The Stanford marshmallow experiment was 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 provided immediately or two small rewards (i.e., a larger later reward) if they waited for a short period, approximately 15 minutes, during which the tester left the room and then returned. (The reward was sometimes a marshmallow, but often a cookie or a pretzel.) 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.
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Origins
The experiment has its roots in an earlier one performed in Trinidad, where Mischel noticed that the different ethnic groups living on the island had contrasting stereotypes about one another, specifically the other's perceived recklessness, self-control, and ability to have fun. This small (n= 53) study focused on male and female children aged seven to nine (35 Black and 18 East Indian) in a rural Trinidad school. The children were required to indicate a choice between receiving a 1¢ candy immediately, or having a (preferable) 10¢ candy given to them in one week's time. Mischel reported a significant ethnic difference, with Indian children showing far more ability to delay gratification as compared to African students, as well as 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 first "Marshmallow Test" was a study conducted by Walter Mischel and Ebbe B. Ebbesen at Stanford University in 1960 (as the marshmallow test, by Walter Mischel. All facts are told from scientists who worked there.)
The purpose of the original study was to understand when the control of delayed 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 ages 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 treat, the researchers said, but if they waited for fifteen minutes without giving in to the temptation, they would be rewarded with a second treat. Mischel observed that 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.
Subjects
Test subjects were 16 boys and 16 girls attending the Bing Nursery School of Stanford University. Three other subjects 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 (four male and four female) were assigned randomly to each of the four experimental conditions. In each condition each experimenter ran two boys and two girls in order to avoid systematic biasing effects from sex or experimenters.
Conditions
- Both the immediate (less preferred) and the delayed (more preferred) reward were left facing the subject and available for attention
- Neither reward was available for the subject's attention, both rewards having been removed from his/her sight
- The delayed reward only was left facing the subject and available for attention while he or she waited
- The immediate reward only was left facing the subject and available for attention while he or she waited
Procedure
On the table in the experimental room there were five pretzels and an opaque cake tin. Under the cake tin were five pretzels and two animal cookies. There were two chairs in front of the table, on one chair was an empty cardboard box. On the floor near the chair with the cardboard box on it were four battery operated toys. The experimenter pointed out the four toys; before the child could play with the toys, the experimenter asked the child to sit in the chair and then demonstrated each toy briefly and in a friendly manner, saying that they would play with the toys later on. Then the experimenter placed each toy in the cardboard box and 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 four times.
Next the experimenter opened the cake tin to reveal two sets of reward objects to the child: five pretzels and two 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 follow-up studies, Mischel found unexpected correlations 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 second follow-up study, in 1990, showed that the ability to delay gratification also correlated with higher SAT scores.
A 2006 paper to which Mischel contributed reports a similar experiment, this time relating ability to delay in order to receive a cookie (at age 4) and reaction time on a go/no go task.
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 (with a smaller N= 28) 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 ability to wait, not strategic waiting when it makes sense. 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.
In the studies Mischel and colleagues conducted at Stanford University, in order to establish trust that the experimenter would return, at the beginning of the "marshmallow test" children first engaged in a game in which they summoned the experimenter back by ringing a bell; the actual waiting portion of the experiment did not start until after the children clearly understood that the experimenter would keep the promise. Participants of the original studies at the Bing School at Stanford University appeared to have no doubt that they would receive a reward after waiting and chose to wait for the more desirable reward. However, Mischel's earlier studies showed there are many other situations in which children cannot be certain that they would receive the delayed outcome. In such situations, waiting for delayed rewards may not be an adaptive response.
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