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" There is a classic psychology experiment that seems to confirm Brewer's point. Children who enjoy drawing were given marker pens and allowed to go at it. Some were rewarded for drawing (they were given a certificate with a gold seal and a ribbon, and told ahead of time about this arrangement, whereas for others the issue of rewards was never raised. Weeks later, those who had been rewarded took less interest in drawing, and their drawings were judged to be lower in quality, whereas those who had not been rewarded continued to enjoy the activity and produced higher-quality drawings. The hypothesis is that the child begins to attribute his interest, which previously needed no justification, to the external reward, and this has the effect of reducing his intrinsic interest in it. That is, an external reward can affect one's interpretation of one's own motivation, an interpretation that comes to be self-fulfilling. "
― Matthew B. Crawford , Shop Class as Soulcraft: An Inquiry Into the Value of Work