The definition of addiction is murky. In fact, many psychologists prefer the term "pathological use" for excessive consumption of drugs, alcohol and other stuff.

By that fancier definition, about 8.5 percent of youth age 8 to 18 who play video games look to be what most of us would call addicted, a new study finds.

The study is based on a Harris Poll survey of 1,178 U.S. youths.

"This is the first study to tell us the national prevalence of pathological play among youth gamers, and it is almost 1 in 10," said Douglas Gentile, an assistant professor of psychology at Iowa State University

Gentile went into the study not believing that video games could be addictive. He's changed his mind.

"What we mean by pathological use is that something someone is doing — in this case, playing video games — is damaging to their functioning," Gentile said. "It's not simply doing it a lot. It has to harm functioning in multiple ways."

Gentile compared the 2007 Harris Poll survey to standards and symptoms established for pathological gambling — causing family, social, school or psychological damage because of their video game playing habits. Gamers were classified as "pathological" if they exhibited at least six of 11 symptoms.

The pathological gamers in the study played video games 24 hours per week, about twice as much as non-pathological gamers. They also were more likely to have video game systems in their bedrooms, reported having more trouble paying attention in school, received poorer grades in school, had more health problems, were more likely to feel "addicted," and even stole to support their habit.

The study also found that pathological gamers were twice as likely to have been diagnosed with attention problems such as Attention Deficit Disorder or Attention Deficit Hyperactivity Disorder.

Gentile will detail his findings in the May edition of the journal Psychological Science, and the findings were posted online today.

"I started studying video game addiction in 1999 largely because I didn't believe in it," said Gentile, who is co-author of "Violent Video Game Effects on Children and Adolescents: Theory, Research, and Public Policy" (2007, Oxford University Press). "I assumed that parents called it 'addiction' because they didn't understand why their children spent so much time playing. So I measured the way you measure pathological gambling and the way it harms functioning, and was surprised to find that a substantial number of gamers do rise to that level (of pathological addiction)."

Now he's calling for more research to figure out how to treat the condition.

"There is still much we do not know," Gentile said. "We don't know who's most at risk, or whether this is part of a pattern of disorders. That's important because many disorders are co-morbid with others. It may be a symptom of depression, for example. And so we would want to understand that pattern of co-morbidity because that would help us know how to treat it."