Ever wondered what makes siblings so different though they may share a lot of habits? A study finds that the answer lies in the parents’ beliefs about their children — and the comparisons they make may cause differences to be magnified.

“Parents’ beliefs about their children, not just their actual parenting, may influence what their children become,” said lead author Alex Jensen from Brigham Young University, Utah.

The study published in the Journal of Family Psychology, focused on siblings and academic achievement.

They looked at 388 teenage first and second-born siblings and their parents from 17 school districts in a northeastern state.

The researchers asked the parents which sibling was better in school.

The majority of parents thought that the first-born was better, although on average, siblings’ achievement was pretty similar.

“That may not sound like much. But over time those small effects have the potential to turn into siblings who are quite different from one another.”

By the time siblings reach the teenage years, parents may have formed their beliefs about siblings’ relative smarts from years of experiences.

So when parents compare adolescent siblings to each other, it may be based on differences that have existed for years.

“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.

“To help all children succeed, parents should focus on recognising the strengths of each of their children and be careful about vocally making comparisons in front of them.”