John B. Watson was a pioneer in the field of psychology during the early 1900's. He is credited with founding the school of thought known as behaviorism, which believes that much of human behavior originated from classical and operant conditioning that we were exposed to over the course of our lives courtesy of our environment. A firm believer of this doctrine, he once said that given the proper environment he could shape an infant child into any sort of specialist he desired, regardless of the child's abilities and genetic factors. Together with his wife Rosalie Rayner, Watson conducted an experiment in 1919 to try and prove that a fear response can be produced in a human infant by way of classical conditioning. They succeeded. The experiment became one of psychology's most well known and publicized endeavors. Little Albert (the case subject) was conditioned to exhibit a fear response when he touched a small furry object with his hand.
Although the ethics of the experiment make it unlikely to be reproduced or expounded upon today, at the time it provided excellent insight on inciting a conditioned emotional response.
Watson wanted the child to exhibit a fear response when in contact with a variety of small furry objects. These objects included: a white rat, a rabbit, a dog, a fur coat, cotton wool, and a Santa clause mask. Watson conditioned the fear response by sharply striking a metal bar with a hammer which produced a loud clang behind the infant when the infant's hand touched the furry object. In terms of classical conditioning, the conditioned stimulus (furry object) was presented to the subject. This produced the unconditioned response of Little Albert playing with the object. The loud clang acted as the unconditioned stimulus when it was paired with the furry object's presentation.