The theory that hospitals charge private insurers more because public programs pay less is known as cost shifting. What underlies this theory is that a hospital’s costs — those for staff, equipment, supplies, space and the like — are fixed. A procedure or visit simply takes a certain amount of time and requires a specific set of resources. Therefore, if Medicare, say, does not pay its full share of those costs, a hospital is forced to offset the loss with higher prices demanded of private insurers.

The cost shifting theory goes back decades. But economists have long been skeptical of it, pointing to two key weaknesses. One is that it assumes hospital costs are immutable. We should be just as suspicious of such claims in health care as we would be for any other industry.

Jeffrey Stensland, Zachary Gaumer and Mark Miller — who serve on the commission that advises Congress on Medicare payment policy — offered a different view in a 2010 article in Health Affairs. Hospital costs, they said, can change and do so in response to market forces. They found that hospitals that face little competition are less efficient and have higher costs. With few competing hospitals to turn to, private insurers have little choice but to cover those high costs. But Medicare’s prices are fixed and are therefore low relative to the high costs of these inefficient hospitals.

Conversely, hospitals in more competitive regions are more efficient and can earn a profit on Medicare prices. But, because of competition, they must charge lower prices to private insurers. Put it together and it is hospitals’ underlying costs, driven by competition — not cost shifting — that lead to differences in prices charged to insurers and Medicare shortfalls or profits. This theory was conveyed in a report to Congress in 2011.

Another weakness of the cost shifting theory is that it runs counter to basic economics. Hospitals that maximize profits, or even maximize revenue to fund charity care, would not raise private prices in response to lower public ones. In fact, such a hospital would already be charging the highest possible prices to all payers. And, instead of raising them to one insurer if another paid less, they’d do exactly the opposite. Prices charged to two types of customers would move together, not in opposition, for the same reason it does so in other industries.