The experiment has since been repeated and some of the results questioned, particular with regard to the role played by socio-economic status and parental education. But the underlying principle, most people seek instant gratification even at the cost of losing a larger reward, has been shown to be pervasive. From an evolutionary point of view, this makes sense. Our ancestors are those who were successful in foraging. With no guarantee of a reward at some future point, delayed gratification didn’t make a lot of sense. Eating what you have and worrying about tomorrow, tomorrow, was the key to survival.