Marc Potenza, a psychiatrist at Yale and the director of the school's Program for Research on Impulsivity and Impulse Control Disorders, has been treating addiction for more than two decades. Early in his career, he, like most others studying addiction at the time, focused on substance-abuse problems —  cocaine and heroin addicts, alcoholics, and the like. Soon, however, he noticed patients with other problems that were more difficult to classify. There were, for example, the sufferers of trichotillomania, the inescapable urge to pull your hair until it falls out. Others had been committed for problem gambling : they couldn't stop no matter how much debt they had accumulated. It was to this second class of behaviors — at the time, they were not called addictions—that he turned his attention. Were they, he wondered, fundamentally the same?

　　In recent years, however, Potenza has been increasingly treating a new kind of problem： people who come to him because they can't get off the Internet. In some ways, it seems exactly like the behavioral addictions that he has been treating for years, with much of the same consequences. There's something different, and more complicated, about Internet addiction, though. Unlike gambling or even trichotillomania, it's more difficult to pin down a quantifiable, negative effect of Internet use.