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We ran field experiments inspired by the classic marshmallow test. The experiments took place at six kindergartens (five high-income schools and one lower-income school) from September to December 2012 in Florianopolis, a city in southern Brazil. A total of 141 children (67 males and 74 females), ages 4 to 6 took part (data available at http://dx.doi.org/10.6084/m9.figshare.1160513). Children between 4 and 6 already consider themselves to be autonomous individuals, capable of counting and understanding quantities, and able to realize that events may have a cause. The research was approved by the Ethical Committee at the Federal University of Santa Catarina (approval number: 057/08; approval date: April 10, 2008; decision published: April 28, 2008). Parents were asked to allow their children to participate before providing consent in writing. Rather than offering the children marshmallows or Oreo cookies, we used candies that are popular in Brazil, as shown in Fig. 1.
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0114394.g001 Candies used in the experiment. Each child was given one candy at the beginning of class. The experimenter (B.M.) then explained to the children that they had to wait for the teacher's permission before eating the candy. Next, the teacher told the children they could eat the candy at any time during class. Additionally, the teacher also explained that, if they resisted the temptation to eat the first candy and waited for further instructions, then they would be given another candy as a reward. After 20 minutes, the teacher offered a second candy to those children who had resisted temptation and refrained from eating the first candy. In contrast to the classic experiment by Mischel et al. [1], the children were not tested alone, but were seated next to each other at small tables. This is by no means a drawback of our experiment. On the contrary, this means that our test is actually more stringent than the classic one, because self-control is more difficult when children imitate peers. Indeed, in a previous study, Nisan [17] offered children a choice between an immediate food reward and a delayed, but larger reward. The children made the decision either alone or in a small, same-sex peer group. Choices made by girls did not differ, irrespective of whether deciding alone or in groups, whereas when the boys were making decisions in groups, the first choice that was suggested prevailed as the final decision. More recently, McCabe and Brooks-Gunn [18] found that children are less able to demonstrate self-control in peer groups, compared to when they are assessed individually. Children in groups tended to imitate the behavior of those around them, and were less capable of exerting self-control than children tested alone. The 2D:4D ratio was measured by the experimenter after tracing the contour of each child's right hand on a sheet of white paper. This is far from ideal and was the result of a compromise with schools' principals. Measuring with a caliper (probably the most accurate method) was seen as time consuming, and most principals tended to view the caliper as too intrusive, which added little in terms of pedagogical value. Even if calipers were allowed, we presume we would experience difficulty in measuring, due to the short attention span of the children. Furthermore, photocopying or scanning young children's hands would likely result in many blurred and unusable images. In the end, tracing around the fingers is probably unacceptable for adult participants, but may represent a best-practice solution for 4 to 6 year olds. Indeed, the children seemed very relaxed during the task, reflecting the playful aspect of the chosen measuring technique. The experimenter also noted cooperation on behalf of the children. This may be related to the fact that this was not the first time they engaged in the task of tracing the contour of their hands, that is, this activity was previously introduced by the teachers. The finger lengths measured from the tracings showed roughly the same results across the six kindergartens. More importantly, there was repetition in the measurements with regard to a previous measurement of an earlier study we conducted in the same six kindergarten classes [19]. In short, the measurement seemed robust across kindergarten classes and across time. Children's anonymity was preserved, and only their gender was recorded by the experimenter. We also noted the children's handedness. Many left-handed people have been reported to have IQs greater than 140 compared to right-handed people, and are associated with musical talent and athleticism. This may be partly because left-handed people have an intrinsic neurological advantage over right-handed people [20]. Because gratification delay in children can be a predictor of higher scores on intelligence tests later in life, we hypothesized that left-handedness might be linked to the ability to delay gratification. However, this hypothesis failed to be validated. The experimenter first asked teachers to report the handedness of the participants and then confirmed their answers by asking a child to make a drawing on a sheet of paper. Using the hands for drawing has been argued to be the best way to assess child's hand preference [21]. In our sample, 105 children were right-handed (62 females and 43 males) and 36 were left-handed (12 females and 24 males). This result of 75.5 percent of right-handed children matches the one in reference [21], where 73 percent were right-handed (in which the children's hand preference was also verified through drawing). We estimate the model parameters and the influence of candidate explanatory variables (gender, digit ratio and handedness) on the probability of delaying gratification as follows:(1)where πi is the probability of failing to delay gratification for child i; βj is the coefficient vector capturing the impact of changes in the explanatory variables on πi; g is a dummy for gender (female = 0; male = 1); di is the 2D:4D digit ratio for child i; h is a dummy for handedness (left- handed = 0; right-handed = 1); and εi is an error term. An explanatory variable selection was conducted using stepwise, backward and forward regressions. It was found that neither gender or handedness were significant for explaining πi.
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