The more adolescent boys absorb violence in media such as movies, television shows and video games, the less sensitive certain areas of their brains become to these images, researchers report.
And those areas of the brain are the ones involved in controlling aggression, notes a study publishedi n the journal Social Cognitive and Affective Neuroscience.
It's possible that these boys might become more aggressive later in life (although the study didn't actually track this), but a larger societal desensitisation may be taking place that might be even more troublesome, the researchers said.
"There are always going to be people who are violent no matter what they're exposed to," said study senior author Jordan Grafman. "What's even more dangerous is when society accepts such behaviours. . . If something becomes acceptable, then those who are creating the violence and aggressive behaviour are allowed to get away with it more because society is not going to police it as much."
How the mechanisms play out in brain
Prior studies have indicated that violent media can make people more violent, but this is one of the first studies into how that mechanism plays out in the brain.
And studies in Vietnam veterans revealed that those who had brain injuries affecting the frontal lobes of the brain were more likely to become aggressive, said Grafman, who is chief of the cognitive neuroscience section at the US National Institute for Neurological Disorders and Stroke.
"The frontal lobe is important for controlling aggression," he explained, and in adolescent males, this part of the brain is still developing.
For this study, 22 boys aged 14 to 17 watched a series of four-second segments of violent videos, not "the most violent you can imagine but certainly moderate," Grafman said.
The boys rated the degree of violence they saw in each clip in relation to the clip before it.
The boys' responses were measured in two ways, the first being skin conductance responses, basically a measure of sweat levels. "We know that when people are shown an emotionally provocative picture or video they have exaggerated sweat responses," Grafman explained.
The participants' brains were also scanned to measure blood flow in different areas, the idea being the more blood flow, the more active that part of the brain.
"They became desensitised over time to these mild and moderate aggressive scenes so they, in essence, had much less sweat over time," Grafman noted.
And brain activity in the orbital frontal cortex, the part of the brain involved in modulating aggression, decreased.
Fragile kids at risk
"Based on prior studies, we know that the size of those brain regions vary depending on their exposure, which means you're shaping a very plastic brain in adolescents for their future," Grafman said. "There's a likelihood that you're shaping their behaviour in the real world as well."
"There's a debate going on as to whether or not violent video games are cathartic or inure teenagers to violence," said Alan Hilfer, director of psychology at Maimonides Medical Center in New York City.
"It's generally the understanding with most of us that if kids are playing them excessively, they do tend to make a certain kind of a kid - a more fragile, more vulnerable kid - less sensitive to violence. We all have a bit of a concern as to the extent that teenagers play video games and are engaged in watching violent acts . . . This study reinforces that concern."
Craig S. Fabrikant, of the department of psychiatry at Hackensack University Medical Center in New Jersey, recommends that parents "monitor what your kids are doing and watch. Be aware."