But many economists say that the emphasis on emergency room use, both in policy and in political speeches, is misplaced, as it makes up only a small part of health care costs in the United States. A federal government health survey found that emergency departments accounted for about 4 percent of total health spending in 2010, far less than inpatient hospital visits, which accounted for about 31 percent. Certain populations, however, like low-income people with chronic illnesses, have much higher rates of use.

Dr. Baicker and Amy Finkelstein, an economist at the Massachusetts Institute of Technology, another author, said the increased use of emergency rooms is driven by a basic economic principle: When services get less expensive, people use them more. Previous studies have found that uninsured people face substantial out-of-pocket costs that can put them in debt when they go to the emergency room. Medicaid reduces those costs.

Medicaid coverage also reduces the costs of going to a primary care doctor, and a previous analysis of data from the Oregon experiment found that such visits also increased substantially.

“This is just one piece of an increase we found across every type of care,” said Bill J. Wright, an author of the new study who is the associate director of the Center for Outcomes Research and Education in Portland, a part of Providence Health and Services, a large health care provider.

The study’s authors emphasized that Medicaid had many benefits. Previous analyses from the experiment found that gaining coverage reduced the incidence of depression and increased feelings of financial stability.

The study drew on data from the Oregon Health Insurance Experiment that included about 90,000 low-income Oregonians and randomly assigned about 30,000 of them access to Medicaid. Health experts say the experiment’s design — random assignment of coverage through a lottery — allowed them to isolate and evaluate the effects of the program. Such designs are the gold standard in medical research, but are rarely used for domestic health care policy.

Some experts noted that the study measured only the first 18 months after people gained coverage, and that old habits of relying on the emergency room are often hard to shake. It also takes time to find a primary care doctor and make an appointment.