There are many theories behind the long-standing problem of women being under-represented in the top levels of scientific fields.

Among the popular ideas: they needed more encouragement?. They need more help with?childcare. They need advice in competing for?. They suffer from?,??and forms of?.

Some factors, like the biases, are?. Others, such as psychological factors, are less clear. But whatever the causes and possible solutions, a new study suggests a fairly straightforward strategy: give women more female colleagues.

The study, by researchers at Ohio State University, covered all 2,541 students who enrolled in 33 doctoral programmes in scientific fields at six Ohio public universities between 2005 and 2016. It found that higher numbers of women starting out in a particular doctoral programme led to higher graduation rates in that programme.

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In classes with only one female student, the woman was 12 percentage points less likely than her male classmates to graduate on time, meaning within six years. But for each additional 10 per cent increase in the number of women in a class, the male-female gap in on-time graduation rates narrowed by more than 2 percentage points, the study found. Classes in the study had an average size of 17 students, and an overall average of 38 per cent women.

The analysis ¨C by Ohio State¡¯s Valerie Bostwick, a postdoctoral researcher in economics, and Bruce Weinberg, a professor of economics ¨C leaves unanswered the question of how exactly to get more women enrolled in scientific doctoral programmes.

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But it does suggest that having more female colleagues in a class helps women, regardless of the specific obstacles female scientists face in their classrooms and in their lives, and therefore gives universities a focus in their bid to?tackle gender gaps in science.

The most direct solution, Dr Bostwick said, would involve universities boosting their recruitment efforts to attract more qualified women into their programmes, especially those programmes with very low shares of female students.

¡°We can only speculate about what it is in the climate that is making it more difficult for women,¡± Dr Bostwick said. ¡°It may be hard to feel like you belong when you don¡¯t see other women around you. There may be subtle discrimination. We don¡¯t know.

¡°But it highlights the fact that women need support, particularly if they are the only ones entering a doctoral class. They need to know about resources that could help them, particularly in that first key year.¡±

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paul.basken@timeshighereducation.com