Though Obamacare remains unpopular, an influential slice of the liberal commentariat insists that it is succeeding not just in expanding coverage, but in “bending the cost curve,” i.e., lowering the rate of health-care-cost inflation. But how much credit does Obamacare really deserve? And could there be another explanation for the slowdown in cost growth that is rather less convenient for Obamacare’s champions? If you guessed “Not very much” and “Yes,” you’re right on the money.

Policymakers have long seen bending the cost curve as critically important. Health-care inflation has been exceeding the rate of economic growth and overall inflation for decades. That Americans spend quite a lot on medical care is not intrinsically objectionable — rich countries, like rich people, will as a general rule spend more money on improving quality of life than will their poorer counterparts. Yet as the cost of medical care has increased, so too has the role of government in financing medical expenditures. As Yuval Levin has observed, virtually the entire projected growth in federal spending over the coming decades can be attributed to the rising cost of health-care entitlement programs. This in turn implies that spending on everything else that government does, from fighting and winning wars to building and maintaining the interstate highways, will get squeezed.

There have been moments when it seemed as though Americans had broken the back of health-care inflation. The early 1990s, for example, saw a pronounced decline in health-care inflation, albeit one that started to reverse itself by the mid 1990s. Health-care inflation then nearly doubled, from 5.3 percent in 1996 to 9.7 percent in 2002, renewing fears that rising medical expenditures would bankrupt us all. Mercifully, health-care inflation has been falling since then. Last year, remarkably, it fell to 3.6 percent.

The great selling point of health-care reform in the early days of the Obama presidency was that it would bend the cost curve. Peter Orszag, who then served as President Obama’s budget director, insisted that the creation of an expensive new health-care entitlement would, when packaged with a welter of tax increases and Medicare tweaks, actually improve America’s long-run fiscal position by revolutionizing the way medical care is delivered.

One thing we do know is that the health-care cost-inflation picture has indeed improved, at least for now. The Centers for Medicare and Medicaid Services recently projected that U.S. health-care spending will be $4 trillion in 2019, which is half a trillion less than the 2019 estimate it offered in 2010. Despite the disastrous rollout of the Obamacare exchanges, David Cutler, a Harvard health-care economist and a champion of the law, took to the Washington Post last fall to argue that a combination of restraining the growth of Medicare payments, emphasizing value over volume in medical care, and spreading accountable care organizations (ACOs), among other provisions of the new law, had already started to work its magic. Cutler acknowledged that a few other developments, including the growing prevalence of high-deductible insurance plans, had played a role, but he soft-pedaled them.

In January, Jason Furman, chairman of the White House Council of Economic Advisers, wrote an op-ed for the Wall Street Journal that made a broadly similar case. Furman was careful to note that the slowdown in health-care costs began in the mid 2000s, but he made no effort to explain why health-care inflation declined in the Bush era. Like Cutler, he touted the ACA’s role in curbing Medicare expenditures. Unlike Cutler, he claimed that “there is good reason to believe that at least some of these gains are spilling over to the private sector,” citing studies that tentatively suggest that Medicare’s efforts to reduce excessive payments might help private insurers do the same and that Medicare’s scale enables it to introduce reforms that a large part of the private sector can later adopt.

It is telling that Furman neglects another possibility — that Medicare’s scale, combined with its reliance on a rigid fee schedule, has done much more to stymie private-sector innovation than it has to contribute to it. Although Medicare’s per capita costs have slowed down in recent years, they had grown more rapidly than per capita GDP, and more rapidly than privately funded health-care costs, from 1975 to 2005. As James Capretta, a fellow of the Ethics and Public Policy Center and a veteran of President Bush’s Office of Management and Budget, has argued, the fact that Medicare accounts for roughly a fifth of medical expenditures means that Medicare has an outsized influence on how medical care is delivered. If anything, this understates the case, as government expenditures, including those on Medicaid, account for almost half of all medical expenditures, and Medicare has a great deal of influence over this entire slice of the pie. While private employers have made various attempts to deliver medical care more cheaply and efficiently, they’ve met with resistance from medical providers, who have been shielded from market pressures by a traditional fee-for-service Medicare program (Medicare FFS) that has long been willing to finance expensive, inefficient, and fragmented care.

This landscape is changing for the better. But it is not Obamacare and its modest Medicare reforms that are driving that change. Rather, it is two developments that merit closer attention. The first is rising enrollment in Medicare Advantage (MA) plans, which Medicare beneficiaries may choose in lieu of Medicare FFS. Unlike Medicare FFS, in which the federal government reimburses health-care providers for each service rendered, Medicare Advantage is administered by private insurance companies and health-care organizations, which receive a fixed payment from the federal government for each patient. When Obamacare reduced those payments, many observers expected that Medicare beneficiaries would flock back to Medicare FFS. Instead, enrollment in MA plans has increased by 30 percent since 2010. Facing reduced payments, MA plans have made a number of adjustments to cut costs, including narrowing their provider networks and introducing more cost-sharing. This in turn has allowed them to charge lower monthly premiums. As of 2013, 28 percent of Medicare beneficiaries were enrolled in these private MA plans; and they are poised to dominate Medicare in a few short years, because newly eligible Medicare beneficiaries are choosing private plans over traditional Medicare in ever larger numbers.

As of now, the expansion of Medicare Advantage has not generated substantial savings for the federal government. MA plans are overpaid relative to Medicare FFS, and they use this surplus to reduce copayments for Medicare beneficiaries. When a 2012 paper in The Journal of the American Medical Association compared Medicare FFS and MA plans on an apples-to-apples basis, however, it found that MA plans can deliver the same package of benefits for 10 percent less than Medicare FFS on average. MA plans are achieving these cost savings by adopting integrated managed-care systems through a trial-and-error process. The Obama administration, in contrast, has been promoting provider integration through its tightly regulated ACO initiative, which offers providers incentives to organize care in specific ways prescribed by the federal government. It should come as no surprise that MA plans are succeeding while ACOs are failing to live up to their promise.

The second development has been the astonishing speed with which employers have shifted their employees to high-deductible health-insurance plans. The percentage of covered workers in plans with deductibles of $1,000 or more has gone from 10 percent in 2006 to 38 percent in 2013, and the increase has been even more dramatic for firms with fewer than 200 workers. High-deductible health insurance discourages the purchase of unnecessary medical care; given that 44.5 percent of Americans are covered by employer-based health insurance, it’s not surprising that the transformation of this sector is restraining health-care inflation overall.

What do these two developments have in common? They are both products of the Medicare Modernization Act of 2003, the much-maligned Bush-era law that, among other things, greatly expanded the use of health savings accounts, which in turn made high-deductible health-insurance plans more attractive. It also greatly improved how the Medicare program treated private plans, which in turn fueled enrollment in Medicare Advantage. Many on the right objected to the Medicare Modernization Act for having created a new prescription-drug benefit. One decade on, however, the law is being vindicated as a catalyst for cost-saving reforms across the entire health-care sector.

Fortunately, Obamacare hasn’t halted the advance of high-deductible health insurance or Medicare Advantage plans. But let’s be clear: The Medicare Modernization Act has done much more to promote a more cost-effective health-care system in the United States than Obamacare, and the health-care reform the country needs will build on its achievements.