The causes of mental disorders are poorly defined, and so various faiths describe how mental disorders are caused. Some ascribe them to brain injuries; others, to demonic possession; others to lineage, and so on. Beliefs that a person’s mental disorders are due to demons possessing them - and that exorcism will treat them - actually cause such exorcism to work, and relieve them of their mental disorders. If you believe injuries cause mental disorders, you can treat these disorders by healing them. Beliefs that mental disorders can’t be treated will simply perpetuate the mental disorder and prevent treatment. And the opposite is also true: if you believe demons cause mental disorders, you can imprint demons onto people to inflict mental disorders on them; and if you believe injuries cause mental disorders, you can inflict mental disorders by inflicting brain injury, and so on.
As the mind seems a black box, its workings are poorly defined; and so various faiths describe how alterations or damages to the brain may impact behavior. Some faiths would have brain damage result in amnesia, others in autism, others in retardation, others in paralysis. The local prevalence of these faiths determines what the brain damage actually results in; so for example, a society that believes brain damage causes epilepsy will see that happen instead of other consequences, for people with brain damage.