AMNA NAWAZ: More than one in five adults in the U.S. live with a mental illness, but only about half receive treatment.

Many struggle to find a therapist that will accept their insurance.

An NPR-ProPublica report aimed to figure out the reasons why.

Stephanie Sy has that story.

STEPHANIE SY: The investigation found that many therapists who initially join insurance networks end up leaving because of difficulties getting reimbursed by insurers.

That leaves a lot of patients paying out of pocket or being unable to access care.

For more, I'm joined by Annie Waldman, who covers health care at ProPublica.

Annie, thanks so much for joining the "News Hour."

So, for this report, you and your colleagues interviewed, I understand, hundreds of psychologists, psychiatrist, therapists.

And you did find that it's insurers who often ultimately decide the length of care and ultimately who gets covered treatment.

Is that right?

ANNIE WALDMAN, ProPublica: Yes, that's right.

In the most simple terms, we really wanted to understand, why is it that, even when you have insurance, it can be so hard to access mental health treatment?

And what people don't realize is that insurance companies play a big role in this and have a big say in how much treatment you get, how often you get treatment, how frequently you get treatment, and what kind of treatment you get.

And providers also told us that they experience a lot of red tape dealing with insurers, delayed and low payments, audits and reviews.

Therapists really told us that they tried to stay in network.

They are committed to their patients, are committed to delivering mental health care.

But these practices from insurance companies are squeezing them out.

STEPHANIE SY: So there's red tape, there's bureaucracy in having to file these claims.

And there's also what you describe as below-market-rate reimbursement from insurers, therapists that describe having their rates not shift in years.

And, obviously, we all know inflation has been a factor, but that reimbursement rate has stayed the same.

ANNIE WALDMAN: Yes, that's right.

In a lot of our interviews, therapists told us about how their rates have been largely stagnant and notoriously low.

Therapists on average earn about $98 for a 45-minute session of therapy from commercial insurers.

But if they went out of network, they could get doubled that.

So a lot of providers say that it's just not sustainable for them to stay in network and get these rates that are really low in which all their expenses of their office, their own health insurance, they can't keep up with them.

STEPHANIE SY: So there are also many claims that get denied on the basis of what the industry calls medical necessity.

You spoke to a therapist who experienced that.

Here's what Anna DiNoto, who provides therapy for children with autism, had to say.

ANNA DINOTO, Psychologist: They started to say that the amount of time we spent with the patient wasn't supported by the documentation.

And so it's things like that where they say, well, you're not demonstrating medical necessity, but then they won't give you what the rubric is for medical necessity.

STEPHANIE SY: Annie, in disputes over what is medically necessary, I understand that there are not clear guidelines in mental health care, the way there may be in physical health care.

Is that the crux of the problem?

ANNIE WALDMAN: Yes, the term medical necessity is frequently used by insurers to deny claims.

Many people assume that, if your mental health provider believes that you need a certain type or length of treatment, that they are the expert and they should have the final say.

But our reporting found that's not entirely true, that insurance companies generally face few limitations on how they define what kind of care is medically necessary.

And they sometimes even create their own internal standards, instead of relying on the ones developed by nonprofits or professional medical societies.

And these standards can be used to challenge diagnoses or treatment plans, which can lead to denials of care.

STEPHANIE SY: You also report instances when the therapists were told by insurers to spend less time with patients who the therapist, as you call them, the experts, felt needed long-term care.

Again, here's what Anna DiNoto said.

ANNA DINOTO: It's really concerning and unethical in many ways, in my opinion, because it's like saying to a surgeon, we're going to give you four hours to be able to do open-heart surgery, and, sorry, not sorry, like, if you can't finish.

We don't know what to tell you.

STEPHANIE SY: Anna DiNoto was not alone.

You interviewed dozens of therapists, some of whom felt that insurers were reducing care for patients who were even on the edge of self-harm, even suicide.

So what impact did the therapist describe these insurance policies could have on their patients?

ANNIE WALDMAN: Yes, what's so striking from what Anna DiNoto experienced is that, under the Mental Health Parity and Addiction Equity Act, which is also known as MHPAEA, it's a federal law that requires insurance companies to provide the same access to mental health and physical health care.

But we heard numerous stories from patients and providers about how mental health care was discriminated against and often scaled back in ways that medical care may not be.

You know, for example, if you have cancer and your doctor says that you need chemotherapy, an insurance company likely wouldn't say, well, you can only have that chemotherapy if you first try a less aggressive, less intensive, less costly medication first and show that it doesn't work.

But with mental health care, we heard stories of scaling back care for acute disorders often, someone in the midst of a mental health crisis who needed intensive treatment, and then the insurer will only improve coverage for a less intensive treatment first.

Such limitation means patients can risk bodily harm or even death through overdose or suicide.

STEPHANIE SY: What do insurance companies tell you by way of explaining these reimbursement and care issues?

ANNIE WALDMAN: Yes, we reached out to several insurance companies for our reporting, and they told us that they are committed to ensuring access to mental health providers.

They also emphasize that their plans are in compliance with state and federal laws.

Insurers also said that they have practices in place to make sure reimbursement rates reflect the market value to support and retain providers, which they're continually trying to recruit.

But that doesn't match up with what we heard from therapists or the previous analyses that policy experts have done and mental health advocates have done.

STEPHANIE SY: Annie Waldman with ProPublica, thank you so much for joining the "News Hour" and sharing your reporting with us.

ANNIE WALDMAN: Thank you for having me.