We’ve found that co-payments do not resolve behavioral hazard. They make it worse. They reduce the use of a drug that is already underused.

A recent experiment by a team led by Niteesh Choudhry, a professor of medicine at Harvard, quantifies the problem. The experiment involved nearly 6,000 patients who had just suffered a heart attack, and were prescribed drugs known to reduce the chance of another one — statins, beta-blockers, angiotensin-converting-enzyme inhibitors or angiotensin-receptor blockers. Half had their co-pays for these drugs waived; the other half paid the usual fee.

As expected, more people in the zero co-pay group took the drugs, and their health improved. Those in the zero co-pay group were 31 percent less likely to have a stroke, 11 percent less likely to have another major “vascular episode” and 16 percent less likely to have a myocardial infarction or unstable angina. None of these benefits came at a net monetary cost. The insurers did not spend more in total. By some measures, they spent less.

Behavioral hazard affects all of the drugs listed above. They are all highly effective, and yet adherence to taking them is a problem. This is a major financial issue. The New England Healthcare Institute has estimated that solving non-adherence could save $290 billion a year, or 13 percent of total annual medical spending in the United States. A number this large is surely open to quibbling, but divide it by 10 and it is still a large figure.

I’m not proposing to make all health care free, or arguing for a return to so-called Cadillac health plans. Moral hazard is all too real for many treatments, and in some cases, behavioral hazard reinforces it. Just as people underrespond to inconspicuous symptoms, they can also overrespond to highly noticeable ones. People may seek too much care for back pain, for example, resorting to ineffective or possibly even harmful treatments. Behavioral hazard suggests that we need even larger co-payments for these overused drugs.

My proposal is targeted: Take drugs that are shown to be of very high benefit to some people, and make those drugs free for them.