Pretty thought-provoking article. TL;DR: A rat in a cage alone prefers the water bottle laced with heroin or cocaine over the one with plain water. A group of rats in a nice cage with toys, tunnels, good food, etc. prefer the plain water to the drug-laced version. Vietnam War: According to a study published in the Archives of General Psychiatry, 20% of soldiers in Vietnam became addicted to heroin. When they returned home, 95% of those soldiers simply stopped, many without rehab.