The exact cause is not known, but scientists suspect genes and environment both play a role. Inside the brain, levels of the chemical messengers dopamine and glutamate may be out of balance. And brain structures may be abnormal, too. For example, brain scans of identical twins show that the fluid-filled "ventricles" can be larger in a twin with schizophrenia, compared with a twin who does not have the illness. Activity levels can also be higher or lower than normal in some areas of the schizophrenic brain.