The most interesting rat study I’ve encountered was the Rat Park experiment by Bruce Alexander of Simon Fraser University.
Distilling it down:
The study focused on how rats chose to consume when presented two options:
Water.
Water infused with morphine.
He did the experiment with two study groups.
The first group of rats lived in small cages with no social interaction. When presented the two options, they very quickly began choosing the morphine infused water and became addicted to it.
The second group lived in Rat Park. It was a large enclosure that included toys and other rats to socialize with. There were more food options. They could mate. Everything about it was more livable.
When presented the two options, this group almost universally rejected the morphine infused water, choosing to stay with the clean water.
Alexander had an ambitious thesis: drugs do not cause addiction; rather, drug addiction is driven by the environment.
Whether he proved it or not, his experiment is a very interesting corollary to human predisposition to addiction and other undesirable behaviors when living in poor conditions.