Rat Park. In these experiments, both sets of rats had access to a pair of drinking bottles. The first bottle contained only water. The other bottle contained morphine—an opiate that rats process in a similar way to humans and that behaves just like heroin when it enters their brains. At the end of each day, Bruce or a member of his team would weigh the bottles to see how much the rats had chosen to take opiates, and how much they had chosen to stay sober.
