American physicians dole out lots of unnecessary medical care to their patients. They prescribe things like antibiotics for people with viral infections, order expensive CT scans for patients with transitory back pain, and obtain screening EKGs for people with no signs or symptoms of heart disease. Some critics even accuse physicians of ordering such services to bolster their revenue.

So what happens when uninsured patients make it to the doctor’s office with coughs, low back pain, or other problems? Do physicians stop ordering all these unnecessary tests and services, out of recognition that most of these patients won’t be able to pay?

A study out of Harvard by Michael Barnett and colleagues provides a rigorous answer to this question. The researchers evaluated how often patients received any of a slew of unnecessary services. They compared patients with private insurance to those with Medicaid (which generally reimburses physicians much less generously than private insurance), and also to those with no insurance.

They found that almost 20% of privately insured patients receive unnecessary services, a staggeringly disturbing number. But even more disturbingly, the same percent of Medicaid enrollees and uninsured patients also receive unnecessary services.

In short, there’s way too much wasteful care, regardless of what kind of insurance people have (or don’t have).

That does not mean that physicians treated all these patients the same way. While the rate of unnecessary care was equal across these three groups of patients, the type of unnecessary services they receive varied. For example, about 1/2 of privately insured patients with upper respiratory infections receive unnecessary antibiotics, as do 1/2 of Medicaid enrollees. But a full 3/4 of uninsured patients with such symptoms receive unnecessary antibiotics.

In addition, among people with back and neck pain, 1/4 of privately insured patients receive narcotics (which are not generally effective for those conditions). The percent receiving narcotics rises to over 1/3 of Medicaid enrollees, and almost 1/2 of uninsured patients.

Almost half!

By contrast, unnecessary CT and MRI scans are less common among uninsured patients than among those with either Medicaid or private insurance.

So what’s going on here? I have a few guesses.

1. When physicians are in the habit of ordering EKGs, prescribing antibiotics, and the like, they don’t switch out of that habit based on the insurance status of their patients. They do what they do, regardless of insurance coverage.

2. Patients are powerless. In theory, uninsured patients should push back when their doctors order unnecessary services, because they’ll bear the cost. However, giving patients “skin in the game” won’t do much to curb such waste. When doctors tell patients that they need antibiotics or CT scans, they usually go along, even if those interventions will be a financial burden. They do so because they don’t realize the interventions are unnecessary.

3. Contingency antibiotics. When seeing privately insured patients who have been under their care for a long time, physicians might give them a tincture of time for their sinusitis or cough to get better, and encouragement to come back if things persist. But when uninsured patients show up in their office for the first time with the same symptoms, they might fear that such patients will never come back again. In such cases, the risks of unnecessary antibiotics might seem justified.

4. Narcotic insanity. I have no idea why Medicaid enrollees and uninsured patients were more likely to receive unnecessary narcotics than people with private insurance. That is very disturbing. I would be eager to hear the thoughts of clinicians who care for these populations.

5. It’s not about physician greed. These findings suggest that it is not greed that is driving physicians to order unnecessary tests and procedures. After all, physicians kept ordering those tests even for patients who probably wouldn’t be able to pay for them.

But don’t dismiss the role of financial incentives. Another study showed that hospital-based physician groups order more unnecessary serves than community-based practices. These findings hint at the likelihood that hospitals’ financial interests influence physician behaviors. Health care providers are under tremendous financial pressure. Eliminating wasteful care would put lots of people out of business. It is naive to think that the 20% rate of unnecessary care has nothing to do with financial interests.

We have an epidemic of unnecessary care in the U.S. Giving patients a financial stake in their health care — exposing them to more out-of-pocket costs — isn’t likely to be a quick cure for this problem. Solutions to prevent unnecessary medical care should primarily focus on helping physicians change their habits (instead of giving patients more skin-in-the-game).

Peter Ubel is a physician and behavioral scientist who blogs at his self-titled site, Peter Ubel and can be reached on Twitter @PeterUbel. He is the author of Critical Decisions: How You and Your Doctor Can Make the Right Medical Choices Together. This article originally appeared in Forbes.

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