Do single-sex schools make girls more competitive?

•This study identifies the causal effect of single-sex schooling on competitiveness.•There are no systematic differences in performance across gender or school type.•Conditional on performance, girls are less competitive than boys.•Single-sex schooling does not reduce the gender-gap in competitiveness.

We examine the effect of single-sex schooling on students' competitiveness by studying middle school students in Seoul who were randomly assigned to either single-sex or coeducational schools within their school districts. Contrary to popular belief and existing studies, our results suggest that single-sex schooling does not reduce the gender gap in competitiveness conditional on student and parental characteristics.