I think 51% are predetermined before we are born, 49% are formed during our growing up after we are born. We are born to be different, you will find that children who grow up in the same environment also have different personalities is a proof. I don't deny the power of change, but to be honest, to change one's personality may be one of the hardest things in the world. We are born with some unique natural instincts, what we may not able to change in the further life stage. As we grow up, the environment we are in, the people around us, influence us invisibly in daily life. What we experienced finally shaped us. One can try to change his personality to adapt to the changing environment, but this change will happen based on who he is first. Sometimes, it is not he has changed but he finds a better way to communicate with the world, he is still him.