Expanded coverage for young adults under the Affordable Care Act substantially raised inpatient hospital visits related to mental health, finds a new study by researchers at Indiana and Purdue universities.

That looks like good news: Better access to care for a population with higher-than-average levels of mental illness that too often endangers them and people nearby.

But it might not be the best result, said Kosali Simon, an economist at Indiana University and one of the authors. Greater hospital use by the newly insured might be caused by inadequate outpatient resources to treat mental-health patients earlier and less expensively, she said.

The health law let people under age 26 stay on their parents’ group insurance plans starting in late 2010. Other research shows the provision raised coverage for young adults. The paper by Yaa Akosa Antwi, Asako S. Moriya and Simon, based on a big national sample from community hospitals, found it also increased their hospital use.

Total inpatient visits for those aged 19 to 25 increased 3.5 percent compared with people aged 27 to 29, who couldn’t be on their parents’ plans. Mental health admissions increased much more — by 9 percent.

That’s a little puzzling. When Massachusetts broadened medical coverage with its 2006 state-based health reform law there were no big increases in mental-health hospitalizations for young adults there.

But psychiatrists, psychologists and other mental-health providers are more available in Massachusetts than elsewhere, Simon said. Perhaps community caregivers largely met the needs of newly insured Bay Staters, saving them from crisis visits to the emergency room.

In the national study, a large portion of psychiatric admissions came through the emergency department.

Neither the Massachusetts study nor the national study looked at the volume of outpatient mental-health care. But even if they had, they wouldn’t have produced a final answer on the effectiveness of expanded coverage.

That will take more research. Increasing insurance and treatments aren’t necessarily the same as helping patients.

“We eventually judge all these [insurance] expansions based on the final outcomes that we care about,” said Simon. “How did this affect the well-being of young adults? Is there a measurable improvement of health status? Does it appear that there is better mental health as a result of this increase?”