Supporters of President Obama’s health care law had predicted that expanding insurance coverage for the poor would reduce costly emergency room visits because people would go to primary care doctors instead. But a rigorous new experiment in Oregon has raised questions about that assumption, finding that newly insured people actually went to the emergency room a good deal 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 during their first 18 months with insurance.

The pattern was so strong that it held true across most demographic groups, times of day and types of visits, including those for conditions that were treatable in primary care settings.