From the title of the question you can obviously discern I'm not a huge sports fan. I don't hate them by any means, they just never interested me as a spectator activity. I do like engaging in physically strenuous exercise for the health benefits, but when it comes to following every little stat...
I would have to say a mix of both. Mostly, however, it would be the environment the child grew up in. He/she would see the world and deal with everyday life the way he/she learned it as a child (general rule). Of course, it goes without saying that individual choices play the most important part...