An important demonstration of the importance of time preference comes from the famous Stanford marshmallow experiment, conducted in the late 1960s. Psychologist Walter Mischel would leave children in a room with a piece of marshmallow or a cookie, and tell the kids they were free to have it if they wanted, but that he will come back in 15 minutes, and if the children had not eaten the candy, he would offer them a second piece as a reward. In other words, the children had the choice between the immediate gratification of a piece of candy, or delaying gratification and receiving two pieces of candy. This is a simple way of testing children's time preference: students with a lower time preference were the ones who could wait for the second piece of candy, whereas the students with the higher time preference could not.

Ammous, Saifedean. The Bitcoin Standard