It's a mixture of genetics and environment, so most of the recent text books say. Although I think a lot of recent research points toward genetic factors. That's not to say you can't change your personality, because you can change your emotional and behavioural responses to different environments. The idea of a blank slate, that two people will react the same in a given environment, is not popular anymore in contemporary psychology. A lot of research on this, good place to start is http://search.apa.org/search?query=personality%20genetic%20environment
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