IN THE LATE morning of April 20th 1999 a pair of teenagers, Dylan Klebold and Eric Harris, walked into the cafeteria at Columbine High School in Colorado and began gunning down their classmates. The two senior-year students killed 13 people in a 45-minute rampage before turning their weapons on themselves. The massacre remains the deadliest high-school shooting in American history.

In the days after the killings it emerged that, besides enjoying violent movies, the two liked playing “Doom”, a gory video game from the mid-1990s in which the heavily armed players use shotguns and rocket launchers to dispose of legions of zombies and demons. Parents, politicians and psychiatrists fretted that exposure to virtual violence had prepared the ground for the real-world killings. Two years later the parents of some of the victims sued dozens of gaming companies, including id Software, the developers of “Doom”, alleging that their products had contributed to the murders.

The massacre fed long-standing worries about video games, particularly in America, the industry's biggest national market. Governments from California to Switzerland have tried to ban the sale of violent games to children, and most countries have an age-rating system similar to that for films.

However, since gaming has become more mainstream, the proportion of violent games has fallen. According to vgchartz, a website that tracks games sales, the ten bestselling console games of 2010 included just three violent shooters. The rest were inoffensive sports and fitness titles, a Super Mario platform-jumping game and a Pokémon product, a cartoony franchise of games based on a Japanese TV series for children. Many games that do feature violence serve up a slapstick version. The sort of gruesomely realistic killings found in serious war films are rare.

Still, many games require the player to dispose of great numbers of Nazis, gangsters, aliens and other bad guys. A few games serve up stylised violence for its own sake. And the critics say there is a crucial difference between films, plays or books, where the players are just passive onlookers, and video games, where they are active participants in the simulated slayings. That, the argument goes, makes it more likely that they will resort to violence in the real world, too.

It's all in the mind