I saw a television program about a woman who was terrified of spiders. "They" scanned her brain looking for the part that was firing most when she was shown spiders. They then proceeded to acclimatize - reprogram her. The part that was firing ended up firing significantly less. Conclusion was that with sufficient training the brain seems to rewire itself. This process does not ever stop, but the speed with which this change takes place slows as we age.

People that regularly stretch their mental faculties get alzheimers much much later than those that don't exercise their minds. Use it or lose it. The brain seems to be very adaptable. Change is possible barring incapacitating trauma.

>>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.
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>>Why do you ask?
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>But can you do it simply by free will alone? Can you turn around essentially who you are simply by wanting to do it? (to quote a portion of my response to Hilmar)
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>There are portions of my personality that I wish were different. I'm sure I'm not alone. At some age, is it too late? Yes, I can decide to say hello every morning and smile at someone I detest, but that doesn't mean I think any different of them. Can that part be changed by wanting it to? I used to sell books door-to-door years ago, and one thing that the company gerus pressed was to "act enthusiastic to become enthusiastic." Granted, their motives were somewhat suspect, but I never "felt" enthusiastic. I did act it though, because that made sales.