the top, and women have lower average rankings. Moreover, comparing millions of individual games, we find a small but robust micro gender gap: women’s scores are about 2% lower than expected when playing a man rather than a woman with an identical rating, age and country. Using a simple theoretical model, we show how this small micro gap may affect women’s long-run human-capital formation. By reducing effort and increasing the probability of quitting, both effects accumulate to explain a larger share of the macro gap.