Serotonin hypothesis of depression

A post from The Twenty-First Floor on the serotonin hypothesis of depression:

The main aetiological explanation for depression in the public consciousness is undoubtedly the “serotonin hypothesis”. This probably manifests more popularly as the idea that depression is somehow the result of a “chemical imbalance” in the brain, and therefore that sufferers of depression (whose suffering is not in question) are somehow the passive victims of an organic condition, like victims of diabetes, for example, and that this can be righted with medication. It’s a neat explanation, which, I guess, is why it’s so appealing. However, the evidence, as it so often does, suggests that depression is nowhere near this simple.

While I agree that the evidence for the serotonin hypothesis of depression (or even the rather wider monoamine hypothesis) is fairly weak, he links to this study finding mixed evidence for an association between suicide, impulsivity, and depression and serotonin metabolite levels. It is worth noting that a review by Mann et al (1989) is often taken as demonstrating that serotonin (or its metabolites) are reduced in the brainstem (source of serotonergic projections in the brain) in suicides, independent of underlying diagnosis. Although this actually only shows reduced serotonin in depressed suicides (if you look at the individual studies) that would actually make it stronger evidence for the hypothesis.