Men with moustaches significantly outnumber women in academic medical leadership positions in the US, says a study.

Thirteen percent of department leader positions at top academic medical institutions in the US are held by women, while nearly 20 percent are held by men with mustaches, the findings showed.

"The lack of women in leadership roles in medicine is well-documented, but despite the eccentricities of the study, our results show that even when you focus solely on men with mustaches - which are rare - women are still outnumbered across various specialties," said lead author Mackenzie Wehner from the University of Pennsylvania Health System.

The number of women in medicine has risen significantly in recent times. Almost 50 percent of US medical students are women, but the proportion of women in academic medicine is still low with only 21 percent full professors being women, the study pointed out.

The lack of women in leadership positions is a problem because of the "strong ethical argument for equality", the researchers said.

They analysed 1,018 medical department leaders by searching the institutional websites of the selected medical schools to identify leaders, such as the chair, chief or head of each specialty.

In the study, the number of women was compared to the number of men with moustaches in academic medical leadership positions.

The team looked at moustaches because these are rare, and wanted to see if women were even rarer. The team measured the proportion of women and men with moustaches across institutions and specialties.

Results showed that women accounted for 13 percent of department leader positions, while moustachioed men accounted for 19 percent of department leader positions.

Only five specialties had more than 20 percent women department leaders, and they were obstetrics and gynaecology, pediaetrics, dermatology, family medicine, and emergency medicine.

The study was published in the BMJ (British Medical Journal).