Schizophrenia is thought to occur when a person has overactivity within this particular neurotransmitter system?

Dopamine.

Schizophrenia is a mental illness that can cause hallucinations, delusions, and impaired thinking. It is thought to be caused by an imbalance of neurotransmitters in the brain, including dopamine. Dopamine is a neurotransmitter that is involved in reward, motivation, and movement. In schizophrenia, it is thought that there is too much dopamine activity in certain areas of the brain, which can lead to the symptoms of the illness.