• Done in 1978 Canada
• Completely changes the way how drug addiction was viewed. It showed that it was the psychology that was the main feed for one's addiction, not the drug itself.
• Alexander constructed Rat Park with wheels and balls for play, plenty of food and mating space, and 16-20 rats of both sexes mingling with one another. He tested a variety of theories using different experiments with Rat Park to show that the rat’s environment played the largest part in whether a rat became addicted to opiates or not.
• In the experiment, the social rats had the choice to drink fluids from one of two dispensers. One had plain tap water, and the other had a morphine solution.
• The caged rats ingested much larger doses of the morphine solution – about 19 times more than Rat Park rats in one of the experiments.
• The Rat Park rats consistently resisted the morphine water, preferring plain water.
• Even rats in cages that were fed nothing but morphine water for 57 days chose plain water when moved to Rat Park, voluntarily going through withdrawal.
• No matter what they tried, Alexander and his team produced nothing that resembled addiction in rats that were housed in Rat Park.
• Prior to Alexander’s experiment, addiction studies using lab rats did not alter the rat’s environment. Scientists placed rats in tiny, isolated cages and starved them for hours on end. The “Skinner Boxes” the rats lived in 24/7 allowed no room for movement and no interaction with other rats.
• Using the Skinner Boxes, scientists hooked rats up to various drugs using intravenous needles implanted in their jugular veins. The rats could choose to inject themselves with the drug by pushing a lever in the cage. Scientists studied drug addiction this way, using heroin, amphetamine, morphine, and cocaine. Typically, the rats would press the lever often enough to consume large doses of the drugs. The studies thus concluded that the drugs were irresistibly addicting by their specific properties.
• Look at Switzerland and Portugal

Source: https://www.summitbehavioralhealth.com/blog/overview-rat-park-addiction-study/