Stanford marshmallow experiment
The Stanford marshmallow experiment refers to a series of studies on delayed gratification in the late 1960s and early 1970s led by psychologist Walter Mischel then a professor at Stanford University. In these studies, a child was offered a choice between one small reward (sometimes a marshmallow, but often a cookie or a pretzel, etc.) provided immediately or two small rewards if he or she waited until the experimenter returned (after an absence of approximately 15 minutes). In follow-up studies, the researchers found that children who were able to wait longer for the preferred rewards tended to have better life outcomes, as measured by SAT scores, educational attainment, body mass index (BMI) and other life measures. However, recent work calls into question whether self-control, as opposed to strategic reasoning, determines children's behavior.
Original experiment 
The experiment has its roots in an earlier one performed on Trinidad, where Mischel noticed that the different ethnic groups living on the island had contrasting stereotypes of one another, specifically, on the other's perceived recklessness, self-control, and ability to have fun. This small (n= 53) study of male and female children aged 7 to 9 (35 Black and 18 East Indian) in a rural Trinidad school involved the children in indicating a choice between receiving a 1c candy immediately, or having a (preferable) 10c candy given to them in one week's time. Mischel reported a significant ethnic difference, large age differences, and that "Comparison of the "high" versus "low" socioeconomic groups on the experimental choice did not yield a significant difference". Absence of the father was prevalent in the African-descent group (occurring only once in the East Indian group), and this variable showed the strongest link to delay of gratification, with children from intact families showing superior ability to delay.
Stanford experiment 
The purpose of the original study was to understand when the control of deferred gratification, the ability to wait to obtain something that one wants, develops in children. The original experiment took place at the Bing Nursery School located at Stanford University, using children age four to six as subjects. The children were led into a room, empty of distractions, where a treat of their choice (Oreo cookie, marshmallow, or pretzel stick) was placed on a table, by a chair. The children could eat the marshmallow, the researchers said, but if they waited for fifteen minutes without giving in to the temptation, they would be rewarded with a second marshmallow. Mischel observed as some would "cover their eyes with their hands or turn around so that they can't see the tray, others start kicking the desk, or tug on their pigtails, or stroke the marshmallow as if it were a tiny stuffed animal", while others would simply eat the marshmallow as soon as the researchers left.
In over 600 children who took part in the experiment, a minority ate the marshmallow immediately. Of those who attempted to delay, one third deferred gratification long enough to get the second marshmallow. Age was a major determinant of deferred gratification.
The first “Marshmallow Test” was a study conducted by Walter Mischel and Ebbe B. Ebbesen at Stanford University in 1970.
16 boys and 16 girls attending the Bing Nursery School of Stanford University. Three other subject were run, but eliminated because of their failure to comprehend the instructions. The children ranged in age from 3 years, 6 months to 5 years, 8 months (with a median age of 4 years, 6 months). The procedures were conducted by two male experimenters. Eight subjects (4 males and 4 females) were assigned randomly to each of the four experimental conditions. In each condition each experimenter ran 2 males and 2 females in order to avoid systematic biasing effects from sex or experimenters.
The conditions 
1) both the immediate (less preferred) and the delayed (more preferred) reward facing was left facing the subject and available for attention
2) Neither reward was available for the subject’s attention, both rewards having been removed from his/her sight
3) Delayed reward only was left facing the subject and available for attention while he or she waited
4) Immediate reward only was left facing the subject and available for attention while he or she waited
On the table in the experimental room there were 5 pretzels and an opaque cake tin. Under cake tin were 5 pretzels and two animal cookies. There were 2 chairs in front of table, on one chair was an empty cardboard box. On the floor near the chair with the cardboard box on it, were 4 battery operated toys. The experimenter pointed out the 4 toys, before the child could play with the toys, the experimenter asked the child to sit in the chair, he then demonstrated each toy briefly and in a friendly manner, saying that they would play with the toys later on – the experimenter placed each toy in the cardboard box & out of sight of the child. The experimenter explained to the child that the experimenter sometimes has to go out of the room but if the child eats a pretzel the experimenter will come back into the room. These instructions were repeated until the child seemed to understand them completely. The experimenter left the room and waited for the child to eat a pretzel – they did this 4 times. Next the experimenter opened the cake tin to reveal 2 sets of reward objects to the child 5 pretzels and 2 animal crackers. The experimenter asked which of the two the child liked better (preferred reward), and after the child chose, the experimenter explained that the child could either continue waiting for the more preferred reward until the experimenter returned or the child could stop waiting by bringing the experimenter back. If the child stopped waiting, then the child would receive the less favored reward and forgo the more preferred one. Depending on the condition and the child’s choice of preferred reward, the experimenter picked up the cake tin and along with it either nothing, one of the rewards, or both. The experimenter returned either as soon as the child signaled him to do so or after 15 minutes.
Follow-up studies 
In the follow-up study that took place many years later, Mischel discovered there existed an unexpected correlation between the results of the marshmallow test and the success of the children many years later. The first follow-up study, in 1988, showed that "preschool children who delayed gratification longer in the self-imposed delay paradigm, were described more than 10 years later by their parents as adolescents who were significantly more competent".
A 2011 brain imaging study of a sample from the original Stanford participants when they reached mid-life showed key differences between those with high delay times and those with low delay times in two areas: the prefrontal cortex (more active in high delayers) and the ventral striatum (an area linked to addictions) when they were trying to control their responses to alluring temptations.
A 2012 study at the University of Rochester altered the experiment by dividing children into two groups: one group was given a broken promise before the marshmallow test was conducted (the unreliable tester group), and the second group had a fulfilled promise before their marshmallow test (the reliable tester group). The reliable tester group waited up to four times longer (12 min) than the unreliable tester group for the second marshmallow to appear.. The authors argue that this calls into question the original interpretation of self-control as the critical factor in children's performance, since self control should predict an inability to wait, not strategic waiting when it makes sense. The authors suggest that the correlations between marshmallow performance and later life success may therefore be confounded, with successful children being raised in reliable situations. Prior to the Marshmallow Studies at Stanford, Walter Mischel had shown that the child's belief that the promised delayed rewards would actually be delivered is an important determinant of the choice to delay, but his later experiments did not take this factor into account or control for individual variation in beliefs about reliability when reporting correlations with life successes.
- Mischel, Walter; Ebbe B. Ebbesen, Antonette Raskoff Zeiss (1972). "Cognitive and attentional mechanisms in delay of gratification.". Journal of Personality and Social Psychology 21 (2): 204–218. doi:10.1037/h0032198. ISSN 0022-3514.
- Mischel, Walter; Yuichi Shoda, Monica L. Rodriguez (1989). "Delay of gratification in children.". Science 244: 933–938.
- Ayduk, Ozlem N.; Rodolfo Mendoa-Denton, Walter Mischel, Geraldine Downey, Philip K. Peake, Monica L. Rodriguez (2000). "Regulating the interpersonal self: Strategic self-regulation for coping with rejection sensitivity". Journal of Personality and Social Psychology 79: 776–792.
- Schlam, Tanya R.; Nicole L. Wilson, Yuichi Shoda, Walter Mischel, Ozlem Ayduk (2013). "Preschoolers’ delay of gratification predicts their body mass 30 years later". The Journal of Pediatrics 162: 90–93.
- Shoda, Yuichi; Mischel, Walter; Peake, Philip K. (1990). "Predicting Adolescent Cognitive and Self-Regulatory Competencies from Preschool Delay of Gratification: Identifying Diagnostic Conditions". Developmental Psychology 26 (6): 978–986. Archived from the original on October 4, 2011.
- "Marshmallow Test Revisited". University of Rochester. October 11, 2012.
- W. Mischel. (1958). Preference for delayed reinforcement: An experimental study of a cultural observation. The Journal of Abnormal and Social Psychology, 56, 57-61
- Mischel, Walter; Ebbesen, Ebbe B. (October 1970). "Attention in delay of gratification". Journal of Personality and Social Psychology 16 (2): 329–337. doi:10.1037/h0029815.
- Eigste, Inge-Marie; Zayas, Vivian; Mischel, Walter; Shoda, Yuichi; Ayduk, Ozlem; Dadlani, Mamta B.; Davidson, Matthew C.; Aber, J. Lawrence et al. (2006). "Predicting Cognitive Control From Preschool to Late Adolescence and Young Adulthood". Psychological Science 17 (6): 478–484. Archived from the original on June 22, 2007.
- "Marshmallow Test Points to Biological Basis for Delayed Gratification". Science Daily. September 1, 2011. Archived from the original on October 4, 2011. Retrieved October 4, 2011.
- Casey, B. J.; L. H. Somerville, I. H. Gotlib, O. Ayduk, N. T. Franklin, M. K. Askren, J. Jonides, M. G. Berman, N. L. Wilson, T. Teslovich, G. Glover, V. Zayas, W. Mischel, Y. Shoda (August 29, 2011). "From the Cover: Behavioral and neural correlates of delay of gratification 40 years later". Proceedings of the National Academy of Sciences 108 (36): 14998–15003. doi:10.1073/pnas.1108561108. ISSN 0027-8424. Archived from the original on October 4, 2011.
- Kidd, Celeste; Holly Palmeri, Richard N. Aslin (2013). "Rational snacking: Young children’s decision-making on the marshmallow task is moderated by beliefs about environmental reliability". Cognition 126: 109–114.
- Mischel, Walter (1961). "Father absence and delay of gratification: Cross-cultural comparisons". Journal of Abnormal and Social Psychology 63: 116–124.
- Mischel, Walter (1966). "Theory and research on the antecedents of self-imposed delay of reward". In B. A. Maher. Progress in Experimental Personality Research. New York: Academic Press. pp. 85–131.
- Mischel, Walter; Ervin Staub (1965). "Effects of expectancy on working and waiting for larger rewards". Journal of Personality and Social Psychology 2: 625–633.
- Mischel, Walter; Joan Grusec (1967). "Waiting for rewards and punishments: Effects of time and probability on choice.". Journal of Personality and Social Psychology 5: 24–31.