The number of psychiatric beds in state hospitals has dropped to a historic low, and nearly half of the beds that are available are filled with patients from the criminal justice system.

Both statistics, reported in a new national study, reflect the sweeping changes that have taken place in the half-century since the United States began deinstitutionalizing mental illness in favor of outpatient treatment. But the promise of that shift was never fulfilled, and experts and advocates say the result is seen even today in the increasing ranks of homeless and incarcerated Americans suffering from serious mental conditions.

Researchers for the Treatment Advocacy Center, a national nonprofit organization, found that states’ psychiatric bed total had fallen by 17 percent since 2010 — from 43,318 in 2010 to 37,559 this year. That has left just 11.7 beds per 100,000 people, far below the count in other developed countries.

“The numbers are so bad that people almost don’t believe them,” said TAC's executive director, John Snook, who considers the precipitous decline to be “disastrous for our nation and those most in need.” He supports funding for more research on bed needs and policy changes to reverse the situation.

The study, which was released last month, recommends reforming Medicaid and Medicare regulations that it says have contributed to bed shortages. It also calls for greater use of interventions that could connect people with treatment before they need “the last resort of a state hospital bed.” Doing so — through court-ordered outpatient treatment, use of mobile crisis teams and special “de-escalation” training for police — would not just keep individuals out of emergency rooms but potentially also out of jails, the study notes.

The diversion of beds to criminal justice needs keeps increasing. Of states’ nearly 38,000 psychiatric beds early this year, 17,601 were only available for forensic cases, whether arrested suspects, individuals in local jails or state prison inmates. That was several thousand beds more than in 2010.

A few states, including California, Michigan and North Carolina, added publicly funded psychiatric beds over the past six years, according to the study. California’s count, for example, increased by nearly 12 percent, growing from 5,283 in 2010 to 5,905 this year.

Jan Emerson-Shea, a spokeswoman for the California Hospital Association, said that while there has been no new funding to specifically increase bed capacity, a 2013 mental health law included grants for crisis treatment to reduce hospitalizations and encounters with law enforcement.

Psychiatrist Renee Binder, a professor at the University of California at San Francisco School of Medicine, who sees patients at one of its teaching hospitals, said they are often acutely suicidal, combative or unable to function.

“The patients who come to the ER can’t be released,” she said. “They sit in the middle of the emergency room. It’s not good for the patients, because they aren’t getting the treatment they need.”

Yet the vast majority of states have lost beds since 2010. In nine, 16 state hospitals either shut down or were merged.

New York saw the sharpest decline, shedding more than a third of the 4,958 beds it had available in 2010. It now has 3,217, according to the study. The decrease is part of the state’s continuing effort to move mental health care to community-based models, according to the New York Department of Mental Health. The agency’s most recent plan notes that inpatient psychiatric costs amount to about half of public spending on mental health.

At University Hospital in Syracuse, chief executive John McCabe said getting inpatient psychiatric care for a mentally ill patient first seen by the emergency department is often difficult. If no bed is available at the hospital, McCabe said that it can take days to two weeks to find one at another facility. The lag for adolescent patients can be even longer, he added.

A three-year, $75 million demonstration project begun as part of the Affordable Care Act suggests that certain policy changes can be part of the remedy, according to the study. The project allowed 10 states plus the District of Columbia to get Medicaid reimbursement for patients receiving mental health care in private psychiatric institutions. One question was whether patient care and discharging practices, as well as the number of mental health patients waiting in emergency rooms, would improve.

A 2013 congressional report said the participating states all indicated that the demonstration had helped them expand inpatient care. This year, the project was extended three more years, although funds have not yet been appropriated.

Rep. Tim Murphy (R-Pa.), the prime sponsor of mental health reform legislation pending in Congress, isn’t convinced the demonstration is making a difference.

“We don’t know if it’s been successful or not. We don’t know the impact it’s had on cost and outcome,” he said. “There is no data on where people went and what the cost was.”

Part of his bill would make the Centers for Medicare and Medicaid Services collect data on the Medicaid mental health project. “We paid for something, but we don’t know what we got,” he said.

Under current law, Medicaid won’t cover inpatient treatment in mental health facilities with more than 16 beds. Murphy’s Helping Families Act would allow the program to reimburse a standalone psychiatric hospital of any size for up to 15 days of treatment provided to Medicaid managed-care patients, who are 80 percent of all Medicaid enrollees.

“The key is compassion in dictating what we can do for people,” Murphy said.