Dr. Hall in Psych of Personality talked about the biological basis of personality. Have you ever seen Awakenings, with Robin Williams? It as based off of the book by the neuropsychologist Oliver Sacks, who also wrote The Man who Mistook his Wife for a Hat.
Anyway, Dr. Hall mentioned a few striking cases where insult or damage to the brain produced personality changes, including
"The Last Hippie", one of Sacks’ case studies.
What happened was that a young man went off to college and stopped communicating with his parents, who became worried. Apparently, he had become involved with a fringe group that restricted access, but his parents eventually managed to track him down and went to visit their commune.
His parents found their son with a stupid smile continuously plastered on his face. He made bizarre comments, showed little deep emotion, seemed disoriented, and was now blind. His parents took him to see a neurologist, and an enormous midline tumor the size of a grapefruit was found. It had destroyed his pituitary gland and optic chiasm (where the nasal portions of the optic nerve crossover in the brain). There was also some frontal and temporal lobe damage. He couldn’t form long term memories, and he spent the rest of his life hospitalized.
And the tumor was benign, but the damage it caused was irreversible.