It does not matter if the procedure is big or small, learned in a decade of training or a weeklong course. In fact, minor procedures typically offer the best return on investment: A cardiac surgeon can perform only a couple of bypass operations a day, but other specialists can perform a dozen procedures in that time span.

That math explains why the incomes of dermatologists, gastroenterologists and oncologists rose 50 percent or more between 1995 and 2012, even when adjusted for inflation, while those for primary care physicians rose only 10 percent and lag far behind, since insurers pay far less for traditional doctoring tasks like listening for a heart murmur or prescribing the right antibiotic.

By 2012, dermatologists — whose incomes were more or less on par with internists in 1985 — had become the fourth-highest earners in American medicine in some surveys, bringing in an average of $471,555, according to the Medical Group Management Association, which tracks doctors’ income, though their workload is one of the lightest.

In addition, salary figures often understate physician earning power since they often do not include revenue from business activities: fees for blood or pathology tests at a lab that the doctor owns or “facility” charges at an ambulatory surgery center where the physician is an investor, for example.