Infant Behav Dev 2019 May 16;55:100-111. Epub 2019 Apr 16.
Institute for Fiscal Studies, London, UK.
Given the large numbers of families with more than one child, understanding similarities and differences in siblings' behaviors and in parents' interactions with their sibling infants is an important goal for advancing more representative developmental science. This study employed a within-family design to examine mean-level consistency and individual-order agreement in 5-month-old sibling behaviors and maternal parenting practices with their firstborns and secondborns (ns = 61 mothers and 122 infants). Each infant was seen independently with mother. Firstborn infants were more social with their mothers and engaged in more exploration with objects than secondborn infants; firstborn and secondborn infants' behaviors were correlated for smiling, distress communication, and efficiency of exploration. Mothers engaged in more physical encouragement, social exchange, didactic interaction, material provisioning, and language with their firstborns than with their secondborns. Notably, only maternal nurturing (e.g., feeding, holding) did not differ in mean level when mothers were with their two infants. However, mean differences in mothers' social exchange and material provisioning with their two children attenuated to nonsignificance when controlling for differences in siblings' behaviors. Individual-order agreement of mothers' behaviors with firstborn and secondborn infants (across an average of almost 3 years) was only moderate. These findings suggest that mother-firstborn interactions may differ from mother-secondborn interactions. Future research should move beyond studying mother-firstborn dyads to understand broader family and developmental processes.