The Evil Monkey in Neurotopia:
Ok, the serotonin theory of depression may not be wrong. But it is definitely incomplete. One might ask why we use serotonergic drugs to treat depression if the theory behind it is wrong. A good question, but to this I say: because it worked.
(I love the Zoloft depressed marshmallow. He's so cute!)
The original antidepressants, the monoamine oxidase inhibitors and tricyclic antidepressants, were originally used to treat other diseases, such as tuberculosis and psychosis, and found to be effective for depression as a sideline. Did people know how they worked? Nope, but they appeared to work (though only in a subset of the population), and so they came into use. Some people might get up in arms about this, and yell about how we shouldn't use drugs unless we know how they work. But if we spent our lives doing that, no one would have ever made asprin. Or morphine. Heck, no one would have patented Ritalin. We know THAT Ritalin works, and we know what Ritalin does in the brain, but do we know why Ritalin calms down people with ADHD when it's really a stimulant? Not really, no. But it's still out there, because it works.