You've probably heard of the marshmallow study on delayed gratification that was done back in the 1960s. The research done at Stanford University found that children that were able to delay eating the marshmallows were more likely to be successful later in life.
Researchers at University of Rochester revisited this study by adding the element of uncertainty and were able to demonstrate that delay gratification is influenced by environment as well as by innate self-control. "Children who experienced reliable interactions immediately before the marshmallow task waited on average four times longer - 12 versus three minutes - than youngsters in similar but unreliable situations." Thus, being able to trust that the reward will, in fact, be given affects the children's motivation to wait.