The “Little Albert” experiment was an experiment conducted by John Watson and Rosalie Rayner, with a 9-month old boy named Albert. Inspired by Ivan Pavlov’s discovery of classical conditioning, the researchers paired a benign object (a white rat, that Albert had recently taken pleasure in playing with), with reaction-evoking loud noises. When the rat was presented later without the noises, Albert showed a startle response to the rat.

Source:

Watson, John B. & Rayner, Rosalie. (1920). “Conditioned emotional reactions“. Journal of Experimental Psychology, 3, 1-14.