Classical conditioning is a mechanism that is thought by many psychologists, particularly behaviorists, to explain human behavior. The best-known experiments in classical conditioning were conducted by the Russian physiologist and psychologist, Ivan Pavlov, and American psychologist John Watson, who is remembered as the "father of behaviorism". Classical conditioning traditionally is thought to explain behaviors and physiological functions that are largely involuntary, whereas operant conditioning explains voluntary behavior.
Pavlov's studies were conducted at the end of the 19th century. While conducting research on digestion in animals, Pavlov realized that dogs automatically salivated when food was present. He then paired the presence of food with a bell. Pavlov repeatedly paired these two stimuli such that the bell was sounded shortly before the food was present. He then found that sounding the bell alone also produced salivation. In classical conditioning terminology, the food is referred to as an unconditioned stimulus and the bell as a conditioned stimulus. Salivation in response to the presence of food is called an unconditioned response, whereas salivation in response to the bell alone is a conditioned response.
Watson's most famous experiment utilized a young orphan in the institution in which Watson worked, who is known as Little Albert. Watson used classical conditioning to cause Albert to fear a white rat scaring him with a loud noise while Albert was playing with the rat. Watson discovered that this procedure also caused Little Albert to fear objects that were visually similar to the rat, such as a white rabbit and a Santa Claus mask with a furry white beard. This process, by which a response to one stimulus leads to similar responses to similar stimuli, is known as stimulus generalization.
Classical conditioning processes have been proposed to underlie certain psychiatric disorders, such as phobias. For example, if a person was once mugged in an alley, he or she may develop an irrational fear of alleys in general, and refuse to enter them.