New evidence suggests that depression may have evolutionary significance in enhancing the body’s ability to fight off infections or attacks to the immune system.

A group of psychiatrists said that previous depression research has largely focused the social and behavioral aspects of the pathological mental illness. They speculate that there may be an alternative reason to why depression is more prevalent among people with chronic conditions like obesity, cardiovascular disease, diabetes, asthma and cancer, as well as among people exhibiting unhealthy behaviors such as smoking, physical inactivity and binge drinking.

Co-authors Dr. Andrew Miller and Professor William Timmie of psychiatry and behavioral sciences at Emory, believe that behaviors linked to depression may have offered an evolutionary advantage that helped our ancestors fight infection.

Previous studies have associated depression with inflammation or overactive immune response, and that depressed individuals tended to have higher levels of inflammation even when their bodies were not fighting infections, but researchers said that high levels of inflammation are not “an inevitable consequence” depression.

“Most of the genetic variations that have been linked to depression turn out to affect the function of the immune system,” Miller said in a statement released on Thursday. “This led us to rethink why depression seems to stay embedded in the genome.”

The study authors explained that depression and the genes that promote it may have been very adaptive for helping people survive infections in the ancestral environment, especially in young children, even though depressive behaviors are maladaptive for social relationships.

For someone to pass on his or her genes, they needed to survive infections, which was the major cause of death in humans' early history, the authors explained. Thus it may be possible that

Infection was the major cause of death in humans' early history, so surviving infection was a key determinant in whether someone was able to pass on his or her genes. Thus it would be reasonable to suggest that evolution and genetics have combined depressive symptoms and useful physiological responses like fever, fatigue, inactivity, social avoidance and anorexia for containing and reducing mortality from infections.

The authors wrote that their theory provided a new explanation for why stress is a risk factor for depression because they can be thought of as a “byproduct” of the immune response process which preps the body to anticipate injury.

Furthermore, Miller said that disrupted sleep patterns are seen in booth mood disorders and when the immune system is activated, a physiological reaction, which may have adapted from the need to stay on alert to fend off predators after injury.

The study authors suggest that the new theory could pave the way for new research on predicting how well some individuals will respond to various depression treatments and whether certain drugs, normally used to treat auto-immune diseases can be just as effective in treating treatment-resistant depression.

The research proposal outline is in the journal Molecular Psychiatry.

At the beginning of the year, a new government report indicated that in the past year about one in five American adults have suffered mental illnesses.

The Substance Abuse and Mental Health Services Administration reported that women were more likely than men to suffer a mental illness at 23 percent compared to 16.8 percent, and the people between the ages of 18 to 25 were two times more likely to suffer a mental illness than those who were 50 and older.

SAMHSA Administrator Pamela Hyde had said that mental disorders often coincide with other chronic disorders and effectively treating mental illnesses has been shown to reduce the effects of other diseases an individual might have.

Mental illness is a significant public health problem in itself, but also because it is associated with chronic medical diseases such as cardiovascular disease, diabetes, obesity, and cancer, as well as several risk behaviors including physical inactivity, smoking, excessive drinking, and insufficient sleep,” said Ileana Arias, Principal Deputy Director of the Centers for Disease Control and Prevention in a statement.

The economic impact of mental illness in the U.S. accounted for about $300 billion in 2002, and according to the World Health Organization mental illness accounts for more disability in developed countries than any other illness, including cancer and heart disease.