The Marshmallow Test might just be measuring social class

A 2012 experiment re-created Mischel's marshmallow test, telling children they could have two marshmallows instead of one if they waited for fifteen minutes. But this time the experimenter added a condition: in one case, the adults repeatedly broke promises to the child  subjects, while in the other condition they were shown to be reliable. The children in the "reliable adult" condition waited far longer. The study's lead author, Celeste Kidde at the University of Rochester, was inspired by her work at a homeless shelter. She wrote in the paper that the results imply that children rationally consider whether delaying gratification will actually pay off. In an unstable environment, such as that created by extreme poverty, "the only guaranteed treats are the ones you have actually swallowed." In other words, what looks like impulse control, like IQ before it, may be little more than a proxy measure for social class.

From The Test by Anya Kamenetz