I read an interesting book about introverts : 'Quiet : The Power of Introverts in a World That Can't Stop Talking' ; here's a quote from the book :

"Schwartz's research suggests something important: we can stretch our personalities, but only up to a point. Our inborn temperaments influence us, regardless of the lives we lead. A sizable part of who we are is ordained by our genes, by our brains, by our nervous systems. And yet, the elasticity that Schwartz found in some of the high-reactive teens also suggests the converse: we have free will and can use it to shape our personalities.

These seem like contradictory principles, but they are not. Free will can take us far, suggests Dr. Schwart's research, but it cannot carry us infinitely beyond our genetic limits. Bill Gates is never going to be Bill Clinton, no matter how he polishes his social skills, and Bill Clinton can never be Bill Gates, no matter how much time he spends alone with a computer.

We might call this the "rubber band theory" of personality. We are like rubber bands at rest. We are elastic and can stretch ourselves, but only so much."