In the study, published inSocial Development, 55 mothers were observed interacting with their first child at the age of 20 months and again, using the same procedures, when their second child was 20 months.

First-born children tended to be more sociable and emotionally available to mothers than second-born children. Mothers' behaviors with first-born and second-born children were not similar in rank order (for example, mothers who engaged in a lot of play with their first-borns did not necessarily engage in a lot of play with their second-borns), but there was no systematic average difference in the amounts or qualities of mothers' interaction directed toward first and second children.

These findings suggest that, despite relatively consistent parenting beliefs over time, siblings behave differently from one another by 20 months of age, and mothers behave differently when interacting with their two children at the same age.