“Absent a clear definition of addiction, researchers will continue finding it very difficult to determine addiction prevalence rates, etiology, or the necessary and sufficient causes that stimulate recovery.” —Howard J. Shaffer, director of the Harvard Medical School Division on Addictions

Stephen started using heroin when he was fourteen. His parents discovered his heroin use when his grades began to drop, and they sent Stephen to a rehabilitation center. After treatment, Stephen began to use again and became involved in repeated confrontations with the law. At sixteen, Stephen’s parents asked him to leave home. Unable to keep a job, Stephen turned to prostitution to obtain money to buy heroin. Despite his estrangement from family and friends, and in spite of the risk of contracting AIDS from needle sharing and prostitution, Stephen continues to use.

Maryann had been married for fifteen years when she began...