Too much screen-based entertainment is bad for childrens' developing brains, according to a recent US study.

Children who spend hours in front of screens every day, whether they are watching TV programs or playing video games, end up having trouble concentrating in school.

Douglas Gentile, who worked on the study, said that although he and his colleagues had formed firm conclusions, he had not discovered a biological basis for the effect of screen time on people's education.

He said that, however, too much screen time had also been linked to increased aggression and childhood weight gain.

For the recent study, the researchers looked at how video games affected children's ability to concentrate.

For the study, the researchers followed about 1,300 children who were already of school age.

The children logged their TV and gaming hours, with the help of their parents, for just over a year.

The researchers then quizzed teachers about how various children behaved in school, and in particular if any of them were prone to interrupt others or get distracted.

The children who watched a lot of TV had slightly more problems concentrating on their school-related tasks, as did children who played a lot of video games.

Children who spent more than two hours a day in front of screens were 67% more likely to have more attention problems than their peers.

Children who have extreme levels of attention difficulty are sometimes diagnosed with attention-deficit hyperactivity disorder (ADHD), and 3-7% of school-age children suffer from the condition.

In another related study, the researchers also tested college undergraduates.

The researchers wanted to know how many college undergraduates were likely to have ADHD, according to the Adult ADHD Self-Report Scale.

In college undergraduates, exceeding a two-hour daily maximum of screen time also caused problems with attention.

Gentile said that, although it would be difficult to find causal links at this stage, it was common sense that attention problems did not cause television watching.

He also said that the impact of screen time on people's personalities was a complicated thing, with many factors to consider.

Miriam Mulsow of Texas Tech University in Lubbock, Texas, an ADHD expert who was not involved in the study, disagreed with the finding, saying that there were parents in the world who did the best they could, and who could not afford child care.

She said that she thought parents should not worry about causing their childrens' ADHD, and that if a child had attention problems, sitting in front of the TV and not getting enough exercise would exacerbate them.

Gentile said the findings also sent a positive message to parents, giving them a first line of defense on something they could control.

He also said that not every child would be similarly influenced by the same amount of screen time.