Are Oldest Kids Smarter? Are Younger Siblings More Outgoing?

By Sarah D. Sparks — July 17, 2015 2 min read
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That straight-laced overachiever in class; he’s an oldest kid, right? The playful artist dreaming in the corner—she’s got to be a little sister. And don’t get me started on those spoiled only children ...

Yeah, no. Like so many stereotypes, it turns out birth order is not a good way to predict a child’s personality or intelligence. (I’m not just saying that because I’m an only child.) The largest birth-order study to date, of 377,000 American high school students, found “no meaningful differences” between students born first and later. The Journal of Research in Personality released the study online this morning, in advance of its October issue.

Psychologists at the University of Illinois at Urbana-Champaign controlled for students’ age, sex, and number of siblings, as well as family structure and wealth. That’s critical, as other studies have shown that both wealthier parents and more-educated ones tend to have fewer children.

Birth Order Did Affect Teens, But Not Enough

This is an interesting example of the difference between research significance and practical meaning. It’s not that there were no differences between oldest and younger siblings. There were, and they broke down exactly as common wisdom would have you expect:

  • Oldest siblings had IQs slightly more than 1 point higher than children born later in the pecking order. In particular, oldest children had a little better language skills.
  • First-born students were also consistently a little less anxious and a little more outgoing.

So doesn’t that prove that these truisms are truth? No. As professor Brent Roberts, who led the study, noted, the connections between birth order and personality and intelligence were “infinitesimally small.” They were significantly smaller than the differences based on overall age and parents’ education levels, for example.

“If a drug saves 10 out of 10,000 lives, for example, small effects can be profound,” Roberts said in a statement. “But in terms of personality traits and how you rate them, a 0.02 correlation doesn’t get you anything of note. ... You’re not going to be able to sit two people down next to each other and see the differences between them.”

This is probably not the end of the debate over the effects of birth order, but it’s a good gut check for researchers, educators, and (like me) parents of siblings. Many parts of a student’s background—from birth order to race or sex—can have a big effect on one student’s identity and none on his classmate’s. This is an area that begs us to consider the whole child, and not just his older sister.

Photo source: Getty Images


A version of this news article first appeared in the Inside School Research blog.