I've lost count of the number of studies that have been done on this- some say they will warp little Timmy's/ Judy's mind and turn them into hardened criminals while others say "it's just a game- lighten the **** up".
Myself, I'm of the second persuasion. If a game is going to influence someone to do something they shouldn't then that person likely had some sort of mental instability to begin with. That being said, while I do disagree with the ones that say it will influence the kids, I still believe those that are under the age it is rated for should not play it UNLESS the parents ok it.