Parents' beliefs about sibling differences weren't influenced by past grades, but future grades by the teenagers were influenced by the parents' beliefs. The child parents believed was smarter tended to do better in the future. The child parents believed was less capable tended to do relatively poorer the next year. Specifically, that belief translated to a 0.21 difference in GPA among study participants. 'That may not sound like much,' Jensen said. 'But over time those small effects have the potential to turn into siblings who are quite different from one another.'
'A mom or dad may think that oldest sibling is smarter because at any given time they are doing more complicated subjects in school,' Jensen said. 'The firstborn likely learned to read first, to write first, and that places the thought in the parent's mind that they are more capable, but when the siblings are teenagers it leads to the siblings becoming more different. Ultimately, the sibling who is seen as less smart will tend to do worse in comparison to their sibling.'
The one exception in the study was when the firstborn was a brother and the second born a sister. In that case, parents believed the sister was more academically competent.
'Parents tend to view older siblings as more capable, but on average older siblings are not doing better in school than their younger siblings,' Jensen said. 'So in that case parents' beliefs are inaccurate. Parents also tend to think their daughters are more academically competent than their sons, and at least in terms of grades that seems to be true.'
So what should parents do to set up all of their children for success?
'It's hard for parents to not notice or think about differences between their children, it's only natural,' Jensen said. 'But to help all children succeed, parents should focus on recognizing the strengths of each of their children and be careful about vocally making comparisons in front of them.'
Alexander C. Jensen, Susan M. McHale. What makes siblings different? The development of sibling differences in academic achievement and interests.. Journal of Family Psychology, 2015; 29 (3): 469