Tim Harford Columnist

It’s 1963. A young psychologist, Bob Rosenthal, conducts an experiment in which his assistants place rats in mazes and then time how long it takes them to find the exit. They are housed in two cages: one for the smartest rats and one for rodent mediocrities. The assistants are not surprised to find that the smart rats solve the mazes more quickly. Their supervisor is — because he knows that in truth both cages contain ordinary lab rats.

Prof Rosenthal — he would go on to chair Harvard’s psychology department — eventually concluded that the secret ingredient was the expectations of his assistants: they treated the “special” rats with care and handled the “stupid” rats with disdain. When we expect the best, we get the best — even if we expect it of a rat...

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