In the 1950s and '60s, scientists were alarmed by a series of experiments in which laboratory rats were given unlimited doses of opiates. The rats wasted away, not even ingesting easily accessible food or water. The implications were ominous: What would happen if people became hooked like the rats? The takeaway was that drugs are so addictive that people need to be kept away from them at all costs. But the story wasn't so clear-cut after all. Take heroin, one of the most addictive drugs, for example: Only 23% of users become dependent. By some measures nicotine is more addictive, possibly because tobacco companies made cigarettes more addictive by design. The actual rates for dependency are 15% for alcohol, 17% for cocaine, 23% for heroin, and 32% for nicotine. Marijuana clocks in at just 9%.