Well, it turns out, according to a US study, that you might be right.
"Those born earlier perform better in school"—and if these researchers are correct, it's all because of the parents.
It turns out that mums and dads simply go easier on their later-born kids, according to data analyzed by economists V. Joseph Hotz and Juan Pantano, and as a result, first-born children tend to receive both the best parenting and the best grades.
Here are some reasons why this could be the case, according to researchers:
1) The Divided-Attention Theory: Earlier-born siblings enjoy more time, care and attention than later-born siblings because attention is divided between fewer kids.
2) The Bad-Genes Theory: The strong evidence of higher IQs among first children leads some to believe that later kids are receiving diminished "genetic endowment."
3) The I've-Had-It-With-Kids! Theory: Some parents decide to stop having more children after a difficult experience raising one. In that case, the poorer performance of later children isn't genetic, so much as selection bias: Some parents keep having children until they have one that's so problematic it makes them say "enough."
4) The No-One-to-Teach Theory: This is the idea that older siblings benefit from the ability to teach their younger brothers and sisters. Building these teaching skills helps them build learning skills that makes them better in school.
5) The Divorce Theory: Family crises like divorce are far more likely to happen after the first child in born (first marriage, then divorce, then a first child is not a common sequence) and they can disrupt later kids' upbringing.
6) The Lazy-Parent Theory: The general idea here is that first-time parents, scared of messing up their new human, commit to memory the first chapter of Battle Hymn of the Tiger Mother but by the second or third child, they've majorly chilled out.