Dopamine is widely believed to be involved in schizophrenia because it is featured in the functioning of brain systems related to the symptoms of schizophrenia.
High dopamine activity in subcortex (central areas of the brain) used to be associated with hallucinations and poverty of speech (e.g. excess of DA receptors in pathways linking from subcortex to Broca's area). This was used to explain specific symptoms e.g poverty of speech and auditory hallucinations.
Now low levels of dopamine in the prefrontal cortex (responsible for thinking), could explain negative symptoms. This explains origins of abnormal DA - genetic variations and early experiences of stress make some people more sensitive to cortical hypodopaminergia and hence subcortical hyperdopaminergia (Howes)
