Congress may not be the only polarized entity in the U.S. An analysis of campaign donations by physicians showed a gap in political preference between high-income specialties such as cardiac and thoracic surgery compared with less lucrative specialties as well differences by sex and employment type.

The study was published in the August issue of JAMA Internal Medicine.

Adam Bonica, PhD, of the political science department at Stanford University in Palo Alto, Calif., and colleagues pored through contribution filings between 1991 and 2012, which are publicly available for gifts over $200. To identify physician contributors, they accessed the National Provider Identifier files. Their goal was to determine differences in contributions by party over time.

Overall, the number of physician donors and the amount donated to election campaigns skyrocketed between the two election cycles. In 1991-92, 11,801 physicians contributed $20 million; in 2011-12, 67,852 gave money for a total of $143.2 million. Slightly more than 50 percent of contributions went to Republicans in 1992 but by 2012, Democrats had a slight edge.

The percentage of male physicians donating to Republicans during the study period was almost double that of female physicians—57 percent vs. 31 percent.

Both male and female cardiac and thoracic surgeons as well as cardiologists were more likely to prefer Republicans compared with physicians as a whole, but the gender gap persisted in these specialties. In cardiac and thoracic surgery, 75 percent of the men donors and 49 percent of the women donors gave to a Republican campaign. In cardiology, 57 percent of men and 38 percent of women contributed to Republicans.

There was an almost 20 percent gap between cardiac and thoracic surgeons and cardiologists in the amount of donations to Republicans. The gap was even wider between cardiac and thoracic surgeons and pediatric cardiologists. Cardiac and thoracic surgeons were also among the highest paid specialties, with six-year average earnings more than twice the income for pediatric cardiologists.

Physicians working in for-profit entities were more likely to donate to Republicans than physicians in small nonprofits.

“A profession once firmly allied with Republicans is now shifting toward the Democrats,” Bonica et al wrote. “Indeed, the variables driving this change—sex, employment type, and specialty—are likely to continue to be active forces and to drive further changes.”