In 1920, Johns Hopkins professor John B. Watson conducted an experiment on a baby (known as "Little Albert") to see if he could make him afraid of nice things. He succeeded.
At the beginning of the experiment, Albert was presented with a white rat, a dog, a white rabbit, and a mask of Santa Claus among other things. The lad was unafraid of everything and was, in fact, really taken with the rat. Then every time the baby touched the animals, scientists struck a metal bar behind him, creating a startlingly loud bang. The sound freaked out the child and soon, like Pavlov's dogs, Little Albert grew terrified of the rat and the mask of Santa and even a fur coat. The particularly messed up thing about the experiment was that Watson didn't even both to reverse the psychological trauma he inflicted.