A few years ago, Mark Lepper and David Greene, two social psychologists, spent time observing children during free play in the classroom and noted that some took an interest in drawing. These kids sought the chance to draw and spontaneously asked for crayons and paper.

The experimenters then took these youngsters and placed them in two different conditions. One group of children were each shown a glossy certificate, adorned with their name, and asked if they would like to receive it in return for drawing. A second group were not promised anything at all, but continued to draw anyway.

Two weeks later, the psychologists returned to observe the children from these groups during free play. The no-reward group continued to seek out pens and paper,