A UCLA study published this week found that the proportion of physicians who are Black in the United States has increased by only four percentage points over the past 120 years, and that the share of doctors who are Black men remains unchanged since 1940.

The research also spotlights a significant income gap between White and Black male physicians—a disparity that could reflect a combination of pay discrimination and unequal access for physicians to pursue careers in more lucrative specialties, according to the study’s author, Dr. Dan Ly.

“These findings demonstrate how slow progress has been, and how far and fast we have to go, if we care about the diversity of the physician workforce and the health benefits such diversity brings to patients, particularly minority patients,” said Ly, an assistant professor of medicine at the David Geffen School of Medicine at UCLA.

Ly analyzed data from surveys administered by the U.S. Census Bureau from 1900 to 2018. The sample included about 150,000 physicians, including about 3,300 Black male physicians and 1,600 Black female physicians, according to the study published in the peer-reviewed Journal of General Internal Medicine.

Ly found that in 1900, when 11.6 percent of the nation’s population was Black, 1.3 percent of physicians were Black. In 1940, when 9.7 percent of the total population was Black, 2.8 percent of physicians were Black—2.7 percent of were Black men and 0.1 percent were Black women. By 2018, when 12.8 percent of the total population was Black, 5.4 percent of U.S. physicians were Black – 2.6 percent Black men and 2.8 percent Black women.

Although the percentage of Black women physicians increased 2.7 percentage points between 1940 and 2018, the proportion of physicians who are Black men during the same period has remained essentially unchanged.