Sometimes doctors choose to do surgery not because it is absolutely preferable to other treatments but because they get reimbursed for it, a new study suggests.

Researchers looked at patients with a narrowed artery in the neck, a condition called carotid artery stenosis that can be treated with surgery or managed with medicine and lifestyle changes. The choice is often a judgment call.

Some were treated in a fee-for-service system, which pays doctors for every procedure they do. Others were treated by doctors on salary at a military hospital. After adjusting for socioeconomic and other variables, they found that over all, patients in the fee-for-service system were 63 percent more likely to have surgery than those in the salary system. Patients with symptoms were especially likely to receive surgery, though those without symptoms also contributed to the difference. The study is in JAMA Surgery.

“Fee-for-service doctors are incentivized to do more because they are paid by the procedure,” said the lead author, Dr. Louis L. Nguyen, an associate professor of surgery at Harvard. “And salaried physicians are incentivized to do less. That’s human behavior. What we need is a system that incentivizes doctors to be aggressive in treating patients in a timely fashion, but not to create procedures for reimbursement.”