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evlPanda said..We've known this since the 70s:
en.wikipedia.org/wiki/Rat_ParkThat somewhat infamous "rat given an unlimited supply of drugs" experiment that saw the rat eventually dose itself to death was done with the lone rat in a cage with nothing else to do.
The same experiment done in a cage with other rats, toys and the like saw completely different results.
It's the environment.
There have been lots of interesting experiments with mice and rats in the past, social type experiments. I watched a utube video on one once where mice were provided with what was effectively a mouse paradise. There was ample food, water, activities for them, space and what was considered to be an interesting environment for them.
Within a year the mice were all dead. They died out due to completely crazy behaviour and social systems that lead to their demise.