Long hours in front of the television, whether channel surfing or gaming, could make it difficult for kids to concentrate in school, psychologists said Monday.
While researchers are still divided on the issue, the findings jibe with most earlier work on the effects of television watching in kids, they said.
"What we don't know at this point is why TV and video games really would cause attention problems," said Douglas A. Gentile, who worked on the study.
Gentile, who runs the Media Research Lab at Iowa State University in Ames, added that too much screen time had also been linked to increased aggression and, perhaps less surprisingly, expanding waistlines.
He said the new study, published in the journal Pediatrics, was the first to follow over time how video games may impact kids' concentration skills.
While the research doesn't directly prove that long screen time causes the psychological issues, "we know that earlier television watching was not caused by later attention problems," Gentile told Reuters Health.
The researchers followed a group of more than 1,300 school-age children, who, assisted by their parents, logged their TV and gaming hours over a year. They then asked teachers to answer questions about how the children behaved in school — whether they had difficulty staying on task, for instance, or often interrupted others.
Even after accounting for attention problems when children entered the study, those who watched a lot of TV or played a lot of video games had slightly more problems concentrating on schoolwork.
Specifically, those children who spent more than two hours per day in front of the screen — the limit recommended by the American Academy of Pediatrics — increased their odds of exceeding the average level of attention problems by 67 percent.
Extreme cases of attention difficulty sometimes lead to a diagnosis of attention-deficit/hyperactivity disorder (ADHD), which between 3 and 7 percent of school-age children suffer from. The researchers did not diagnose any kids with that condition, however.
They also tested undergraduate students, this time using psychological questionnaires designed to reveal ADHD, such as the Adult ADHD Self-Report Scale.
In these students, exceeding two hours of daily screen time doubled their risk of landing above average in attention problems, although they weren't diagnosed with ADHD.
Gentile said the impact of TV and video games depended on lots of factors, and wasn't necessarily dramatic.
"Not every kid is going to be influenced to the same amount," he said. "No one thing causes our behavior. It's a combination of all the pushes and pulls that we get — the media is just one variable."
Miriam Mulsow, an expert in ADHD who was not involved in the study, said she did not think TV or video games could cause attention problems or ADHD.
"There are parents out there who are doing the best they can, but are working multiple jobs and can't afford child care," said Mulsow, of Texas Tech University in Lubbock. "What worries me is that those parents will think they cause their children to have ADHD. I don't think that's the case, and I don't think those parents should feel bad."
However, she added, "if a child has a tendency toward attention problems, then sitting in front of the TV not getting enough exercise would exacerbate it."
She said she agreed a child shouldn't be allowed to watch more than two hours of TV a day. "I didn't even allow my kids to watch that much," she told Reuters Health.
Gentile said the findings also send a positive message to parents whose kids are plagued by attention problems.
"This study perhaps gives parents a first line of defense because (screen time) is something they can control," he said. "The research suggests that parents actually are in a more powerful position to help their children than they realize."