Giving Leads to Happiness in Children Figure 2. Five phases of the main experiment. Toddlers were (a) introduced to a puppet and (b) given eight treats. Then, in counterbalanced order, each toddler (c) watched as the experimenter gave one treat to the puppet, (d) was asked to give a ‘‘found’’ treat to the puppet, and (e) was asked to give one of their own treats to the puppet. doi:10.1371/journal.pone.0039211.g002 Discussion While previous research has demonstrated spontaneous helping among toddlers in the absence of explicit or implicit social demands [23], the present study provides the first evidence that giving to others makes young children happy-even happier than when they are receiving treats themselves. In addition, children’s differential happiness during costly and non-costly giving suggests that they (1) distinguished between the two, and (2) derived more happiness when giving involved sacrificing their own resources. This result is especially significant in that the ‘puzzle’ of prosocial behaviour toward non-kin requires explaining costly prosocial behaviour in particular; from our theoretical perspective, the emotional benefits of this form of giving may support such behavior despite its costs, whereas non-costly prosocial behaviour could be sustained in the absence of a powerful proximate mechanism. While the role of socialization can almost never be completely ruled out, the present results support the argument that humans have evolved to find prosocial behavior rewarding. Although children may be socialized to engage in helping and sharing behavior before the age of two through praise and other forms of reinforcement, it seems unlikely that children’s nonverbal responses in this experiment solely reflected their anticipation of external rewards. Indeed, children’s happiness was unrelated to the puppet’s enthusiasm, and previous research has shown that external reinforcement of prosocial behavior actually undermines toddlers’ subsequent prosocial acts, both immediately [23] and two years later [33]. Finally, from a socialization perspective, it is difficult to explain our finding that the emotional benefits of giving are strongest when giving is costly; it seems unlikely that parents provide toddlers with differential reinforcement for costly versus non-costly forms of giving. Future work examining whether giving leads to happiness in young children should replicate these findings with a larger sample. While the present research suggests the impact of giving on happiness is large (effect sizes between 0.46–1.35), replicating these findings with an additional sample would further support these claims. Future work should also examine whether giving to some targets produces greater emotional rewards than giving to others; for instance, perhaps toddlers would be happiest after giving to relatives or to people who had provided help in the past. Finally, given research suggesting that adults experience greater emotional benefits when they freely choose to help others than when they feel obligated to do so [34], research should also explore whether the emotional benefits of giving are greater when children give spontaneously, as opposed to giving in response to an experimenter’s suggestion. We have argued that the warm glow of giving represents a proximate mechanism that encourages individuals to engage in prosocial behavior, even, or perhaps especially, when doing so is costly. In addition to simply reinforcing prosocial behavior, positive emotions facilitate constructive mindsets [35,36], promote success in a wide range of domains [37], and have been linked to longevity [38], making happiness an especially adaptive proximate mechanism. For example, individuals who report experiencing more frequent positive emotion are more likely to seek out creative solutions to novel problems, excel professionally, get married, and live longer. In sum, the present research speaks to the origins of human prosociality towards non-kin, a puzzle that has intrigued scientists for decades. By documenting the emotionally rewarding properties of costly prosocial behavior among children in the second year of life, this research provides foundational support for the claim that experiencing positive emotions when giving to others is a proxi- mate mechanism for human cooperation and prosociality. Methods All participants were healthy full-term toddlers recruited from local libraries, hospitals, and community events in Vancouver, Canada. Families were contacted and asked to bring their child into the lab when their children fell within the appropriate age window for a study. Public transportation or parking costs were paid for by the lab and children received either a t-shirt or small toy for their participation. All data collection was approved by the University of British Columbia’s Behavioral Research Ethics Board (H10–01808). Age and gender were unrelated to happiness ratings in all study phases (all ps ..18). Preliminary Study Eleven toddlers (5 boys; Xage = 22 months, 1 day, range 21;16 to 22;20) participated in the giving interaction; 12 in the non- giving interaction (6 boys; Xage = 19 months, 27 days, range 19;13 to 20;8). Children were recruited for two separate studies investigating what infants offer to someone else (giving interaction) [39] and whether infants can distinguish between formerly helpful and unhelpful individuals when they need help (non-giving interaction) [40]. Giving and non-giving interactions represented the warm-up phase for each study, respectively. Children were allocated to the two studies based solely on their age. Children were later randomly assigned to different conditions within their experiment. Importantly, however, this condition assignment occurred after the giving and non-giving warm-up, meaning that participants were not treated differently based on condition during the time at which their responses were captured. Main Study Twenty toddlers (11 boys, Xage = 22 months, 26 days, range 22;7 to 23;17) participated. Ten additional children were excluded for technical/experimental errors (5), fussiness (3), failure to complete the warm-up because of shyness (1), and refusal to share PLoS ONE | www.plosone.org 3 June 2012 | Volume 7 | Issue 6 | e39211