Previously I posted about the much-maligned serotonin theory of depression and tentatively defended it, while making it clear that "low serotonin" was certainly not the whole story. Critics have noted that the serotonin-is-happiness hypothesis has become folk wisdom, despite being clearly incomplete, and this is generally ascribed to the marketing power of the pharmaceutical industry. What's also interesting is that a predecessor and rival to the serotonin hypothesis, the noradrenaline theory, failed to achieve such prominence.