Conditioning accounts for a lot of learning, both in humans and nonhuman species. However, biological factors can limit the capacity for conditioning. Two good examples of biological influences on conditioning are taste aversion and instinctive drift.
Psychologist John Garcia and his colleagues found that aversion to a particular taste is conditioned only by pairing the taste (a conditioned stimulus) with nausea (an unconditioned stimulus). If taste is paired with other unconditioned stimuli, conditioning doesn’t occur.
Similarly, nausea paired with most other conditioned stimuli doesn’t produce aversion to those stimuli. Pairing taste and nausea, on the other hand, produces conditioning very quickly, even with a delay of several hours between the conditioned stimulus of the taste and the unconditioned stimulus of nausea. This phenomenon is unusual, since normally classical conditioning occurs only when the unconditioned stimulus immediately follows the conditioned stimulus.
Example: Joe eats pepperoni pizza while watching a movie with his roommate, and three hours later, he becomes nauseated. He may develop an aversion to pepperoni pizza, but he won’t develop an aversion to the movie he was watching or to his roommate, even though they were also present at the same time as the pizza. Joe’s roommate and the movie won’t become conditioned stimuli, but the pizza will. If, right after eating the pizza, Joe gets a sharp pain in his elbow instead of nausea, it’s unlikely that he will develop an aversion to pizza as a result. Unlike nausea, the pain won’t act as an unconditioned stimulus.
Instinctive drift is the tendency for conditioning to be hindered by natural instincts. Two psychologists, Keller and Marian Breland, were the first to describe instinctive drift. The Brelands found that through operant conditioning, they could teach raccoons to put a coin in a box by using food as a reinforcer. However, they couldn’t teach raccoons to put two coins in a box. If given two coins, raccoons just held on to the coins and rubbed them together. Giving the raccoons two coins brought out their instinctive food-washing behavior: raccoons instinctively rub edible things together to clean them before eating them. Once the coins became associated with food, it became impossible to train them to drop the coins into the box.