The reality is that we have not yet successfully defined violence and aggression, whether when analyzing the content we consume, or investigating the potentially resultant aggressive behaviour. Because individual studies define these notions differently, the goal posts are constantly moving for anyone who is trying to get a big picture look at the situation. The difficulty of quantifying aggression and violence in a strict way makes it nearly impossible to accurately answer the question “Does media violence cause people to commit violence?”
Back in 1994 Andrea Martinez at the University of Ottawa conducted a comprehensive review of the scientific literature on media violence for the Canadian Radio-television and Telecommunications Commission (CRTC). She concluded that the lack of consensus about media effects reflects three "grey areas" or constraints contained in the research itself. These grey areas still apply today.
First, media violence is notoriously hard to define and measure. Some experts who track violence in television programming, such as the late George Gerbner, defined violence as the act (or threat) of injuring or killing someone, independent of the method used or the surrounding context. As such, Gerber included cartoon violence in his data-set. But others, such as University of Laval professors Guy Paquette and Jacques de Guise, specifically excluded cartoon violence from their research because of its comical and unrealistic presentation. (How they would view some of the increasingly realistic violence in many of today's cartoons aimed at teens – such as the gruesome injuries suffered by many of the characters on South Park and Family Guy – is an open question.)
Second, researchers disagree over the type of relationship the data supports. Some argue that exposure to media violence causes aggression. Others say that the two are associated, but that there is no causal connection (that both, for instance, may be caused by some third factor) while others say the data supports the conclusion that there is no relationship between the two at all.
Third, even those who agree that there is a connection between media violence and aggression disagree about how the one affects the other. Some say that the mechanism is a psychological one, rooted in the ways we learn. For example, L. Rowell Huesmann argues that children develop "cognitive scripts" that guide their own behaviour by imitating the actions of media heroes. As they watch violent shows, children learn to internalize scripts that use violence as an appropriate method of problem-solving.
Other researchers argue that it is the physiological effects of media violence that cause aggressive behaviour. Exposure to violent imagery is linked to increased heart rate, faster respiration and higher blood pressure. Some think that this simulated "fight-or-flight" response predisposes people to act aggressively in the real world.
Still others focus on the ways in which media violence primes or cues pre-existing aggressive thoughts and feelings. They argue that an individual’s desire to strike out is justified by media images in which both the hero and the villain use violence to seek revenge, often without consequences.
In her final report to the CRTC, Martinez concluded that most studies support "a positive, though weak, relation between exposure to television violence and aggressive behaviour." Although that relationship cannot be "confirmed systematically," she agrees with Dutch researcher Tom Van der Voot who argues that it would be illogical to conclude that "a phenomenon does not exist simply because it is found at times not to occur, or only to occur under certain circumstances."
With that in mind, based on a number of recent studies published in peer-reviewed academic journals, there are some things we can say:
What’s the good news?
What’s the bad news?
What else needs to be considered?
A number of older studies and the criticisms about them remain relevant today as well. Ever since the 1950s, laboratory experiments have consistently shown that exposure to violence is associated with increased heartbeat, blood pressure and respiration rate, and a greater willingness to inflict pain or punishment on others. However, this line of enquiry has been criticized because of its focus on short term results and the artificial nature of the viewing environment.
A number of surveys indicate that children and young people who report a preference for violent entertainment also score higher on aggression indexes than those who watch less violent shows. L. Rowell Huesmann reviewed studies conducted in Australia, Finland, Poland, Israel, Netherlands and the United States and reported that "the child most likely to be aggressive would be the one who (a) watches violent television programs most of the time, (b) believes that these shows portray life just as it is, [and] (c) identifies strongly with the aggressive characters in the shows."  However, it may equally be that youth with tendencies towards violence are more likely to enjoy violent media.
In a study conducted by the Kaiser Family Foundation in 2003  nearly half (47 per cent) of parents with children between the ages of four and six reported that their children had imitated aggressive behaviours from TV. However, it is interesting to note that children are more likely to mimic positive behaviours — 87 per cent of kids do so.
Kansas State University professor John Murray  concluded in his research that "the most plausible interpretation of this pattern of correlations is that early preference for violent television programming and other media is one factor in the production of aggressive and antisocial behavior when the young boy becomes a young man."
A number of studies have reported that watching media violence frightens young children,  and that the effects of this may be long lasting.
In 1998, Professors Singer, Slovak, Frierson and York  surveyed 2,000 Ohio students in Grades three through eight. They reported that the incidences of psychological trauma (including anxiety, depression and post-traumatic stress) increased in proportion to the number of hours of television watched each day.
A 1999 survey of 500 Rhode Island parents led by Brown University professor Judith Owens  revealed that the presence of a television in a child’s bedroom made it more likely that the child would suffer from sleep disturbances. Nine per cent of all the parents surveyed reported that their children had nightmares because of a television show at least once a week.
Tom Van der Voort  studied 314 children ages nine through 12 in 1986. He found that although children can easily distinguish cartoons, westerns and spy thrillers from reality, they often confuse realistic programs with the real world. When they are unable to integrate the violence in these shows because they can’t follow the plot, they are much more likely to become anxious. This is particularly problematic because the children reported that they prefer realistic programs, which they equate with fun and excitement. Similar studies,  have since been conducted in the 90s with results corroborating Van der Voort’s findings. As Jacques de Guise  reported in 2002, the younger the child, the less likely he or she will be able to identify violent content as violence.
In 1994, researchers Fred Molitor and Ken Hirsch  found that children are more likely to tolerate aggressive behaviour in the real world if they first watch TV shows or films that contain violent content.
George Gerbner conducted the longest running study of television violence. His seminal research suggests that heavy TV viewers tend to perceive the world in ways that are consistent with the images on TV. As viewers’ perceptions of the world come to conform with the depictions they see on TV, they become more passive, more anxious, and more fearful. Gerbner called this the "Mean World Syndrome." 
Gerbner’s research found that those who watch greater amounts of television are more likely to:
André Gosselin, Jacques de Guise and Guy Paquette decided to test Gerbner’s theory in the Canadian context in 1997 . They surveyed 360 university students, and found that heavy television viewers are more likely to believe the world is a more dangerous place. However, they also found heavy viewers are not actually more likely to be more afraid.
A number of studies since then suggest that media is only one of a number of variables that put children at risk of aggressive behaviour. For example, a Norwegian study  that included 20 at-risk teenaged boys found that the lack of parental rules regulating what the boys watched was a more significant predictor of aggressive behaviour than the amount of media violence they watched. It also indicated that exposure to real world violence, together with exposure to media violence, created an "overload" of violent events. Boys who experienced this overload were more likely to use violent media images to create and consolidate their identities as members of an anti-social and marginalized group.
On the other hand, researchers report that parental attitudes towards media violence can mitigate the impact it has on children. Huesmann and Bacharach conclude, "Family attitudes and social class are stronger determinants of attitudes toward aggression than is the amount of exposure to TV, which is nevertheless a significant but weaker predictor."
What should be apparent to us when we look at these kinds of claims and studies is that media violence is a highly complex and nuanced issue. There are clearly concerns with regards to violent media content such as age-appropriateness, saturation, desensitization, and instilling fear or unease in viewers. At the same time, many of the media products through which we are exposed to violent imagery provide benefits as well. Games and movies may expose young people to some violent content, but studies increasingly show that they also offer positive benefits. There is no way to completely shut out violent content, or to guarantee that children will never play video games that are rated as too old for them, or to make certain that everyone’s feelings on what is inappropriate content will coincide with industry self-regulation practices. What concerned adults and parents can do, however, is promote critical engagement with the media that young people and children consume, monitor their children’s media use, and discuss and establish rules at home to let young people understand what is or is not appropriate. More on how to talk about media violence with children can be found in the subsection Critically Engaging with Media Violence. If you are interested in legislation and industry tools that can help you to understand laws or give you a better idea of what to look out for, see our Government and Industry Responses to Media Violence.
 “The School Shooting/Violent Videogame Link: Causal Relationship or Moral Panic?” Journal of Investigative Psychology and Offender Profiling Volume 5, Issue 1-2, Article first published online: 9 DEC 2008. (http://onlinelibrary.wiley.com/doi/10.1002/jip.76/pdf)
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 “The Roots of Youth Violence”: Government of Ontario. 2008.
 Dr. Lawrence Steinberg, Developmental Psychologist. Temple University. Qtd. In: Grand Theft Childhood. Kutner, Lawrence and Cheryl K. Olson. Simon & Shuster, NY, 2008.
 Grand Theft Childhood:Kutner, Lawrence and Cheryl K. Olson. Simon & Shuster, NY, 2008.
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 Rideout, V et al. Zero to Six. Kaiser Family Foundation. 2003
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 Karlsen, Faltin and Trine Syvertsen. Media Regulation and Parents. Statens Filmtilsyn. 2004
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