Does having health insurance make people healthier? It's widely assumed that it does.
Obamacare advocates repeatedly said that its expansion of Medicaid would save thousands of lives a year. Obamacare critics seldom challenged the idea that increased insurance coverage would improve at least some people's health.
Now, out of Oregon, comes a study that casts doubt on the premise that insurance improves health.
In 2008, Oregon state government had enough Medicaid money to extend the program to 10,000 people but many more were eligible. So the state set up a lottery to determine who would get coverage.
That created a randomized control trial (RCT), to compare the health outcomes of about 6,000 people who won the lottery with a similar number who lost.
RCTs are the best way to test the effects of public policies, as Jim Manzi argues in his recent book "Uncontrolled: The Surprising Payoff of Trial-and-Error for Business, Politics and Society."
Other studies compare the effect of policies on populations that may differ in significant ways -- apples and oranges. RCTs compare apples and apples.
The only previous RCT on health care policies was conducted by the RAND Corporation between 1971 and 1982. It found no statistically significant difference in health outcomes from having more insurance. But health care has changed a lot since then.
The Oregon Health Study, published last month in the New England Journal of Medicine, found much the same thing. Comparing three important measures -- blood sugar levels, blood pressure and cholesterol levels -- It found no significant difference after two years between those on Medicaid and those who were uninsured.
It did find lower levels of reported depression among the group on Medicaid. And it found, unsurprisingly, that they did save significant money. Those findings may not be unrelated.
The findings have serious implications for Obamacare. Half of its predicted increase in insurance coverage comes from expansion of Medicaid.
Obamacare supporters have assumed that those eligible for Medicaid -- poorer, sicker and less steadier in habits than the general population -- would have great difficulty getting health care without insurance.
The Oregon Health Study is evidence that at least in that state Medicaid-eligible people without insurance -- a "pretty sick" population, one state official said -- nevertheless managed somehow to get care that produced results about as good as those who won the lottery.