The researchers used existing data from a national US survey to study the amount of television watched by 1266 four-year-olds. Then they compared that amount with follow-up reports - by the children's mothers - on whether the children bullied or were "cruel or mean to others" when they were between six and 11 years old.

The study showed that four-year-olds who watched the average amount of television - 3.5 hours per day - were 25% more likely to become bullies than those who watched none. And children who watched eight hours of television a day were 200% more likely to become bullies.

Desensitised to violence
The study did not probe what types of programmes the children were watching, but Zimmerman suggests they were mainly animated videos and cartoons. He says such shows may follow a trend seen in movies and cites a recent study showing the average G-rated kids' movie contains (U-rated in the UK) about 9.5 minutes of violence - up from 6 minutes in 1940.

"What I suspect is these violent animated shows are causing kids to become desensitised to violence," he told New Scientist. "Parents should understand that, just because a TV show or movie is made for kids, it doesn't mean it's good for kids - especially four-year-olds."

He suggests parents follow guidelines set by the American Academy of Pediatrics, which recommends no television for children under two and no more than two hours a day for older kids. "We have added bullying to the list of potential negative consequences of excessive television viewing, along with obesity, inattention, and other types of aggression," write the authors in the Archives of Pediatrics & Adolescent Medicine.

“Double whammy”
The study also looked at two other factors thought to decrease the likelihood of bullying - cognitive stimulation and parental emotional support. It found that children whose parents regularly exposed them to ideas - by reading aloud or taking them to museums, for example - were a third less likely to become bullies, as were those whose parents provided them with emotional support - by eating meals together and talking.

"Each of these things has an independent effect," says Zimmerman. "So parents who are not going to read to their children and who put their kids in front of the TV instead [represent] a double whammy" for their children's chances of becoming bullies, he says.

Some would argue that parents of children genetically predisposed to bad behaviours and bullying may simply be putting them in front of the TV to reduce the stress of dealing with this negative behaviour, rather than the TV itself being a causal factor.

But because the effects of cognitive stimulation, emotional support and television viewing can be teased apart and examined separately, Zimmerman says the chances of the bad behaviours coming before the excess TV viewing are generally reduced.

Journal reference: Archives of Pediatrics and Adolescent Medicine (vol 159, p 384)