There is evidence for a laterborn sibling advantage in some social skills, although this has not been investigated in children's early capacities for cooperation. Using a within-family design, this study compared firstborn and laterborn (i.e., middle and youngest) siblings on their cooperative abilities when they were aged around 3 years. Further, the study investigated whether the association between children's birth order and cooperative abilities was dependent on the prosocial behaviour of other siblings in the home. The sample included 288 ethnically and sociodemographically diverse children clustered within 144 families. Cooperation was directly assessed using a problem-solving paradigm requiring two simultaneous and complementary actions of the child and adult tester to achieve a joint goal. Parents reported on the prosocial behaviour of up to four siblings in the home. Results of a multilevel analysis indicated that youngest children were more advanced in their cooperative abilities, compared to firstborn children, irrespective of their siblings' prosociality. Middle children, in contrast, were only advantaged over firstborn children if their siblings showed high levels of prosociality. The analysis accounted for a number of potential family-wide confounds, providing evidence that this is a child-specific effect related to birth order. Findings are discussed from a social constructivist perspective with an emphasis on the sibling relationship as a context for cooperative interactions that facilitate sociocognitive development. Statement of contribution What is already known on this subject? There are individual differences in children's early capacities for cooperation. Children's early cooperation has not been considered in relation to birth order and/or sibling interaction quality. What does this study add? Youngest children are advantaged in their cooperation as compared to firstborn children. Middle children are also advantaged, but only if their siblings are prosocial. The use of a developmentally sensitive within-family design reduces confounds common to birth order studies.
Keywords: birth order; collaborative problem-solving; cooperation; prosocial behaviour; siblings; social cognition.
© 2017 The British Psychological Society.