Learning allows behavior to change as a result of experience, and psychologists have described several distinct mechanisms through which this occurs. Classical conditioning, discovered by Ivan Pavlov, is a form of learning through association. A normally neutral stimulus is repeatedly paired with an unconditioned stimulus that naturally triggers a response, until the formerly neutral stimulus alone elicits the same reaction. The vocabulary of classical conditioning distinguishes the unconditioned stimulus, the natural trigger, and the unconditioned response, the natural reaction, from the conditioned stimulus, the originally neutral trigger that has been learned, and the conditioned response, the learned reaction. A famous demonstration of classical conditioning in humans was Watson and Rosalie Rayner's "Little Albert" experiment, in which a baby was conditioned to fear a white rat by pairing the rat with a loud noise, showing that emotional responses can be classically conditioned.
A second major form of learning is operant conditioning, formalized by B.F. Skinner, in which behavior is strengthened or weakened by its consequences. Behaviors followed by pleasant consequences tend to be repeated, while behaviors followed by unpleasant consequences tend to diminish. In positive reinforcement, a pleasant stimulus is added after a behavior, as when giving a treat to a dog that has just sat on command, making the behavior more likely to occur again. In negative reinforcement, an unpleasant stimulus is removed after a behavior, such as taking aspirin to remove a headache, which likewise increases the likelihood of the behavior. By contrast, punishment decreases the likelihood of a behavior. Skinner studied these processes extensively using the "Skinner box," demonstrating how reinforcement and punishment shape an animal's voluntary responses.
Beyond direct experience, much of human learning occurs by watching others. Observational learning, or modeling, was famously studied by Albert Bandura, whose Bobo doll experiment showed that children imitate aggressive behavior they have observed in adults. Together, classical conditioning, operant conditioning, and observational learning form three complementary accounts of how experience shapes the way organisms behave.