The study I found is called the Stanford Marshmellow Experiment, and it was a study done in the late 1960's on the delayed gratification in children, when it develops, and what conditions it develops in.. They found that children used many methods, such as closing their eyes, hurting themselves or thinking of dark thoughts that would allow them to wait. It the test, children were told that either they could have one marshmallow now, or two in fifteen minutes. In follow up experiments, they noticed that children who were able to delay there gratification tended to have better life outcomes, such as higher SAT scores and healthier BMI's.
The original question asked when does the control of delayed gratification develop in children. The hypothesis of the experiment was if different socioeconomic backgrounds, ethnicity's and age effect the way that children think, then delayed gratification will develop in children later in life and those who come from better economic backgrounds. This hypothesis was based on the prior knowledge of a similar test done in Trinidad in which different ethnicity's effected the results and children from poor backgrounds were more likely to gain the immediate reward rather then wait and fear losing the reward.
https://en.wikipedia.org/wiki/Stanford_marshmallow_experiment
http://theness.com/neurologicablog/index.php/the-marshmallow-test/
