A lot of people who take nootropics assume, or at least they used to, I don't know if people still believe this, that depression is caused by a small hippocampus, which is caused by decreased neurogenesis of unknown cause (perhaps excessive free-radical burden), which anti-depressants uniquely reverse, through some extraordinarily complex downstream mechanism. Therefore why not take anti-depressants if you aren't depressed, since it is in their nature to induce neurogenesis. Sure, people quibble about the neurogenesis effect to be shown only in depressed paitients, but what makes depressed people so special?
Why isn't it much, much, more natural and obvious to conclude that the anti-depressants lift the depression, and that depression in its nature is characterized by low neurogenesis? If you take the core nature of depression to be a withdrawl from the world into a self-reflective state, whether the mood which characterises this self-reflective state is melancholy or anxious or just plain flat, it seems clear that the less the brain is stimulated by the outside world, the less it is stimulated to grow new neurons. This effect has been shown in young brains, in the correlation between brain weight and enviornmental richness in rat pups. It seems likely that the same mechanism of neurogenesis persists into adulthood. As an aside, perhaps vanity, also a withdrawl into the self, is a disorder fundamentally similar to depression.
My own opinion of how anti-depressants work is that they upset the balance of the intricately interrelated mood-related neurotransmitters, which activates the brain's homeostatic mechanisms, moving the configuration of the brain into a different state, hopefully a good one, but . So they work on a principal similar to homeopathy. But it is not the homeostatic mechanisms which cause an increase in neurogenesis, rather it is depression in its functional nature which causes a decrease.
