Garcia and Koelling's Taste Aversion Experiments

In 1966 the psychologists John Garcia and Robert Koelling published a series of experiments with rats that identified a phenomenon now known as the Garcia effect or conditioned taste aversion, in which the researchers paired a distinctively flavored, bright and noisy drinking water with either mild electric shock or radiation exposure that induced delayed nausea, and then measured which pairing produced longer-lasting avoidance of which specific stimulus. Rats that experienced nausea after drinking the flavored water developed a strong and durable aversion to the taste itself but showed little avoidance of the accompanying light and noise, while rats that experienced shock instead developed avoidance of the light and noise but showed little aversion to the taste, even though in both groups the taste, light and noise had all been presented together as a single compound stimulus. This selective pattern, in which animals readily associate taste specifically with subsequent illness but not with pain, and readily associate external stimuli like light and sound with pain but not as easily with illness, ran counter to the prevailing behaviorist assumption that any stimulus could be conditioned to any consequence with equal ease. Garcia and Koelling's findings demonstrated that learning is constrained by an organism's evolutionary history, since an animal that can rapidly learn to avoid a food that made it sick gains an obvious survival advantage, and the study became a foundational challenge to general-process learning theory and a founding contribution to the biological study of learning.