With substance abuse considered by many to have reached epidemic proportions, researchers are attempting to identify the factors that make people susceptible to drug addiction. One theory that has emerged is the “gateway” hypothesis: the belief that the use of alcohol, tobacco products, or marijuana often leads to experimentation with harder drugs. Some analysts believe that marijuana users are extremely likely to use—and become hooked on—highly addictive and often debilitating drugs such as heroin and cocaine. The reasons offered for this phenomenon range from claims that marijuana use alters the chemistry of the brain, thereby “priming” it for other drugs, to the assertion that marijuana use introduces people to a subculture in which hard drugs are readily accessible.

The gateway hypothesis has been the basis for a government ad campaign that portrays marijuana as...