Rat Park

Wikipedia:

"The colony was built to examine Alexander's hypothesis that drug addiction is a myth and that continued drug use, particularly the use of heroin, is largely the product of unhappiness, not neurophysiological compulsion. Alexander hypothesized that the addiction to morphine commonly observed in laboratory rats exposed to it is attributable to the conditions in which they are normally kept, and not to any addictive property of the drug itself."

Fascinating article. Apparently, rats that are kept in tiny little cages with nothing to do all day are 20 times more likely to become "addicts" than rats that have a little freedom, some toys, and some rat-chicks to have sex with.