Interactive gaming, where youngsters played with their friends in the same room or with ''11 million friends online'' had increased the risks of problem gaming because it made it more attractive, Dr Gentile said.

It was not hard to understand how some children became addicted, because gaming gave them a sense of control, belonging and competency, answering three basic human needs, he said.

Although 90 per cent of youngsters developed no serious problems with gaming, many spent much more time staring at screens than was recommended by the American Academy of Paediatrics - six hours instead of the recommended two.

Dr Gentile, who met his wife playing an online game, said gaming could have positive effects: ''It's not like crack cocaine.''

However, further research he had done showed video games involving violence prompted children to think and behave more aggressively.

Parents were not powerless to set limits on the amount of screen time for their children, he said. Other strategies might be needed with teenagers, such as setting a weekly limit they could manage themselves.

''It's a matter of balance,'' he said. ''But one thing we know is that there's no such thing as too much reading.''