The Marshmallow Experiment was developed by psychologist Walter Mischel in the 60s and 70s, when he was a professor at Stanford University.
Walter Mischel had tested the ability of preschoolers (with an average age of 4 years and a half) to resist the temptation of eating a marshmallow, they could eat it right away or wait 15 minutes until the experimenter returned. If they waited, they would get an extra marshmallow. A child out of three was able to resist and wait the time set to get a second marshmallow. Here’s an experiment conducted after the model of the 60s: