There might be trouble brewing behind the glassy eyes of kids who spend too much time and energy on video games, according to a controversial new study.

In the 2-year study of more than 3,000 school children in Singapore, researchers found nearly one in ten were video game “addicts,” and most were stuck with the problem.

While these kids were more likely to have behavioral problems to begin with, excessive gaming appeared to cause additional mental woes.

“When children became addicted, their depression, anxiety, and social phobias got worse, and their grades dropped,” said Douglas A. Gentile, who runs the Media Research Lab at Iowa State University in Ames and worked on the study.

“When they stopped being addicted, their depression, anxiety, and social phobias got better.”

He said neither parents nor healthcare providers are paying enough attention to video games’ effect on mental health.

“We tend to approach it as ‘just’ entertainment, or just a game, and forget that entertainment still affects us,” he told Reuters Health in an e-mail. “In fact, if it doesn’t affect us, we call it ‘boring!’”

But an independent expert said the study had important flaws.

“My own research has shown that excessive video game play is not necessarily addictive play and that many video gamers can play for long periods without there being any negative detrimental effects,” said Mark Griffiths, director of the International Gaming Research Unit at Nottingham Trent University in the UK.

“If nine percent of children were genuinely addicted to video games there would be video game addiction clinics in every major city!” he said in an e-mail, adding that the concept is not currently an accepted diagnosis among psychiatrists and psychologists.