Violent imagery in computer games and on TV increases the risk of young children becoming aggressive and emotionally disturbed, a report says.
Parents should monitor what children watch, experts say
The study found the effect was "small but significant" in the short-term and especially relevant for boys.
The effect was less clear for older children and in the long-term, the Birmingham University researchers said.
The review, published in The Lancet, said family and social factors were likely to affect the response too.
The team of researchers reviewed six major studies on the effects of media violence on children.
The report said violent imagery should be looked at in the context of public health and the wider effects on child welfare, families and the community.
Report author Professor Kevin Browne said parents should exercise the same caution with violent imagery as they did with medication or chemicals around the home.
"Carelessness with material that contains extreme violent and sexual imagery might even be regarded as a form of emotional maltreatment of the child.
"Producers also need to recognise the potential effects of their violent images on vulnerable audiences who might not have the capacity or the will to see violence in the context of the story."
The findings have reignited the debate about the effect of computer games and TV violence.
The parents of murdered Leicester teenager Stefan Pakeerah last year blamed his killer's obsession with a violent video game called Manhunt for their son's death.
At the time, industry experts and several academics denied there was a link between violent imagery and aggressive behaviour.
Previous research into the issue has produced mixed results.
But John Bayer, of campaign group Mediawatch UK, said: "It is common sense really.
"If children are watching violence on TV and playing violent computer games, it is not surprising they become aggressive.
"I think the link is especially strong with video games. Children become isolated from the world when they play these games, some of which give you extra points for inflicting more pain, and don't know right from wrong.
"The games literally rot their brains."
However, Professor David Buckingham, director of the Centre for the Study of Children at the Institute of Education, said there was really no consensus on the issue.
"People will tell you different things. It has become a political issue. The media are an easy target and become a scapegoat for politicians when they say they are going to get tough.
"In my view, it is not as simple as monkey see, monkey do.
"Children are not going to commit a violent act just because they see it on TV.
"But what it does perhaps do is alter the form of violence.
"There is much less discussion on the emotional side of it, which this report touches on. That is of more concern."