Exactly why spending on medical care has slowed down is the subject of considerable debate among economists. Over time, health care spending tends to track the economy as a whole, though sometimes with a slight time lag, since people who have less money (or are less insurance) are less likely to seek out medical care. But most experts think other factors are also at play. Some point to changes in private insurance—specifically, the fact that people are paying more and more out of their own pockets, in the form of co-payments and deductibles. Others note that per capita spending in Medicare, the government’s insurance program for the elderly, is also rising slowly. (Margot Sanger-Katz from The Upshot just had a very good article on that.) That would suggest some changes in the law, like lower payments to hospitals and incentives for more efficient care, are also having an effect.

Conservatives love the part about changes to private insurance, while liberals love the part about changes to Medicare. The CMS actuaries, like most health economists I know, think both are playing a role. But the CMS actuaries are wary of assuming these changes will be too big or long-lasting. They seem particularly cautious when it comes to assessing the impact of “payment reforms” in the Affordable Care Act—changes in the way Medicare pays for services, designed to encourage hospitals to deliver care more efficiently.

Are the CMS actuaries right to be so skeptical? There’s no way to be sure, obviously. Plenty of experts, including those at Altarum and the Kaiser Family Foundation, have predicted health care spending would rise more quickly once the economy recovers. And savings have certainly proven ephemeral in the past.

But the slowdown in Medicare spending (which has little to do with the economy or changes to private insurance) is a powerful indicator that health care really is becoming a more efficient enterprise. “The evidence for fundamental health system change grows stronger by the month,” says David Cutler, the Harvard economist and widely respected expert on health care spending. “CMS is skeptical about the lasting power of many of these pieces of evidence. I, and others, remain more optimistic.”