system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are the biological/natural reasons Humans have mental disorders like ADD, ADHD, Bi-Polar, Depression, etc - are there any purposes and/or functions outside a civilized society?
gpt
Evolutionary explanations for a human trait *T* need not take the form, "*T* is beneficial because..." Indeed, very often their form is, "*T* persists in the population despite..."

What's generally going on there is that some combination of genes that do serve some very useful purposes also happen to have a side-effect that is not at all useful, but the benefits of the useful things outweigh the drawbacks of the nonuseful one, at least enough to cause that combination of genes to continue being handed down to successive generations of offspring.