Until recently, a central tenet of biomedical psychiatry was that depression is caused by low serotonin levels. A recent study has debunked that claim, to great publicity. In response, some researchers have called for “doubling down” on the search for biological causes of depression. I want to present something radically different.
What if depression is purposeful, rather than pathological? What if it’s a designed response to a problem of life, not a disease?
Some researchers over the past 30 years have argued that depression isn’t a disease but an evolved adaptation. Though there are a number of specific hypotheses, they all share the idea that depression is nature’s way of telling us that something in our lives isn’t going well—particularly with our social interactions.
One of the first of these hypotheses was the social competition hypothesis. In this view, depression originated in prehistoric competitions over resources. When I find myself “outcompeted” by another person for a prized resource, depression saps my incentive to fight.
A second, more general, view is that depression is nature’s way of helping us detach from unrealistic life goals. Low mood takes away my motivation to strive for unrealistic goals and lets me focus on realistic ones.
A third hypothesis is called the social risk theory. This view sees depression as a response to the threat of exclusion. Through depression, I communicate that I’m a “low social risk” to others, and I convey the need for additional support from close friends and relatives.
A fourth is the analytical rumination hypothesis. It stems from the observation that people with depression often mull over their perceived failures. This suggests that the purpose of depression is to help us focus attention on complex social problems.
Evidence for the Design Hypothesis
Unfortunately, evolutionary hypotheses cannot be tested directly. They require drawing evidence from a large number of different areas. And, the fact is, there’s not a lot of funding available for testing them.
Still, there are some intriguing lines of evidence suggesting that depression is an adaptation, not a disease (see del Giudice 2018 for a review).
Depression occurs worldwide, with a high incidence and strong genetic risk. That’s often a sign that it’s playing some important role in our lives. If it were a disease, we wouldn’t expect it to be so common, especially among people of reproductive age.
It’s long been recognized that depression is more likely to be triggered by perceived social losses, rather than nonsocial losses. It’s more likely to be triggered by factors such as divorce or humiliation, rather than losing a house or car.
Depression shows high comorbidity with other disorders with a strong social component, such as social anxiety disorder. Also, depressed people tend to be highly vigilant to the threat of social rejection.
Even if depression is an adaptation, that doesn’t mean it always has a positive outcome. In some cases, depression can become unregulated and spiral into something quite destructive.
For example, depression can have a “self-reinforcing” character. Depression can lead to social isolation, which can reinforce depression. Rumination over one’s perceived failures can reinforce patterns of negative thinking.
Moreover, even if depression helped our prehistoric ancestors, that doesn’t mean it’s always useful today. For example, if depression is caused by having unrealistic life goals, it doesn’t help that we’re surrounded by media telling us our worth as human beings is connected to our accomplishments.