Does depression offer an evolutionary advantage? Biologists have proposed several theories about the role of depression in evolution, focusing on how it affects behavior in a social context, but two psychiatrists now offer a different explanation: There may be a link between depression and resistance to infection.
Depression is so common – affecting one in 10 adults in the United States, according to the Centers for Disease Control and Prevention – that the possibility of depression seems to be "hard-wired" into our brains.
Charles Raison, M.D., holds a joint appointment as associate professor with the Department of Psychiatry at the UA College of Medicine - Tucson, and as the Barry and Janet Lang Associate Professor of Integrative Mental Health with the Norton School of Family and Consumer Sciences. He previously was with Emory University in Atlanta. Co-author Andrew Miller, M.D., is the William P. Timmie Professor of Psychiatry and Behavioral Sciences and is director of psychiatric oncology at the Winship Cancer Institute at Emory. The two propose that genetic variations that promote depression arose during evolution because they helped our ancestors fight infection.
An outline of their hypothesis, "The evolutionary significance of depression in Pathogen Host Defense (PATHOS-D)," appears online in the journal Molecular Psychiatry.
Studies have found links between depression and inflammation, or overactivation of the immune system, as well as higher levels of inflammation in people with depression even if they're not fighting an infection. Raison and Miller note that in humans' early history, the ability to pass on genes depended on surviving the major cause of death: infection. They propose that depression and physiological responses that reduced mortality from infection – fever, fatigue, inactivity, social avoidance and loss of appetite – were bound together by evolution and genetics.
Raison recently joined the UA to further his research in mind-body medicine. His work focuses on inflammation and the development of depression in response to illness and stress.
"The basic idea is that depression and the genes that promote it were very adaptive for helping people – especially young children – not die of infection in the ancestral environment, even if those same behaviors are not helpful in our relationships with other people," he said. "Moreover, significant data suggest that at least some depressive symptoms actually may help people avoid infection or survive it when it occurs."
Raison serves as CNN Health's mental health expert and frequently appears in other media outlets as well. He is the 2011 Chair of the U.S. Psychiatric and Mental Health Congress.
Co-author Miller said, "Most of the genetic variations that have been linked to depression turn out to affect the function of the immune system. This led us to rethink why depression seems to stay embedded in the genome."
The theory provides a new explanation for why stress is a risk factor for depression, and also for the disruption of sleep patterns seen both in mood disorders and when the immune system is activated. The theory also may impact depression research: For example, the presence of biomarkers for inflammation may predict a person's response to various treatments for depression. Miller and Raison currently are researching the effectiveness of medications normally used to treat autoimmune diseases on treatment-resistant depression.