Humans are extraordinary prosocial beings. No other Great Ape species invests a comparable amount of time and resources into building, maintaining, and repairing social relationships. The degree to which small-scale communities and large-scale societies embrace the human capacity for prosociality and capitalise on human-unique forms of cooperation is variable. The cross-cultural constant, however, is that each individual child undergoes a critical period of prosocial development in the first four years of life. During this period children’s emotions undergo a culture-specific transformation through children’s interactions with adults, siblings, and same-age peers. Studying how emotions shape young children’s social interactions, how they allow children to maintain and repair social relationships, is the developmental psychologist’s study of the psychological origins of human prosociality. This thesis is about studying examples of such so-called prosocial emotions. The main question of the current thesis is: How do emotions regulate children’s social inter- actions and relationships in early ontogeny? Addressing this question requires the assessment of both the internal mechanisms (attention and physiological arousal) as well as the expression (in behaviour or body language) of emotions in young children’s social interactions. At first, I provide a theoretical basis for studying the regulatory function of prosocial emotions in the first four years of children’s development. I suggest that children’s developing prosociality progresses through two distinct phases, one which shapes their attention and physiological arousal to others’ needs in the first two years of life, and subsequently a second phase during which children’s prosocial behaviour emerges and is increasingly regulated by emotions. In contrast to previous work, such a study of emotions as underlying early prosocial development requires broadening the suite of methods through which emotions are objectively measurable and interpretable. The first research aim was to study the development of children’s attention and arousal in response to others’ needs. We found that prosocial attention is a stable phenomenon from toddlerhood through school-age. In both a western and non-western culture, children anticipated how others are best helped. In a clinical comparison study, we found that this anticipatory response was blunted in children with an autism spectrum diagnosis. Young children’s prosocial orientation was not limited to helping contexts but encompassed other domains. Toddlers understood others’ desires and anticipated how those were best met. We further found that two-year toddlers were attentive to same-age peers’ needs and helped a same-age peer in sim- ilar ways and to a similar degree as has been previously reported in studies with child-adult interactions. We further found that toddlers’ attention to others’ needs was not contingent on explicit requests for help. Rather toddlers helped an adult even if helping was anonymous suggesting that toddlers did not help to interact with a competent adult partner. Finally, in a comparative study, we found chimpanzees’ underlying motivation to provide help to be different from that previously documented in young children. Chimpanzees were fastest to complete an action if this resulted in a conspecific’s need being fulfilled but, in contrast to young children, chimpanzees were motivated to carry out the behaviour themselves (which was not the case in non-helping control scenarios). Together, this indicates that a young chil- dren’s prosocial orientation develops during and persists beyond the first two years of life, thus laying the foundation for children’s prosocial behaviour. The second research aim was to study how children’s attention to others’ needs relates to their actual prosocial behaviour. In a first, comparative, study we found that young children regulated their prosocial behaviour more than chimpanzees. Both 3-year-old children and chimpanzees helped others, but children were more strongly motivated to help paternalistically by correcting dysfunctional requests for help that did not align with the requester’s actual need. The implication of these findings is that chimpanzees’ helping is more motivated by concerns to ’get credit’ and comply with others’ request for help whereas children’s helping is based on a, sometimes even paternalistic, concern to see others being helped. In a separate series of studies we developed experimental paradigms that allowed us to mea- sure the underlying emotions through changes in both internal arousal as well as overt body expressions. We found that changes in children’s physiological arousal were predictive of their subsequent helping behaviour. We further studied the function of guilt and gratitude in regulating two- and three-year-olds’ prosocial behaviour. Children’s intrinsic motivation to help an adult changed when they had accidentally caused the adult harm (guilt) or when the adult had previously helped them (gratitude). Specifically, guilt- and gratitude-provoking situations motivated young children to actively provide help. In contrast, and replicating previous work, under conditions without invoked guilt or gratitude, children’s motivation was to see that individual being helped (irrespective of whether they themselves or another adult provided the help). In a separate study we investigated children’s positive emotions, expressed in elevated upper-body posture, resulting from their successful helping behaviour. Children at the age of two years showed a similarly elevated body posture after completing a goal for themselves and after completing an adult’s goal whereas no such elevation was observed when children’s actions did not benefit anyone. Together these findings suggest that emotions invoked in their interactions with others in turn influence children’s motivation to interact, thus serving a regulatory function to repair (guilt) and maintain (gratitude) children’s social relationships. This successful navigation of the social world may contribute to children’s own emotional well-being. In sum, the current thesis explored the developmental origins of prosociality focusing on prosocial emotions that allow already young children to build and maintain functional relationships with adults and peers. The first four years of life give rise to prosocial emotions that critically shape children’s social interactions toward cooperative ends. Together, the re- search presented here adds to our knowledge of the deep ontogenetic roots of human-unique prosociality.:1 Executive summary 1 2 Introduction and background 3 2.1 Children’s prosocial attention and interactions.................... 6 2.2 The regulatory social function of children’s emotions . . . . . . . . . . . . . . . . 8 2.2.1 Children’s emotional response to seeing others needing help . . . . . . . 8 2.2.2 Children’s emotional response to seeing others being helped . . . . . . . 9 3 Theoretical framework and research programs 11 3.1 Revealing the foundation of human prosociality (Theoretical paper 1) . . . . . 11 3.1.1 Precursors and mechanisms........................... 12 3.1.2 Social groups & interaction partners...................... 13 3.1.3 The self in social relationships ......................... 13 3.1.4 The long reach of early development ..................... 14 3.2 ResearchPrograms .................................... 15 3.2.1 Prosocial attention and interactions ...................... 16 3.2.2 The regulatory social function of emotions . . . . . . . . . . . . . . . . . . 17 4 Empirical studies 21 4.1 The development of prosocial attention across two cultures (Study 1) . . . . . . 21 4.1.1 Abstract ...................................... 21 4.1.2 Introduction.................................... 21 4.1.3 Materials and methods.............................. 24 4.1.4 DataAnalysis................................... 26 4.1.5 Results....................................... 28 4.1.6 Discussion..................................... 31 4.2 Prosocial attention in children with an autism spectrum diagnosis (Study 2) . . 35 4.2.1 Abstract ...................................... 35 4.2.2 Introduction.................................... 35 4.2.3 Methods...................................... 38 4.2.4 Data analysis ................................... 40 4.2.5 Results....................................... 44 4.2.6 Discussion..................................... 46 4.3 Desire understanding in 2-year-old children: An eye-tracking study (Study 3) . 50 4.3.1 Abstract ...................................... 50 4.3.2 Introduction.................................... 50 4.3.3 Methods...................................... 53 4.3.4 Results....................................... 57 4.3.5 Discussion..................................... 61 4.4 Toddlers help anonymously (Study4)......................... 65 4.4.1 Abstract ...................................... 65 4.4.2 Introduction.................................... 65 4.4.3 Study1....................................... 66 4.4.4 Study1: Methods................................. 66 4.4.5 Study1: Results.................................. 70 4.4.6 Study1: Discussion................................ 70 4.4.7 Study2....................................... 72 4.4.8 Study2: Method ................................. 72 4.4.9 Study2: Results.................................. 74 4.4.10 General discussion ................................ 75 4.5 Toddlers help a peer (Study5) ............................. 79 4.5.1 Abstract ...................................... 79 4.5.2 Introduction.................................... 79 4.5.3 Method....................................... 81 4.5.4 Results....................................... 86 4.5.5 Discussion..................................... 89 4.6 Chimpanzees are motivated to help others - and to get credit (Study 6) . . . . . 93 4.6.1 Abstract ...................................... 93 4.6.2 Introduction.................................... 93 4.6.3 Study1....................................... 95 4.6.4 Results.......................................100 4.6.5 Study2.......................................102 4.6.6 Results.......................................103 4.6.7 General discussion ................................104 4.7 Chimpanzees comply with requests; Children fulfil others’ needs (Study 7) . . . 106 4.7.1 Abstract ......................................106 4.7.2 Introduction....................................106 4.7.3 General method..................................109 4.7.4 Study1.......................................111 4.7.5 Results.......................................115 4.7.6 Study2.......................................116 4.7.7 Results.......................................119 4.7.8 General discussion ................................119 4.8 Young children’s physiological arousal and their motivation to help (Study 8) . 123 4.8.1 Abstract ......................................123 4.8.2 Introduction....................................123 4.8.3 Methods......................................126 4.8.4 Data analysis ...................................128 4.8.5 Results.......................................130 4.8.6 Discussion.....................................131 4.9 Children’s intrinsic motivation to reconcile after accidental harm (Study 9) . . . 135 4.9.1 Abstract ......................................135 4.9.2 Introduction....................................135 4.9.3 General method..................................137 4.9.4 Study1.......................................142 4.9.5 Study1: Results and discussion ........................145 4.9.6 Study2.......................................146 4.9.7 Study2: Results..................................148 4.9.8 General discussion ................................149 4.10 Toddlers’ intrinsic motivation to return help to their benefactor (Study 10) . . . 152 4.10.1 Abstract ......................................152 4.10.2 Introduction....................................152 4.10.3 Study1:Methods.................................155 4.10.4 Study1:Results..................................161 4.10.5 Study1:Discussion................................163 4.10.6 Study2:Methods.................................164 4.10.7 General discussion ................................167 4.10.8 Conclusion.....................................170 4.11 The fulfillment of others’ needs elevates children’s body posture (Study 11) . . . 171 4.11.1 Abstract ......................................171 4.11.2 Introduction....................................171 4.11.3 Validation study .................................173 4.11.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 4.11.5 Study 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 4.11.6 Study1:Method .................................176 4.11.7 Study1:Results..................................181 4.11.8 Study1:Discussion................................183 4.11.9 Study 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 4.11.10 Study2:Method .................................185 4.11.11 Study 2: Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 4.11.12 General discussion ................................189 5 Integrative discussion and theoretical perspectives 193 5.1 Children’s prosocial attention and interactions . . . . . . . . . . . . . . . . . . . .194 5.1.1 Summary & conclusions.............................194 5.1.2 Future directions .................................195 5.2 Prosocial arousal in children (Theoretical paper 2) . . . . . . . . . . . . . . . .200 5.2.1 Abstract ......................................200 5.2.2 Introduction....................................200 5.2.3 The phenomenon and the debate........................201 5.2.4 Measuring prosocial arousal ..........................202 5.2.5 Children’s prosocial arousal and intrinsic motivation to help . . . . . . . 204 5.2.6 Looking ahead ..................................205 5.2.7 Conclusion.....................................207 5.3 The regulatory social function of children’s emotions . . . . . . . . . . . . . . . .207 5.3.1 Summary & conclusions.............................207 5.3.2 Future directions .................................208 5.4 The development of prosocial emotions (Theoretical paper 3) . . . . . . . . . . 211 5.4.1 Abstract ......................................211 5.4.2 Introduction....................................211 5.4.3 The prosocial functions of emotions......................212 5.4.4 General discussion and future directions . . . . . . . . . . . . . . . . . . .220 6 Bibliography 227 7 Declaration of originality 255 8 Contribution statement 257