Righty or Lefty

I was sitting in government and it was really boring, so I thought that it would be a good time to become ambidextrous. I started taking notes with my left hand (I'm right handed) and realized that I could actually read them. True it looks like a 5 or 6 year old wrote it, but that's not bad for never practicing. It got me to wondering what affects someone becoming right or left handed. I know a whole bunch of righties and not many lefties. So is it hereditary factors that cause you to become certain sided? Or is it environmental? Here's some figures I found:
Only 8-15% of the population is left handed
Obviously this makes it a lot harder for left handers since the majority of the population is right handed. I know some twins. One is right handed and one is left handed. They aren't identical so that could be a factor there, but what's interesting is one parent is right handed and the other is left. I think that becoming right or left handed is largely due to your surroundings. If you have a parent who you are closer to, and they are left handed, it seems natural that you would become left handed too. Then there's always the case that you have a family entirely made up of righties and the kid never encounters someone who's left handed, but they still become left handed...would that be hereditary, or the kids personality? I'm not sure. My personal and extremely unofficial opinion is that 75% of the affects on one hand domination is environmental and about 25% is hereditary. Any thoughts? And if you wouldn't mind taking a minute and voting in this poll, I'm curious to see how many righties and lefties there are.