A natural reaction to receiving help from someone is to help that person in return. In two studies, we investigated the developmental origins of children's motivation to return help. In Study 1, 18- and 24-month-old toddlers were either helped or not helped by an adult, and they could subsequently provide that adult with help or else observe another person providing help. We measured children's internal arousal, via changes in pupil dilation, both before and after help was provided. At both ages, children's internal arousal was higher when they could not help the adult who had previously helped them (and was lower when they could). On the other hand, if the adult needing help had not previously helped children, their internal arousal was equally low regardless of whether they or another person provided the help. Study 2 replicated this result and also found that if children had previously been helped but the person needing help was a different adult (not their benefactor), children's internal arousal was equally low regardless of whether they or another person provided the help. Together, these results suggest that young children are intrinsically motivated to return a received favor specifically to the previous benefactor, perhaps indicating a nascent sense of gratitude.
