In nearly every profession, women are paid less than men. In medicine, where detailed data on doctors' productivity, hours worked, education, and experience are available and gender differences in these have been accounted for, numerous studies still show that a gender gap exists. To put it simply, women earn less than men for the same work.

Many factors explain the gender earnings gap, including implicit and explicit biases (PDF) in the workplace, gender differences in how credit is attributed (PDF), and differences in how men and women negotiate.

But we've identified another factor that could influence the pay women receive: the number of men in their workplace.

In a study (PDF) published in the British Medical Journal, we combined salary data for 18,802 doctors with information on the number of male and female doctors in each workplace. We suspected that female doctors might get paid less in practices overwhelmingly made up of male doctors, perhaps because they could find it harder to negotiate for higher pay with a group of doctor colleagues that is mostly men.

Indeed, they do get paid less.

Among specialist doctors, women who worked in practices with an equal number of male and female doctors had a relatively small difference in pay (women were still paid less, however). But in practices primarily made up of male doctors, female doctors' salaries were substantially lower. This was true even after accounting for differences between men and women in specialty, practice type and geography, years of experience, and clinical revenue.

A good example is surgeons, where women working in practices with an equal number of male and female doctors earned about $46,500 less a year than their male counterparts—a large dollar amount but about 10 percent less than male surgeons. As the proportion of male surgeons in the practice increased, so did the gender pay gap, peaking at a $149,460 difference for female surgeons who worked in practices where 90 percent or more surgeons were men.

Read the full article about workplace diversity and the gender pay gap by Christopher Whaley and Anupam B. Jena at RAND Corporation.