The gender gap in many top jobs has been stubbornly difficult to erase. Decades of anti-discrimination laws and changing social norms have chipped at the glass ceiling, and yet women remain underrepresented and underpaid in many professions.

Overt discrimination against women has no doubt played a role. But, at least when it comes to medicine, there are also more subtle biases that maintain the status quo.

In the April issue of the American Economic Journal: Applied Economics, author Dan Zeltzer says gender “homophily” is partly to blame. The term refers to the human phenomenon of gravitating toward someone with whom you share something in common. It shows up everywhere, from lunch rooms to social media feeds.

But when male doctors—who already dominate the physician ranks—refer patients to their male colleagues, it serves to preserve their advantage over women.

 

Figure 1 from Zeltzer (2020)

Combining data from Medicare and the Dartmouth Atlas Project, Zeltzer analyzed referrals of Medicare patients in 306 US health care markets. All else being equal, there would be no difference in male and female physician referral practices. But the data said otherwise. The scatterplot above shows how referrals to male specialists (plotted on the vertical axis) differed between doctors of different genders. Male doctors (blue circles) were more likely than their female counterparts (red crosses) to send patients to male specialists. 

The pattern holds even after accounting for variation in the share of male specialists in each market, which is plotted on the horizontal axis. Zeltzer says it also holds after accounting for other potential differences between male and female physicians, including differences in medical specialization, experience, and the gender mix of patients doctors see.

The gap appears to close as the share of male specialists increases, because in markets with few female specialists, doctors’ referral choices are too limited to show gender differences. But in markets with more gender-balance among specialists, disparities are wider.