["Economic Inquiry, EarlyView. ", "\nAbstract\nDoes the gender composition of high school peers affect whether students pursue a science, technology, engineering, and mathematics (STEM) major at university? Using administrative data from British Columbia, Canada, I exploit idiosyncratic within‐school variation in gender composition. I find that having larger proportions of female peers has large and strong effects on students' STEM major choice. Such effects differ in both sign and magnitude across gender, high school type, and stage of high school education.\n"]