Evolutionarily speaking, depression (both clinical depression and temporary sadness) is a curious phenomenon. Since being depressed is likely to increase the chance of a person with it to commit suicide, one would think it comes with a negative fitness cost/selection pressure.
I wouldn't bet on the evolutionary reasons for depression being related to genetic drift as it's been observed in several non-human species too (see this post).
The main risk factors of depression include past abuse (physical, sexual, emotional), certain medications, familial conflicts, death or loss, chronic or major illness, and there's even a genetic component (family history). (Berne, 2007). But perhaps more interestingly, even positive events such as graduating, getting married, a new job can lead to depression (Brees, 2008).
- What possible evolutionary reasons are there to explain depression?
- Are there any studies that attempt to address this question?