the influential theory that schizophrenia is caused by an excess of dopamine in the brain, due either to an overproduction of dopamine or a deficiency of the enzyme needed to convert dopamine to norepinephrine (adrenaline). There is some supporting pharmacological and biochemical evidence for this hypothesis, and it is still widely discussed and promoted, particularly in a revised form that postulates the involvement in schizophrenia of both an increased mesolimbic and a decreased prefrontal dopaminergic activity. See also glutamate hypothesis.