| Prosocial acts, such as providing comfort or help, as well as sharing and cooperating with others, are important domains of children's early behavioural repertoire that set the stage for positive social interactions and response to others across the lifespan. While developmental research has provided evidence of associations of childhood prosocial behaviours with later prosociality and various aspects of psychosocial functioning, questions remain regarding patterns in prosocial behaviour development across developmental periods, associations with adjustment, and potential biological forces underlying prosocial responding. The present thesis therefore aimed to identify trends and individual differences in prosocial behaviour frequencies throughout childhood and adolescence, in addition to examining longitudinal patterns of association between prosocial behaviours and externalizing and internalizing problems. Neurochemical bases of prosocial behaviours, specifically serotonergic functioning, were also investigated. Three studies are presented.;Prosocial behaviour development must however be viewed within the broader context of children's behavioural and affective functioning. Study 2 therefore focused on longitudinal associations between prosocial behaviour development and the development of externalizing and internalizing problems. Potential predictors of joint development were also investigated. Using a large, representative sample of Canadian children, prosocial behaviour, physical aggression, anxiety, and depression developmental trajectories spanning ages 2 to 11 were modeled based ratings by the person most knowledgeable about the child. A negative association tended to operate between prosocial behaviour levels and levels of physical aggression and depression, while a more complex relationship emerged with anxiety. Child, mother, and family characteristics significantly contributed to the prediction of joint development, with evidence of maternal depression and low family income increasing the likelihood of displaying high levels of prosocial behaviours with high levels of externalizing and internalizing difficulties.;Study 3 focused on the neurochemical underpinnings of prosocial behaviours, specifically investigating the impact of acute tryptophan supplementation, and by extension increased serotonergic functioning, on children's laboratory aggressive and prosocial behaviours. Boys with a history of relatively high levels of physical aggression were randomly assigned to a tryptophan supplementation or control condition following a double-blind procedure. Results revealed that boys in the tryptophan condition were more likely to sustain competition with a fictitious opponent during a computerized reaction time game. Boys in the tryptophan condition also showed greater perspective taking, as well as greater instrumental help.;Taken together, results of the present thesis provide evidence of both stability and change in prosocial behaviour frequencies during childhood and adolescence. Results also highlight the complex relationship of prosocial behaviours with adjustment, as well as potential contributions of the serotonergic system to the regulation of prosocial responding in children. The present thesis sets the stage for future studies investigating the interplay of psychosocial and biological forces in determining individual differences in prosocial behaviour and related aspects of functioning.;Study 1 longitudinally described prosocial behaviour development from childhood to adolescence, using multiple informants within Canadian and Italian samples. Participants were boys from low SES areas of Montreal, Canada, as well as boys and girls in Genzano, Italy. Developmental trajectories were estimated based on yearly ratings by mothers and teachers (Montreal, ages 10 to 15) and children and teachers (Genzano, ages 10 to 14). Results revealed that the majority of children, across cultures and informants, followed trajectories characterized by stable or declining levels of prosocial behaviours over time. Low to moderate agreement was observed between informants. |
