Dopamine (DA) Hypothesis
The dopamine (DA) hypothesis is a theory that proposes a biological cause of schizophrenia. This is the oldest theory about the cause of schizophrenia and was first presented in the 1960s. It suggests that schizophrenia is caused by too much DA transmission in the brain. Pharmacological evidence exists for this theory in that medications that block DA receptors helped decrease schizophrenic symptoms. DA enhancing medications can induce psychosis and cause schizophrenic like symptoms in individuals.
Modern research has built on this theory and shown that a deficit of DA in the prefrontal cortex is responsible for the 'negative' symptoms of schizophrenia (like flattened affect, social problems, anhedonia) and too much DA transmission is responsible for the 'positive' symptoms of schizophrenia like delusions and hallucinations. Current research is still focusing on this theory but it is believed to only be one component of a more complex set of circumstances that cause schizophrenia to arise.