This will be quick; I'm actually headed off to work.
In his "In Practice" blog, Peter Kramer discusses the issue of whether the concept of a chemical imbalance is still a useful one and he looks at the evidence for and against such a theory, concluding that the concept met a premature death.
"Since 1993, other biochemical contributors to depression have claimed their roles, especially “stress hormones” and factors that influence nerve cell growth. The new overarching biological model of depression (I outline it in Against Depression) integrates all three factors—monoamines, stress, and cell growth—but serotonin dysregulation remains very much on the table as a contributor to depression."
Dr. Kramer talks about PET scans and genes and differential rates of monoamine metabolism, and the stupid little bouncing Zoloft mascot with the smiley face.
For the shrink in the field, so far it doesn't mean much. I can't order a test to find out if someone has too much of one enzyme breaking down any given neurotransmitter and thereby telling me what to prescribe. I'm waiting. In the meantime, what I do have is patients who come in wanting to know what they have. "Do I have a chemical imbalance?" Now what does that even mean? Do you have too much serotonin in some places in your brain and not enough in others? How would I know that? Too much (compared to what?) monamine oxidase breaking down your noradrenergic neurotransmitters? Should we inhibit them and this will make you better? Let me get my probe.
What I do know is that while I don't know what is meant by a "Chemical Imbalance," my patients do. For them it is a term that explains things, that writes the story, that has meaning. There's something socially acceptable about it. "I have poor coping skills" is pejorative and equally unprovable. "I have a chemical imbalance" is somehow explanatory, though still unprovable in a day-by-day psychiatric practice.
So, generally, if a patient with Major Depression asks, "Do I have a Chemical Imbalance?" I simply say "yes." It seems to work.