Addiction is a slippery, hard-to-define concept. The one element of addiction that is commonly agreed upon is that addiction involves “reinforcement”: consumption of the addictive good or activity today increases the desire to consume the good in the future. By this reckoning alone, caffeine, cocaine, alcohol, nicotine, opiates, gambling, sex, exercise, chess, listening to music, watching television, playing video games, eBay, eating Chinese food, reading, and religious observance are addictive for many people. Public policy responds mainly to those potentially addictive substances or behaviors that tend to bring on severe adverse effects: whereas some people read too much, the negative consequences of “readaholism” are insufficiently widespread or dangerous to make book addiction a pressing social concern. But the detrimental effects arising from excessive indulgence in alcohol, nicotine, cocaine, and gambling, among other vices, foster efforts at social (and sometimes governmental) influence or control.

Beyond reinforcement, other markers of addiction include tolerance, withdrawal, and craving. Tolerance is present if a given level of enjoyment of the good or activity requires higher and higher doses over time. Withdrawal implies that a failure to consume the good leads to adverse psychological or physical symptoms. Craving exists if an individual is beset with an almost overwhelming desire for the addictive good or activity; addicts who relapse following a period of abstinence often attribute their backsliding to intense craving. Not all addictions seem to involve tolerance, withdrawal, or craving, however.