A new study by Harvard University and the National Institute of Mental Health (search) claims that 46 percent of all Americans will, at some point in their lives, develop a mental disorder.

But this new statistic has experts arguing over exactly what constitutes a true mental illness.

According to experts, severe mental illnesses like schizophrenia, dementia and manic depression are relatively uncommon. But the updated Diagnostic and Statistical Manual of Mental Disorders (search), or DSM — the standard survey for mental illness — lists conditions like adjustment disorder, passive-aggressive disorder and female sexual arousal disorder as mental illness, reflecting what are claimed to be advances in the mental health profession.

Critics say that's crazy, and that it won't be long before all human quirks and flaws are classified as mental disorders.

"When you say we're going to provide mental health treatment to everyone, then the severely mentally ill usually get the lowest priority," said Dr. Richard Lamb (search), professor of Psychiatry at the University of Southern California.

Others say the DSM should be considered the mental health bible, because if it says that a person is mentally ill, then that person can get the treatment they need — and insurance companies will foot the bill.

"We would never suggest to somebody with one type of cancer that because their type of cancer is not necessarily as fatal as somebody else's that they should withhold treatment so someone else can get it," said Cynthia Focarelli of the National Mental Health Association (search).

The next edition of the DSM will not come out until 2010. The American Psychiatric Association (search) is considering making changes to refine the subclasses of various mental illnesses.

Click in the box near the top of the story to watch a report by FOX News' Trace Gallagher.