Young children help other people, but it is not clear why. In the current study, we found that 2-year-old children’s sympathetic arousal, as measured by relative changes in pupil dilation, is similar when they themselves help a person and when they see that person being helped by a third party (and sympathetic arousal in both cases is different from that when the person is not being helped at all). These results demonstrate that the intrinsic motivation for young children’s helping behavior does not require that they perform the behavior themselves and thus “get credit” for it, but rather requires only that the other person be helped. Thus, from an early age, humans seem to have genuine concern for the welfare of others.

The human pupil is a small opening in each eye that dilates in response not only to changes in luminance but also to novel events. This makes changes in pupil diameter an attractive measure in studies on infants' and young children's physical and social cognition. However, designing and interpreting pupillometry studies for developmental populations comes with its own caveats.Here we give an overview of how psychologically induced changes in pupil diameter have been investigated and interpreted in developmental studies. We highlight the methodological challenges when designing experiments for infants and young children and provide several suggestions to address common problems. The fact that pupillometry provides a sensitive Journal of Cognition and DevelopmentA c c e p t e d M a n u s c r i p t 3 measure of the time course of responses to novelty extends the scope of possibilities for researchers studying infant cognition and development.

Children's instrumental helping has sometimes been interpreted as a desire to complete action sequences or to restore the physical order of things. Two-year-old children (n = 51) selectively retrieved for an adult the object he needed rather than one he did not (but which equally served to restore the previous order of things), and those with greater internal arousal (i.e., pupil dilation) were faster to help. In a second experiment (n = 64), children's arousal increased when they witnessed an adult respond inappropriately to another adult's need. This was not the case in a nonsocial control condition. These findings suggest that children's helping is not aimed at restoring the order of things but rather at seeing another person's need fulfilled.

Much is known about young children's helping behavior, but little is known about the underlying motivations and emotions involved. In 2 studies we found that 2-year-old children showed positive emotions of similar magnitude-as measured by changes in their postural elevation using depth sensor imaging technology-after they achieved a goal for themselves and after they helped another person achieve her goal. Conversely, children's posture decreased in elevation when their actions did not result in a positive outcome. These results suggest that for young children, working for themselves and helping others are similarly rewarding.