When poor people are given medical insurance, they not only find regular doctors and see doctors more often but they also feel better, are less depressed and are better able to maintain financial stability, according to a new, large-scale study that provides the first rigorously controlled assessment of the impact of Medicaid…

Some said that of course it would help to insure the uninsured. Others said maybe not. There was already a safety net: emergency rooms, charity care, free clinics and the option to go to a doctor and simply not pay the bill. And in any case, the argument goes, if Medicaid coverage is expanded, people will still have trouble seeing a doctor because so few accept that insurance…

The new study, published Thursday by the National Bureau of Economic Research, avoided that problem. Its design is like that used to test new drugs. People were randomly selected to have Medicaid or not, and researchers then asked if the insurance made any difference.

Health economists and other researchers said the study was historic and would be cited for years to come, shaping health care debates.