"In the first test, conducted in Northern California in the late 1960s, a researcher named Calvin Edlund selected 79 children between the ages of 5 and 7, all from “low-middle class and lower-class homes.” The children were randomly divided into an experimental group and a control group. First, they all took a standard version of the Stanford-Binet IQ test. Seven weeks later, they took a similar test, but this time the kids in the experimental group were given one M&M for each correct answer. On the first test, the two groups were evenly matched on IQ. On the second test, the IQ of the M&M group went up an average of 12 points—a huge leap."

From Paul Tough's book "how children succeed"