Imagine a man walking down a street one afternoon who slips on a banana peel, causing him to fall and hit his head on an old typewriter someone has left on the curb for the trash collector. Now here's a question that seems simple at first but gets complicated pretty quickly: what's to blame for the bump on the man's head? Is it the banana peel or the typewriter? Or is the man's inattention the problem? After all, it's pretty hard to miss a bright yellow banana peel.

You might be thinking these questions are pretty silly, but I'm trying to get at something important. Even in such a simple-minded situation, it's hard to say exactly what made something happen. Naturally, in more complicated and serious situations, proving causation is much more difficult.

For years, we've suspected that violent video games cause violent behavior and aggressive actions. As the games get better and better - more realistic, more fun to play - their impact on players has been harder and harder to deny. And, for years a growing body of scientific evidence has demonstrated that blood-spatteringly violent games are associated with aggressive tendencies in the kids who play them. But, scientifically speaking, it's pretty difficult to prove that images on a screen do something to your thoughts and impulses. Until now, we've had to admit that just because we see a link between the games and the behavior doesn't mean one causes the other.

I say until now because a study just conducted by researchers at Michigan State University has proven what we've long suspected: violent video games cause aggression. Using MRI brain scans of players who played at least five hours of a popular violent video game each week, the scientists definitively showed that the game caused unmistakably aggressive brain activity while the players were playing. In other words, the players' brains were acting as if they were really engaged in the actions of their onscreen counterparts.

A recent survey of the top-selling video games revealed that fully half of the most popular games contain serious violence. This means that if your child is playing a game, he probably has a one-in-two chance that his brain is undergoing aggressive thought patterns. While the MSU study does not show a long-term relationship of causation between violent games and aggression, the short-term effects are clear.

Everyone who grew up watching cartoons knows that a banana peel on the floor will make you slip and fall. Well, it turns out the generation growing up on video games knows something much more serious: what it feels like to commit brutal acts of violence. So, let's take "watching what our kids watch" more seriously.

David Walsh, Ph.D. is the founder of the MediaWise Movement, a program of the National Institute on Media and the Family (www.mediawise.org). His latest book is Why Do They Act That Way? A Survival Guide to the Adolescent Brain for You and Your Teen is a national bestseller.