TALLAHASSEE, Fla., March 4 (UPI) -- The theory depression is caused by a chemical imbalance is often repeated in the media, but there is little evidence this is true, a U.S. study said.

Jeffrey Lacasse, a doctoral candidate at Florida State University in Tallahassee, and Jonathan Leo of Lincoln Memorial University in Tennessee asked members of the media and others to provide documentation that supported the chemical imbalance theory, but none could.

"The media's presentation of the theory as fact is troublesome because it misrepresents the current status of the theory," Lacasse said in a statement.

The Diagnostic and Statistical Manual of Mental Disorders, used by psychiatrists to diagnose patients, states that the cause of depression and anxiety is unknown, the researchers said.

A review of a full set of trial data published in the journal PLoS Medicine last month concluded that much of the perceived efficacy of several of the most common antidepressants prescribed, called Selective Serotonin Reuptake Inhibitors, was due to the placebo effect, Lacasse said. Other studies indicate that for every 10 people who take those drugs, only one to two people are truly receiving benefit from the medication, Lacasse and Leo said.

The findings are published in the journal Society.