Between 1968 and 1974, more than 600 students at Bing Nursery School in Palo Alto, Calif., took part in the "marshmallow test," one of the most famous studies in psychology. The "test" was the brainchild of psychologist Walter Mischel, then of Stanford University, who set out to understand how children develop the ability to delay gratification, a critical skill for success in life.