A new study by researchers at the University of Pennsylvania has found that health insurance plans available in the marketplaces established by the Affordable Care Act offer fewer choices for mental health care compared with primary care.

The findings suggest that the law still falls short of achieving "parity" for mental health care — the guarantee that these benefits will be covered as fully as other medical benefits, researchers said.

"The ACA was a huge step in expanding the scope and coverage of mental health benefits," said lead author Jane Zhu, an associate fellow at the Leonard Davis Institute of Health Economics at the University of Pennsylvania.

But, Zhu said, the limited provider networks offered by plans on the insurance exchanges may represent another barrier to accessing mental health care.

The researchers looked at data from the plans available on the insurance marketplaces in 2016 and found that, on average, just one of 10 mental health care providers in a given coverage area participated in these plans. By comparison, about one of four primary care doctors were in the plans' networks.

The study was published Tuesday in the journal Health Affairs. While the researchers focused on plans purchased in the ACA marketplaces, which cover about 12.2 million Americans, Zhu said the problem isn't unique to those plans.

"This is an issue that is pretty pervasive in the entire mental health care system," she said. "That's probably because a lot of the underlying roots of these problems are systemic."

An overall shortage of mental health care professionals and low reimbursement rates by insurance plans can lead many of these providers to opt out of insurance networks, researchers said. The study found that only about 43 percent of psychiatrists and 19 percent of other mental health care providers participated in any plan on the ACA exchanges.

Insurance companies may also try to keep their provider networks small to protect their bottom line, according to the Penn researchers. Doing so could have the effect of excluding patients with mental health conditions that are expensive to treat, as these patients may seek alternate plans with more extensive networks of specialists.

Zhu said more research is needed to figure out just how much the "narrow networks" seen in the ACA marketplace plans could undermine the law's requirements for mental health parity.

"We don't really know at this time how narrow a network has to be before it starts impacting access to care and health outcomes negatively," said Zhu.

Figuring that out could help set new standards for how big a network must be to ensure that it provides adequate coverage for mental health conditions.