According to classic definitions, addiction refers to the compulsive use of a habit-forming substance. The two characterizing features of addiction are tolerance—the need for higher doses of a substance in order to produce the same effects—and the occurrence of withdrawal symptoms upon quitting. Long-term users of heroin, for example, no longer experience a high from heroin but keep taking it in order to ward off painful withdrawal symptoms.

While most people agree that heroin and other “hard” drugs are addictive, any consensus about the definition of addiction ends here. The principal controversy over defining addiction is whether a compulsive interest in activities other than drug use— such as gambling, sex, surfing the Internet, or eating—constitutes addiction.

Some experts maintain that any compulsive behavior— whether it involves the use or drugs or not...