Jay,
I guess it depends how you define "personality". Severe physical or mental trauma can certainly alter a person's beliefs and behavior. Type A personalities can "burn out" and turn into bums. Indoctrination before the age of about 7 can seriously mould belief and behavior forever after. However, anecdotally there are twin studies in which twins brought up in completely different environments end up with very similar personalities. There are also numerous examples of siblings brought up identically of whom one turns out to be a monster. And the nastiest bully in the playground may turn into the nastiest bully in adult life, or into a lovely person.
Why do you ask?
Regards
j.R
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
