Players of the video game MadWorld can use their Nintendo Wiis to impale enemies on spikes, gouge out their eyes with street signs, and chop them in half with chain saws. The Mortal Kombat series offers its users similar thrills: ripping foes’ heads from their bodies, tearing their hearts out of their chests, and burning the flesh off their skeletons.

Although their producers argue that these games have no ill effects, a new research article shows that violent media blunt people’s altruistic tendencies. In one experiment, for example, participants who played a violent video game took longer to respond to an emergency than did participants who played a nonviolent game. And in a second study, theater patrons exiting a violent film responded more slowly to a woman in distress than did patrons exiting a nonviolent film.

“Violent media make people numb to the pain and suffering of others,” concludes Brad J. Bushman, a professor at the University of Michigan’s Institute for Social Research and the article’s lead author. His coauthor is Craig A. Anderson, a professor at Iowa State University and an expert on media violence.

Previous research shows that viewing violent media makes both children and adults more physically aggressive. Other studies further indicate that playing violent video games desensitizes people to the violence of others. Yet Bushman and Anderson’s studies are the first to connect the dots between viewing violent media and failing to help people in need, including victims of brutality.

For their first study, Bushman and Anderson randomly assigned 320 men and women to play either a violent video game (e.g., Mortal Kombat, Carmageddon, or Future Cop) or a nonviolent game (e.g., Austin Powers, Tetra Madness, or 3D Pinball). After 20 minutes of play, the researchers gave participants a bogus survey to complete. A staged fracas then erupted outside the lab, replete with thrown chairs, banged doors, and one party loudly complaining about an injured ankle. The researchers found that the players of violent games tarried longer before responding to the emergency, were less likely to report that they heard a fight, and judged the fight to be less serious than did the players of nonviolent games.

Taking their findings into the real world, Bushman and Anderson next planted a female confederate with crutches and a bandaged ankle outside a movie theater. After the confederate dropped her crutches, a hidden assistant measured how long it took bystanders to help her. Once again, the researchers found that people spilling out of a violent movie took longer to help than did people exiting a nonviolent movie.

Many people seem to think that “if violent media don’t make you kill someone, then they have no effect,” says Bushman. As his research findings underscore, however, gratuitous gore and casual cruelty can quietly chip away at civility.

Read more stories by Alana Conner.