According to studies cited by Sonjy Lyubomirsky (http://www.faculty.ucr.edu/~sonja/ ) in her Book "The how of happiness" (a happiness book from a scientific view), the genetic predisposition is about 50%. 10% is your environment, and the rest is what you do with your life.

So, there is room for improvement for most people, although not as much as some happiness-gurus may like them to think.