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Family Dynamics

Younger Siblings Underestimate Their Intelligence

Birth order doesn't matter, usually—unless it affects your self-image.

Birth order isn't destiny.

You may have grown up hearing from your parents that your older sister is bossy because she's the eldest. Or that because you're the youngest, you're the most agreeable, trying to get along with everyone—or more spoiled and selfish.

The truth is that birth order doesn't seem to matter—unless it affects your self-image.

Our ideas about how birth order affects personality largely come from the theories of Alfred Adler, published in 1928. However, recent large studies find that there's no link between birth order and personality at all.

Psychologists see personality in different ways, but one of the most common research paradigms is the Big 5 traits: extraversion, emotional stability, agreeableness, conscientiousness, and imagination. In 2015, a landmark study of more than 20,000 individuals in the United States, Germany, and Great Britain concluded that birth order didn't affect your scores on those measures. A 2019 study backed that up.

Does birth order have an impact on twins? If so, it's because the first baby to emerge is usually the larger of the two. More than other siblings, twins are a little world unto themselves. Also, the second-born may have health problems triggered by trauma at birth, as much research documents.

But in a twist that will please feminists everywhere, small girls with twin brothers tend to lead, deciding on their joint opinions as a pair and announcing them, and this does not seem dependent on being first. This "psychologically and verbally dominant" role may last until their early twenties, according to at least one study. (The boy twin, in that research, becomes more physically dominant as a teen). Could testosterone exposure in the womb make a difference? A good deal of research has been lavished on that subject, but a 2020 review of 23 studies published since 2011 concluded that we don't really know.

I wrote earlier about research suggesting that we don't always know who is Mom's favorite. Overall, I think the message is that it's helpful to re-think some of the ideas about our families we view as gospel.

Untrue ideas often cause pain and narrow our choices. Yes, you may feel you know without a doubt that you didn't have a chance to become a leader because your older sister was so bossy, but maybe it's time to start your own group and see how well you do running it. You may be a leader, after all.

Does your older sister think she's smart (maybe you think she overrates herself) and you don't think you're so smart yourself? The landmark 2015 study did show that people perform less well on intelligence tests the more older siblings they have. But much more interesting to me is that birth order affected people's self-evaluations, and younger siblings underrate themselves. You give yourself lower ratings the more older siblings you have.

That self-assessment pattern showed up independently of the intelligence test scores—in other words, whether or not it was true. Let's put aside questions about whether the tests truly rate intelligence or what we mean by intelligence. The research suggests that even if you easily do geometry and she can't, or you have a big vocabulary and she doesn't, or you know how to charm strangers and she freezes, she is likely to have a higher opinion of her own intelligence.

My goal here isn't to put down first-borns. It's to help us all give ourselves a chance to be who we are, less burdened by our myths. Maybe you're a first-born taking care of your mother and your younger siblings consider you bossy—when the truth is you'd rather one of them took over, already. Your baby sister—if only she had more confidence!