You get a rat and you put it in a cage, and you give it two water bottles: One is just water, and the other is water laced with either heroin or cocaine. The rat will almost always prefer the drug water and almost always kill itself quite quickly.

Professor Alexander built a cage that he called "Rat Park," which is basically heaven for rats. Crucially, they've got loads of friends in there.  

 In Rat Park, they don't like the drug water. None of them ever use it compulsively oroverdose. You go from almost 100% overdose when they're isolated to 0% overdose when they have connected lives.