Researchers Michael Barnett, Zirui Song and Bruce Landon crunch the numbers and find that physician visits that result in a referral to a specialist jumped from 41 million in 1999 to 105 million in 2009, an increase of 159 percent. As the above chart shows, primary care physicians have begun referring to specialists more—and specialists have become more likely to refer patients to other specialists.

The big question is why doctors have become more likely to send their patients to specialists. Part of the answer, Landon finds, has to do with health care becoming more complex, with new technology that demands more speciality care. Physicians have also found themselves with more preventive tests and screenings to handle, which may cut into time to deal with other issues. And, part of it likely has to do with the economics of referrals: Doctors who have an ownership stake in their practice are 50 percent more likely to refer to a specialist, which would increase the total revenue generated by a given patient.