Depression May Have Evolved to Combat Infection

The depressed tend to have higher levels of inflammation, even if they’re not fighting an infection

Depression is common enough to suggest the possibility that it must be ‘hard-wired’ into our brains. This has led biologists to propose several theories to account for how depression, or behaviours linked to it, can somehow offer an evolutionary advantage.

Some proposals for the role of depression in evolution have focused on how it affects behaviour in a social context. Now a pair of psychiatrists addresses this puzzle in a different way, tying together depression and resistance to infection. They propose that genetic variations that promote depression arose during evolution because they helped our ancestors fight infection. An outline of this proposal appears online in the journal Molecular Psychiatry.

According to an Emory University Press release, their work builds upon the observation that for several years ‘researchers have seen links between depression and inflammation, or over-activation of the immune system. People with depression tend to have higher levels of inflammation, even if they’re not fighting an infection’.

“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 behaviours are not helpful in our relationships with other people,” co-author Charles Raison says.

Infection was the major cause of death in early human history, so surviving infection was a key determinant in whether someone was able to pass on his or her genes. The authors propose that evolution and genetics have bound together depressive symptoms and physiological responses that were selected on the basis of reducing mortality from infection. Fever, fatigue/inactivity, social avoidance and anorexia can all be seen as adaptive behaviours in light of the need to contain infection, they write.

The theory provides a new explanation for why stress is a risk factor for depression. The link between stress and depression can be seen as the byproduct of a process that preactivates the immune system in anticipation of a wound, they write.

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