All Placebos are not created equal

Have you ever heard of a Perkins Tractor? If you lived in London in the 18th Century, you could buy a pair of Perkins Tractors (shown above) for the steep price of five guineas, and wave them over an aching part of your body for about twenty minutes as a way of relieving your pain. They worked, sort of. People who bought and used Perkins Tractors

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