Can a physical injury to the brain, resulting from an accident, and not the result of a diagnosed mental illness, affect a person's social behavior?
Socialization is a cognitive and executive brain based function that requires higher level thinking mechanisms usually dependent on the prefrontal coretex. A lot of social cues we obtain from our environment when we are young can shape our brains during its developmental growth (infancy-post puberty) and cause our brains to develop social habits and access specific social functions.
A great text book example of altered social behavior due to brain injury is Phineas Gage. He had a metal rod shot through the prefrontal cortex and it caused a severe change in his behavior. He no longer cared about social cues and people often referred to his behavior as becoming "rude" and "out of context." There are many subtle aspects of behavior that are regulated by brain functions and there are many books on the topic.
Source: Wikipedia page on Phineas Gage and Pyschopharmacology, book by: Meyer and Quenzer DSM-5, text book on clinical psychology
Further Reading on Phineas Gage
- What were the symptoms of Phineas Gage after suffering his brain injury?