How reward representation affect the ability to wait: a comparative study in capuchin monkeys and human children

Introduction The Delay choice task, in which subjects choose between a smaller sooner option (SS) and a larger later option (LL), is frequently used to evaluate delay tolerance. However, when the options consist of visible food amounts, the preference for LL might be due to a difficulty to inhibit a response towards the quantity rather than to a sustained delay tolerance. To clarify this issue, we tested 10 capuchin monkeys and 101 preschool children in a delay choice task with food, low symbolic tokens and high symbolic tokens. Moreover, to assess whether children improve their performances in the delay choice task over development, we tested a sample of the same children two years later. Since in young children and in non-human primates the symbolic representation of the reward helps to direct the attention away from its salient features, we expected the subjects to choose less LL with symbolic tokens than with food, according to the idea that the preference for LL might be due to a failure at inhibiting an impulsive response to the larger quantity. Methods In the first phase of the research, 10 capuchin monkeys and 101 children (51 three-year-olds and 50 four-year-olds) were individually tested in three experimental conditions: Food Delay (FD), Low Symbolic Token Delay (LSTD), High symbolic token delay (HSTD), in which the subject chose between a SS option and a LL option, and in two control conditions: Food Control (FC), High Symbolic Token Control (HSTC), in which both options were immediately available. We employed a within-subject design with capuchins and a between-subject design with children. Two years later, 58 children (28 five-year-olds and 30 six-year-olds) were tested again in one of the four following conditions: FD, HSTD, FC, or HSTC. Results Both capuchins and preschool children chose LL less in HSTD than in FD, although they chose LL to a similar extent in HSTC and FC. Capuchins, but not children, also chose LL less in LSTD than in FD. Considering children longitudinal data, LL was chosen more in HSTC than in HSTD and more in FD than in HSTD; moreover, there was a significant interaction between age and time of the research, with younger children choosing LL more in Phase 2 than in Phase 1. Discussion In capuchins and preschool children, high-symbolic stimuli reduce prepotent responses towards the larger quantity. These findings suggest that in the visible-food version of the Delay choice task, the choice of LL partly depends on an impulsive preference of quantity, and invite to use caution to interpret the results obtained in this kind of task as a measure of delay tolerance. This effect decreases over development, with older children’s behavior becoming more similar to that of adult humans.