[T]he 'addictiveness' of drugs such as opiates and cocaine was established by observing how frequently caged animals would push levers to obtain doses.
[In the followup experiment, 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. [...] It seemed that the standard experiments were measuring not the addictiveness of opiates but the cruelty of the stresses inflicted