In laboratory experiments, fruit flies tend to avoid solutions laced with morphine as much as they do caffeine. If these insects take another sip despite the bitter taste, an even greater aversion to morphine develops over the course of an experiment. If mammals return to take another taste, by contrast, a preference for diet laced with morphine can develop. Rats preferred food laced with bitter morphine over food laced with bitter quinine. And just like humans, other mammals, like rats, that are socially isolated or are under other stresses are prone to increase their preferences for, and
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