Supporters of President Obama’s health care law had predicted that expanding insurance coverage for the poor would reduce costly emergency room visits as people sought care from primary care doctors. 
But a rigorous new study conducted in Oregon has flipped that assumption on its head, finding that the newly insured actually went to the emergency room more often.

 The study, published in the journal Science, compared thousands of low-income people in the Portland area who were randomly selected in a 2008 lottery to get Medicaid coverage with people who entered the lottery but remained uninsured. 
Those who gained coverage made 40 percent more visits to the emergency room than their uninsured counterparts. 

But Dr. Baicker and other economists who worked on the study said the increased use of emergency rooms is driven by a basic economic principle: When things or services get cheaper, people buy and use more of them.