Parenting styles can influence what kind of person a child grows up to be, but beyond actions, the way a parent simply thinks about their child can make an impact. Researchers from Brigham Young University found a significant and often overlooked flaw in the way parents express perceptions of their children. The results of their study, published in the Journal of Family Psychology, encourage parents to stop comparing siblings to one another before it causes a lifetime of harm.

“Parents' beliefs about their children, not just their actual parenting, may influence who their children become,” the study’s lead author Alexander Jensen, a professor at Brigham Young University, said in a press release. “It's hard for parents to not notice or think about differences between their children. It's only natural. 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.”

Researchers looked at 388 teenagers from 17 different school districts who were either firstborn or secondborn siblings, along with their corresponding parents. The researchers asked the parents straight out: Which one of your children is better in school? The majority of parents said their firstborn was better despite the fact that on average, siblings perform the same. Why is the firstborn perceived as smarter when they haven’t earned it?

“A mom or dad may think that the 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.”

By the time siblings grow up, the ones who are thought of as smarter may begin to fulfill their perceived roll. When parents believe in their child, it takes the pressure off of them and places it on their closest competitor — their brother or sister.

“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.”

Source: Jensen AC and McHale SM. What makes siblings different? The development of sibling differences in academic achievement and interests. Journal of Family Psychology. 2015.