Prior research, primarily based on laboratory experiments of children and students, suggests that women might be more averse to competition than are men; women might, instead, be more inclined toward collaboration. Were these findings to generalize to working-age men and women across the workforce, there could be profound implications for organizational design and personnel management. We report on a field experiment in which 97,678 adults from a wide range of fields of training and career stages were invited to join a product development platform. Individuals were randomly assigned to treatments framing the opportunity as either involving competitive or collaborative interactions with other participants. Among those outside of science, technology, engineering, and math (STEM) fields, we find differences in the willingness of men and women to participate under competition. Thus, patterns in non-STEM fields conform to the usual claims of gender differences. However, among those in STEM fields, we find no statistical gender differences. Results hold under a series of alternative specifications, controls, and stratified analyses of 17 narrowly defined STEM subfields. The results are consistent with sorting into fields on the basis of competitiveness, as suggested by prior research, as well as other explanations we discuss. Overall, heterogeneity among women and heterogeneity among men appear to be at least as important as population-wide gender differences.