The reason, Wolpert believes, is so we are more attuned to external events. This can mistakenly lead to erroneous estimations. Wolpert and others discovered this when they asked people to hold out their index finger in front of a force transducer, which applied a small push. When the participants were then told to apply the same amount of force to a second transducer, they consistently pushed back harder and harder, in what could be described as an escalating “tit-for-tat” exchange. Self-generated force is perceived to be as much as half as strong as when it gets generated externally. It
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