https://en.wikipedia.org/wiki/Rat_Park https://en.wikipedia.org/wiki/Rat_Park likely is the study you were thinking about, studying how the willingness/desire o

https://en.wikipedia.org/wiki/Rat_Park https://en.wikipedia.org/wiki/Rat_Park likely is the study you were thinking about, studying how the willingness/desire of rats to self-administer morphine changes depending on their living conditions.