The Negative Effects of Games on Addiction

I wouldn't deny that there's a small group of gamers that could get addicted to games. I think research should be really neutral and have an open eye for what is out there in the field. For example, if I want to have an effective learning game, it's good to have some research on that. How does it work? How can I get better learning effects from games? A lot of stuff that you can learn from the book, even if you're a designer.