EARLIER this summer the actor Jim Carrey, a star of the new superhero movie “Kick-Ass 2,” tweeted that he was distancing himself from the film because, in the wake of the Sandy Hook massacre, “in all good conscience I cannot support” the movie’s extensive and graphically violent scenes.

Mark Millar, a creator of the “Kick-Ass” comic book series and one of the movie’s executive producers, responded that he has “never quite bought the notion that violence in fiction leads to violence in real life any more than Harry Potter casting a spell creates more boy wizards in real life.”

While Mr. Carrey’s point of view has its adherents, most people reflexively agree with Mr. Millar. After all, the logic goes, millions of Americans see violent imagery in films and on TV every day, but vanishingly few become killers.

But a growing body of research indicates that this reasoning may be off base. Exposure to violent imagery does not preordain violence, but it is a risk factor. We would never say: “I’ve smoked cigarettes for a long time, and I don’t have lung cancer. Therefore there’s no link between smoking cigarettes and lung cancer.” So why use such flawed reasoning when it comes to media violence?