The demand for mental health services is growing nationally, and comprehensive mental health legislation is gaining momentum in Congress for the first time in years. But both forces could run up against a counter-force: a shortage of psychiatrists, psychologists, counselors and therapists in much of the country.

More than half of U.S. counties have no mental health professionals and so "don't have any access whatsoever," according to Thomas Insel, director of the National Institute of Mental Health.

The implications are significant.

Nearly one in five adults -- about 43 million people -- had a diagnosable mental disorder within the past year. For nearly 10 million, the condition was serious enough to affect their ability to function day to day. Millions of adolescents also struggle with a debilitating mental disorder.

Many people have become eligible for mental health coverage under the 2010 Affordable Care Act. Yet finding the professionals to deliver that care is increasingly tough.

Merritt Hawkins, a physician search firm, conducted more searches for psychiatrists this past year than at any time in its 27-year history, according to a recent report. Psychiatrists trailed only primary-care doctors on the list of the firm’s 20 most in-demand medical specialties.

States with the highest rates of mental illness and the lowest rates of access to care are in the South and the West, according to the patient advocacy group Mental Health America. Rural areas face some of the biggest deficits.

"It is a serious shortage, especially in certain parts of the country," said Renee Binder, president of the American Psychiatric Association.

The lack of psychiatrists and other mental-health providers is part of an overall shortage of physicians in the United States. Earlier this year, a study by the Association of American Medical Colleges concluded that the nation will face a deficit of between 46,000 and 90,400 doctors within a decade. While primary care doctors will certainly be in short supply, it said, the biggest deficits may be among specialist physicians who care for the elderly, including psychiatrists.

Experts cite inadequate reimbursement from government and private insurance plans as one factor.

"A medical student leaves medical school and residency with the same amount of debt no matter their specialty, yet primary care and psychiatry are professions with some of the lowest annual salaries," said Chuck Ingoglia, senior vice president for public policy at the National Council for Behavioral Health. "Which one would a smart, ambitious young person choose?"

Or put another way: "If you look at the valuation for an hour of therapy, you could say we pay plumbers, carpenters and handymen more than we pay for behavioral health," said Paul Gionfriddo, president of Mental Health America.

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