The effects of changes in the referential problem space of infants and toddlers (Homo sapiens): Implications for cross-species comparisons

Recent reviews have highlighted the tendency in the comparative literature to make claims about species' relative evolutionarily adaptive histories based on studies comparing different species tested with procedurally and methodologically different protocols. One particularly contentious area is the use of the object-choice task, used to measure an individual's ability to use referential cues, which is a core attribute of joint attention. We tested human children with versions of the object-choice task that have been previously used with dogs and nonhuman primates to see if manipulating the setup would lead to behavioral changes. In Study 1, we compared the responses of 18-month-olds and 36-month-olds when tested with and without a barrier. The presence of a barrier between the child and the reward did not suppress performance but did elicit more communicative behavior. Moreover, the barrier had a greater facilitating effect on the younger children, who displayed more communicative behavior in comparison with older children, who more frequently reached through the barrier in acts of direct prehension. In Study 2, we compared the behavior of 36-month-olds when the reward was within reaching distance (proximal) and when it was out of reach (distal). The children used index-finger points significantly more in the distal condition and grabbed more in the proximal condition, showing that they were making spatial judgments about the accessibility of the reward rather than just grabbing per se. We discuss the implications of these within-species differences in behavioral responses for cross-species comparisons.