1st) There is no such thing as a chemical imbalance that can occur in the brain. (stay with me)
2nd) Mental illness IS real but why
Addressing the 1st issue. Every humans chemical structure in the brain is as individuate as there muscle structure. Jim Bob can bench press 200lbs, but Greg 60. Our chemical structure is the same. We have what we need. So giving someone a pill (one size fits all) is like asking every person on Earth to bench press the exact same amount of weight from the start. Also woman's chemical structure is far different than a mans and so they need LESS of some of the same medications. The phrase (chemical imbalance) was coined in Europe prior to Hitler to explain to the public why they needed to drill in your head and drain fluids, the phrase stuck. Real scientific research has turned up some very interesting FACTS.
2nd point. Mental illness if real, such as bi-polar,depression, (most anxiety disorders have been removed as a mental illness. Just difficulty with coping skills now) schizophrenia, borderline personality disorders (BPD is now considered the serious of all mental disorders and has 8 levels.) and other well established mental disorders.
What is the cause? This is fascinating. In a worldwide study done by over 20 nations and repeated since 1950, without collaboration (meaning no chance to fix the results) the 3 hormones (testosterone, estrogen and progesterone) in some cases suffer a level imbalance around the time the embryo's sex is established in the womb. In other words when a boy is suppose to turn into a boy or girl a girl, these hormones are out of balance. The developing embryo's mind attempts to correct, but can't so (here is the important part) the structure and layout of the brain changes. (more later). Another cause. Childhood trauma or experiences that fall out of range of normal. During childhood the brain structure developments from back to front. If key development stages are skipped the structural layout suffers, if trauma is suffered and not properly dealt with, the layout suffers. As adults are brain structure appears to be static but it is not. It is now known it is dynamic. So we suffer trauma or too many negative experiences as adults our structure alters.
What is important about the structure? The method that the brain cells fire in. Our brain cells are designed to fire is a sequenced way, from bottom to top and back to front, in a normal pattern. However if the layout or structure of the brain is altered, then the method of firing suffers resulting is well established mental disorders.
Thus far the real scientific testing has shown that current medications were designed to target chemical structure not layout and firing, so for most people current medications fail.
The current psychiatrist are fighting change, backed by drug companies, loss of money.
Source material: Data from returning vets and their treatment; Scientific American Mind; Dr. Bruce Perry, The Canadian Institute, a compilation of worldwide studies out of a University in England. and many more. I spent the last 5-8 years reading the material and I was allowed to participate in two of the studies. The information is already out, the new DSM reflects some of the changes. In BPD levels 6-8 is were most serial killers fall. The data to support the above is staggering, but the drug companies do not want to reinvest billions in a different path.