There are no biochemical markers, no biological tests, no hard evidence at all, to "prove" the existence of "mental illness." ≥Proof≤ means the ability to demonstrate a reliable association between a clearly specified pattern of observables and other reliably measurable event(s) which operate as antecedents. (This is same level of proof used for TB, cancer, diabetes, etc.)

For those who adhere to the "chemical imbalance" theory,

Which of the neurotransmitting brain chemical(s) is it that is/are out of balance?

What is the nature of the imbalance(s) -- Too much, too little?

In what part(s) of the brain is/are these imbalances occurring?

What is the formula for determining the baseline "normal" amount of the offending chemical(s), given one's gender, age, weight, etc, and where can I find it referenced? I am interested in reading the articles where you obtained this information.