Very few psychological traits seem to remain stable from early childhood into adulthood, and fewer yet have been shown to predict success or failure in life. One of the best predictors of success in life is self-control. During the 1960s, Walter Mischel and his colleagues developed a clever laboratory method for testing children’s capacity to delay gratification. ,  Each child would be shown some treat, such as a marshmallow. The experimenter would explain to each child that the experimenter was going to leave the room but the child could summon him or her back by ringing a bell on the table. As soon as the child did this, the child would receive the treat. However, if the child could refrain from ringing the bell and just wait until the experimenter returned, the child would get a bigger reward (e.g., two marshmallows instead of one). Some children were able to wait and get the larger reward; others succumbed to temptation and rang the bell. Mischel followed up with many of the children years after they had participated in his experiments. The children who were good at delaying gratification when they were just four years old grew into adults who were more popular with friends and family and more successful in universities and jobs than those who had not been able to resist taking the quick marshmallow in his lab. , 
One problem with violent video games is that they discourage players from exercising self-control. For example, in the Grand Theft Auto video games players can steal cars, have sex with a prostitute (then kill her afterwards to get their money back), and kill other characters in the game (including police officers). Rather than being punished for such behaviors, players are often rewarded (e.g., through points, extra health to their character, etc).
We recently conducted a study in which 172 high school students were assigned by the flip of a coin to play either a violent video game (Grand Theft Auto III or Grand Theft Auto San Andreas) or a nonviolent game (Pinball 3D or MiniGolf 3D) for 35 minutes after practicing for 10 minutes.
During the experiment, a bowl containing 100 grams (3.5 ounces) of chocolate M&M candy was placed next to the computer. The teens were told they could freely eat them, but were warned that high consumption of candy in a short time was unhealthy. Those who played the violent games ate more than three times as much candy as did the other teens, the result showed.
After playing the game, the teens were asked to solve a 10-item logic test in which they could win one raffle ticket for each question they got right. The raffle tickets could be used to win cool prizes (e.g., iPads). The teens were told how many answers they got correct and had the opportunity to take the appropriate number of raffle tickets out of an envelope containing many tickets while not being watched. However, we knew how many tickets were in the envelope so we could determine if a player took more than he or she had earned. Results showed that teens who played a violent game cheated more than did those who played a nonviolent game—over 8 times more.
We also measured aggressive behavior. The players competed with an unseen “partner” in a game in which the winner got to blast the other person with a loud noise through headphones. (There was actually no partner, although participants thought there was.) The noise was very unpleasant—a combination of noises that most people hate (e.g., fingernails scratching a chalkboard, a dentist drilling teeth, ambulance sirens). As has been found in many other studies, those who played the violent games chose to blast their ostensible partners with louder noises that lasted longer than did teens who played the nonviolent games.
Teens in the study also completed the Moral Disengagement Scale, which measures how much people hold themselves to high moral standards in all situations. Some sample questions are “Compared to the illegal things people do, taking some things from a store without paying for them is not very serious” and “It is okay to insult a classmate because beating him/her is worse.”
Results showed that for teens who played the violent video games, those who scored higher in moral disengagement were especially likely to cheat, eat more chocolate, and act more aggressively. There were no such differences among those who played the nonviolent games.
Parents may think violent video games rated M for mature players 17 and older are just “harmless fun” for kids, but they are not. Parents should not allow their children to play age inappropriate video games.
 Mischel, W. (1996). From good intentions to willpower. In P. M. Gollwitzer & J. Bargh (Eds.), The psychology of action: Linking cognition and motivation to behavior (pp. 197–218). New York: Guilford Press.
 Mischel, W., & Mendoza-Denton, R. (2002). Harnessing willpower and socio-emotional intelligence to enhance human agency and potential. In L. G. Aspinwall & U. M. Staudinger (Eds.), A psychology of human strengths: Fundamental questions and future directions for a positive psychology (pp. 245–256). Washington, DC: American Psychological Association.
 Shoda, Y., Mischel, W., & Peake, P. K. (1990). Predicting adolescent cognitive and self-regulatory competencies from preschool delay of gratification: Identifying diagnostic conditions. Developmental Psychology, 26, 978–986.
 Gabbiadini, A., Riva, P., Andrighetto, L., Volpato, C., & Bushman, B. J.(in press). Moral disengagement moderates the effect of violent video games on self-control, cheating and aggression. Social Psychological and Personality Science.
 Anderson, C. A., Shibuya, A., Ihori, N., Swing, E. L., Bushman, B. J., Sakamoto, A., Rothstein, H. R., Saleem, M., & Barlett, C. P. (2010). Violent video game effects on aggression, empathy, and prosocial behavior in Eastern and Western countries: A meta-analytic review. Psychological Bulletin, 136(2), 151-173.