Allowing young adults to stay on their parents' health plans is one of the most popular elements of the president's health-care law, but a pair of new studies out today raises questions about the overall impact of the coverage expansion to an estimated 3 million people.

The provision, which allows young adults to stay on their parents' health insurance plans until their 26th birthday, was one of the earliest parts of the law to take effect, in 2010, and researchers are now starting to report on the effects of that expansion. As expected, it increased the rate of health insurance among young adults, who historically had the highest uninsured rates of any age group. But the provision didn't change whether the age group perceived themselves as healthier or whether they thought health care was any more affordable, according to a new study in JAMA Pediatrics.

Further, adults under age 26 reported a significant decline in having a usual source of care between 2009 and 2012 (from 62 percent to 58.8 percent), according to researchers who analyzed the results of two publicly available surveys. Still, that decline wasn't as steep as the 6.6 percentage point decline experienced among people ages 26-34, researchers found. But when compared to that slightly older age group, researchers found the under-26 crowd didn't report a significant difference in several key metrics: going for a wellness visit, getting a flu shot, or being able to afford a doctor's visit, prescription medications or dental care. (Interestingly, these results seemed to contradict earlier findings in the journal's sister publication, the Journal of the American Medical Association.)

Young adults as a group already think of themselves as pretty healthy, which explains why many will choose to skip the cost of health insurance in the first place. And when they do get sick, they're more likely to seek out care at emergency rooms — a much more expensive option. So another significant measure of the under-26 provision is whether ER visits are decreasing for young adults.

In another study out today, Stanford University researchers compared the number of emergency room visits of two different young adult age groups: those who benefited from the new ACA provision, ages 19-25, and those who didn't, ages 26-31. Analyzing 2011 data from California, Florida and New York hospitals, the researchers found that the under-26 group had 2.7 fewer ER visits per 1,000 people than the older group. That added up to about 63,500 fewer ER visits among young adults across these states in just 2011, according to the study published today in the Health Affairs journal.

The researchers had another finding that seems just as important. While the total number of ER visits among the under-26 group was down, about the same number of people still went to the ER. The distinction here is that young adults with chronic conditions, who have greater care needs, probably now had better access to non-ER care settings, so their number of visits to the ER decreased. But the finding also suggests that healthy young adults, who might have shunned health insurance before, still continued to see the ER as a place for seeking out routine care, according to the study. Further, insurance likely makes those ER visits cheaper, which could actually increase how much people use the ER, the researchers wrote.

Past studies have shown that ER use increases immediately after a coverage expansion. Patients could be having a hard time finding new primary care doctors, or it could be that changing old habits is difficult. But everyone agrees that in an ideal health-care system, the ER wouldn't be a place for routine care.

It's also a reminder that providing health insurance is just one part of the equation. Changing someone's behavior can be tougher.