Although depression was classified as a disorder over 50 years ago, it existed long before the advent of modern classification methods. According to these scientific methods that now characterize the disease, nearly 121 million people worldwide are affected by depression. Many of these cases are left untreated. Unlike modern diseases such as cancer, obesity), depression’s origin has been contemplated since the time of philosophers Plato and Aristotle. Depression remains a prevalent and troublesome disorder despite the changes in social and environmental conditions over human history. Thus, it is important to ask: why has natural selection not culled it out through the course of evolution?

The human genome contains about 23,000 coding genes, each of which can be regulated by multiple others. There is no ‘depression gene’ that determines a person’s susceptibility to the disorder. People become depressed for different reasons or for no reason at all, and to different extents. The array of genetics and environment makes it quite difficult to predict why, if and when depression will hit. Some scientists have ideas, but no particular theory holds up to skepticism.

Symptoms of depression include difficulty in concentrating, feelings of guilt, helplessness, social withdrawal, and general loss of interest in life. Why do these severe manifestations of normal emotions persist, and what evolutionary advantage could they possibly offer? A trait must confer a reproductive fitness advantage in order to arise by natural selection, and it is this premise that underlies the claim that depression itself is an adaptation. The social navigation hypothesis proposes that depression evolved to help deal with social problems by allowing the individual to focus their energy on an issue at hand. Also, it could serve as a signal to friends and family to give the depressed one attention, which would increase care giving traits of the loved ones.

Depression affects 10% of the American population – if it improves survival fitness, should it not be more prevalent? However, an adaptive trait does not necessarily have to be expressed unless a particular environment triggers the required response. Therefore, the adaptation can be widespread in the population but expressed only in a minority of individuals. Since depression is highly costly, it should occur in the most crucial cases.

A big problem with this speculation is that depressed patients report causes of their illness to be life events such as death, divorce, or a layoff – things that most people deal with without developing the severe symptoms of depression. What makes some more prone than others? Susceptibility for depression varies among individuals, reflecting the way a certain personality trait, say neuroticism, is variable in different people. Many supposedly negative behavioral traits are thought to have an evolutionary purpose. For example, jealousy informs of a social competitor, eliciting actions to fight the threat. Also, neurotic individuals are more likely to stress over mistakes or low test scores, and as a result try harder to reach a goal. Neuroticism is highly correlated with depression and both are considered good predictors of marital failure. Seeing that marital failure does not necessarily solve social problems like it should under the social navigation hypothesis, the adaptation theory does not explain the disorder as much as regular bad moods.

In order for depression to serve an adaptive purpose, we need to examine whether its absence reduces fitness. Just as the inability to feel physical pain is detrimental and can lead to death, depression also impairs physical and mental health. But does it really have a greater benefit? As mentioned, depression can be thought of as an effort to increase performance when faced with a social dilemma, but more often than not, the non-depressed individuals do just as well. People cope, remarry and find new jobs. Adaptation theories have trouble describing the benefits of severe depression.

In the search to determine why evolution did not wipe depressive traits off the face of the earth, we can contemplate other negative, seemingly ineffective behaviors. As is the case with neuroticism, the spectrum of personalities does include unhealthy outliers. If milder forms of neurotic features are advantageous for competition and achievement, the trait is still fitness-enhancing and selected for. Since variability and degree of a trait largely depend on one’s genes and environment, the extremes are bound to occur.

There are plenty of other evolutionary forces (and maybe adaptations, too) that together interact to create predispositions to depression. Unfortunately, it is almost impossible to account for the myriad of factors that play the game of chance to produce individual differences and likelihood for disease. But as we can see, trying to discern what some of them are is still an exciting endeavor.