What was the natural stimulus in Watson and Rayner's experiment?

In Watson and Rayner's experiment, the natural stimulus used was a rat. This experiment, conducted in the early 20th century, is a classic study in psychology known for its exploration of classical conditioning. The rat was initially a neutral stimulus for the child, Little Albert, meaning he did not show any fear or negative response towards the animal before the conditioning process began.

Through a series of trials, Little Albert was conditioned to associate the presence of the rat with a loud noise, which was a fear-inducing stimulus. Over time, this led to Little Albert exhibiting fear toward the rat even when the loud noise was not present. This demonstrated how a naturally neutral stimulus could be transformed into a conditioned stimulus through association. The use of a rat highlights the principles of classical conditioning, particularly how behaviors can be learned and modified through stimulus association.