I Googled "depression chemical imbalance". Out of the top 8 results 6 or so are somewhat critical of the chemical imbalance explanation saying it is a major simplification of a complex condition. I would suggest that the drugs that meddle with these chemicals don't treat the underlying cause of depression in the majority of cases, and only produce a somewhat superficial and temporary improvement (sort of like a cough lolly for a cough).
I reckon only a minority of cases where for some genetic reason (unrelated to other genetic causes of depression to do with the hard-wiring of the brain) there is a production of a chemical imbalance in the same way a minority of obese people are obese because of a genetic glitch that prevents the correct breakdown of certain elements of food even though they may a have a diet similar to much less obese people.
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Linkys:
http://www.health.harvard.edu/mind-and-mood/what-causes-depression
http://www.portlanddepressiontreatment.com/is-depression-just-a-chemical-imbalance/
http://www.scientificamerican.com/article/is-depression-just-bad-chemistry/
http://psychcentral.com/news/2006/11/09/depressions-chemical-imbalance-explained/398.html
https://www.beyondblue.org.au/the-facts/depression/what-causes-depression
etc...
The term "chemical imbalance"
is a gross oversimplification of the process of depression, but it isn't necessarily the incorrect theory (and, as are so many other ideas in medicine, they are theories, because science doesn't burden its practitioners with the concept of absolutes, unlike mathematics).
When humans are happy, and relaxed, they release certain hormones into their bloodstream. It's actually a similar process to when you hold your loved one in your arms, or when a mother holds her newborn.
But when a mother suffers from postnatal depression, the prevailing theories jot it down to hormonal changes in the mother's body - she's not releasing the correct hormones into her body so she's not feeling the love for her child, even though logically she knows she does love her child.
I don't really see the distinction here - a mother feels empty towards her newborn, and society says, "Hormones - not get fault."
But a man of 30, rich in finance, family and friends, ends his life because he feels nothing towards his beautiful wife and child, and society immediately questions how he could be depressed if his life was that great.
Sure, to people who have regular brain function - but if your brain isn't allowing you to feel the highs of life, wouldn't you reach a point of no return?
As a sufferer of depression for many years, I want to clear up the idea that people must be unhappy when they're depressed.
I've never felt sad - I have had a great life, and I'm truly fortunate.
But I've felt anger at friends and family for totally illogical (to mist people's minds, anyway) reasons, and I've felt complete nothingness towards partners and friends that I logically know mean the world to me.
People, let's process that for a second - imagine looking into the eyes of your wife/husband/partner, and
knowing you love them, but feeling nothing.
There seems to be a taboo with depression and suicide, but I have to day - that inability to feel feelings, despite therapy and medication, nearly pushed me over the edge. It truly is hell.