Many feel that violent video games contributed to the deadly shootings at Columbine High School and the movie massacre in Aurora, Colorado. Some research suggests that most American boys exposed to gory video games tend to be more violent. Studies show that gamers who play violent games are likely to be slightly more violent. It is also important to note that it has not been proven yet that playing violent video games causes people to commit violent crimes. A recent study had 47 undergraduate students play "Mortal Kombat: Deadly Alliance" for 15 minutes. Participants were tested for physical and mental arousal, and whether the students would be more violent. In the end, compared to the non-violent gamers, the 47 students tested were slightly more violent. While there isn't enough good evidence, it is clear to researchers that playing violent video games does affect players. Parent groups, educators, law enforcement, and even game distributors acknowledge that limiting use of violent games is a good idea, and video games can be one common source of those images.