[scns]

You can show via SPECT brainscans that the over- or underactivity in certain parts of the brain correlates strongly with psychological symptoms. For example you can show that people suffering from ADD have a lowered activity in their prefrontal cortex. Or that people with an overactive deep limbic system suffer from depression. Check out Dr Daniel Amen's work. He did over 83000 brainscans.

https://www.youtube.com/watch?v=esPRsT-lmw8

edit: added count and link to video

[mnm1]

That's a far cry from demonstrating that specific neurotransmitter imbalances are the cause of depression.

[scns]

True. It seems depression is created and sustained through distorted thoughts. Check out David Burns' "Feeling Good".