Women who enter a US PhD programme in a science, technology, engineering or mathematics (STEM) field are less likely to graduate if relatively few other women also join, according to a report by economists Valerie Bostwick and Bruce Weinberg of Ohio State University in Columbus (see go.nature.com/2mubhhs). The authors looked at data for 2,541 students starting PhDs at public universities in Ohio from 2005 to 2009. Women accounted for nearly 40% of the sample, but their numbers varied widely between programmes. When a cohort contained just one woman, she was 12% less likely to graduate within 6 years than were her male peers. But as the proportion of women increased, so did each woman’s likelihood of obtaining a degree. The authors suggest that women’s chances of earning a STEM PhD are linked to the ‘female-friendliness’ of that programme. “If there are few or no other women in your incoming class, it can make it more difficult to complete your degree,” says Bostwick.