The medical community used to be a reliable source of campaign contributions for Republican candidates. Used to be. Over the past few election cycles, doctors' contributions have moved more in line with everyone else, according to a new study conducted by researchers from Stanford University, New York University and Columbia University.

You can see the change in the graph at right. By matching data from political giving databases (including the Federal Election Commission) with information from the National Provider Identifier public use file (and other sources), the team tracked giving to Republican candidates over time. The gap that existed in 1992 between medical donors and all donors has all but vanished.

Why? As with all things, it's complex. But the data indicate that the primary motivators are an increase in the number of female physicians and a decrease in the number of doctors that work in small practices.

The difference in giving between male and female doctors is significant. Overall, the team found that the percentage of contributions from male doctors to Republican candidates was 57 percent. The percentage of contributions to Republicans from female doctors? Only 27 percent. And that gender gap has itself increased since 1992.

The difference is not uniform across specialty, though. Below, some of the specialties tracked in the study, with the percentage of contributions to Republicans by gender identified.

Notice that line for ophthalmologists. There are other specialties in which the gap between male and female doctors is smaller, including a number of pediatric specialty professions. But that line still jumped out at us.

Giving is related to how much doctors make; the higher their income, the more likely they are to give to Republican candidates. The graph below shows how annual earnings track with likelihood to give to Republicans (shown on the vertical axis. And, again, note the position of ophthalmologists. (Emphasis added.)

One of the researchers involved in the study, NYU's Howard Rosenthal, told The Post by e-mail that the team hadn't explored why ophthalmologists stood out "with any scientific rigor." But the topic came up among the team. Stanford's Adam Bonica indicated among the possibilities that were considered is that the salary data for ophthalmologists was possibly inaccurate. In other words, the dot on that second graph should be shifted to the right. Bonica also pointed out that ophthalmologists were also outliers when payments to Medicare providers became public last year. It's a question mark.

The team expects that the trend away from contributions to Republicans will continue, given that the trend toward more female doctors is continuing. It would be interesting to see how the pattern holds up for the next presidential cycle. In part, that's because one of the top contenders on the Republican side, Sen. Rand Paul of Kentucky, was a doctor before joining the Senate. An ophthalmologist, no less.