What is the Dopamine Hypothesis and how can I evaluate it?

The dopamine hypothesis is the theory that schizophrenia is caused by too much dopamine in the brain. The theory states that due to dopamine being a neurotransmitter that instigates the firing of neurons in the brain, an excess of this can cause neurons to fire too often, thereby producing many symptoms of schizophrenia such as hallucinations and delusions. To evaluate this theory, it is useful to firstly examine the logic that underlies it: if too much dopamine causes schizophrenic symptoms, then when dopamine levels are increased there should be a corresponding increase in symptoms. On the other hand, when dopamine levels in the brain are decreased, there should be a corresponding decrease in symptoms. In other words, if A is proposed to cause B, and we observe that more of A = more of B, this is a good indicator that A does indeed cause B. If the studies you find in textbooks/google fit well with this logic, the theory may well be valid.

To give you two examples: using PET scans, one study found that schizophrenics had higher levels of dopamine in the brain compared to controls (this supports the theory because more dopamine = more schizophrenia). On the other hand, another study found that antipsychotic drugs which reduce the effects of dopamine actually become less effective when they target the maximum amount of dopamine receptors (as the drugs reduce dopamine, according to the dopamine hypothesis you would expect more drug = less schizophrenia. This is not the case in the study, and while it does not completely prove it wrong, it does show that it is not as simple as more dopamine = more schizophrenia - there must be something else going on too).