The inclination to wait depends on one’s experiences. “For a child accustomed to stolen possessions and broken promises, the only guaranteed treats are the ones you have already swallowed,” remarked a group of social scientists at the University of Rochester. Last year they conducted an experiment in which children were encouraged to wait for “a brand-new set of exciting art supplies” rather than using the well-worn crayons and dinky little stickers that were already available. After a few minutes, the adult returned. Half the kids received the promised, far superior materials. But the other half got only an apology: “I’m sorry, but I made a mistake. We don’t have any other art supplies after all.”

Then it was time for the marshmallow challenge. And how long did the children wait for two to appear before they gave up and ate the one sitting in front of them? Well, it depended on what had happened earlier. Those for whom the adult had proved unreliable (by failing to deliver the promised art supplies) waited only about three minutes. But those who had learned that good things do come to those who wait were willing to hold off, on average, for a remarkable twelve minutes.

—Alfie Kohn
S’More Misrepresentation of Research
From The Myth of the Spoiled Child