As he took me through an interactive map on his computer screen, Oakland physician Nate Gross showed me what a neurosurgeon in my area might expect to make. Scrolling his cursor over the D.C. suburb of Montgomery County, Maryland, a number appeared: $580,000. I knew that, vaguely, but I did gasp.

"But if you move down to South Carolina," Gross continued, scrolling south, "here, you're looking at $645,000."

In an even more drastic example, the average anesthesiologist practicing in Massachusetts would increase her salary by 61 percent if she moved to Wisconsin. Most doctors have a vague idea that they could earn more money if they moved away from big cities and the coasts, where job markets tend to saturate—or if they abandoned preventive care to specialize in anything involving a scalpel or lasers. But in a profession where talk of finances is taboo, job listings rarely include salaries, and compensation models are duly withheld from medical-school curricula. Today Gross's company, Doximity, launched a new tool that lets doctors see exactly what salaries other doctors are earning, by county and specialty. The maps already include anonymous data from more than 18,000 physicians.

"The goal here is to empower doctors with transparency," said Gross. "They're going to be looking for jobs after residency, and they have no idea what they're doing. They weren't taught that kind of stuff in residency. They were taught how to be doctors and surgeons." And in the job search, "They don't necessarily get screwed over, but they don't necessarily get what they deserve, either."