To test his hunch he designed Rat Park... A colony of rats, who are naturally gregarious, were allowed to roam together in a large vivarium enriched with wheels, balls and other playthings, on a deep bed of aromatic cedar shavings and with plenty of space for breeding and private interactions. Pleasant woodland vistas were even painted on the surrounding walls. In this situation, the rats' responses to drugs such as opiates were transformed. They no longer showed interest in pressing levers for rewards of morphine: even if forcibly addicted, they would suffer withdrawals rather than maintaining their dependence.

Nth Position article via 3quarksdaily