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Rupert Georg von Trapp, M.D. (1st), Agathe von Trapp (2nd), Maria Franziska (3rd), Werner Ritter (4th), Hedwig Maria (5th), Johanna Karolina (6th) Martina (7th) born to Agatha Whitehead; Rosmarie (8th), Eleonore (9th), and Johannes (10th) born to Maria Augusta Kutschera. Trapp family singers @ 1949  Image from: Salzburg Museum

Birth order not meaningful to personality or IQ

A study of 377,000 high school students offers some good news: Yes, first-borns do have higher IQs and consistently different personality traits than those born later in the family. However, the differences are so small they have no practical relevance.

The analysis is reported in the Journal of Research in Personality.

"This is a conspicuously large sample size," said University of Illinois psychology professor Brent Roberts, who led the analysis with postdoctoral researcher Rodica Damian, formerly at the University of Illinois at Urbana-Champaign and now a professor of psychology at the University of Houston. "It's the biggest in history looking at birth order and personality."

The analysis found - as a previous large-scale study did - that first-borns enjoy a one-IQ-point advantage over later-borns, Damian said. The difference is statistically significant — but meaningless, she added.

The analysis also revealed consistent differences in personality traits between first-borns and later-borns as first-borns tended to be more extroverted, agreeable and conscientious, and had less anxiety than later-borns. But those differences were "infinitesimally small," amounting to a correlation of 0.02.

"In some cases, if a drug saves 10 out of 10,000 lives, for example, small effects can be profound. But in terms of personality traits and how you rate them, a 0.02 correlation doesn't get you anything of note. You are not going to be able to see it with the naked eye. You're not going to be able to sit two people down next to each other and see the differences between them. It's not noticeable by anybody."

Rodica Ioana Damian PhD, Department of Psychology, University of Illinois at Urbana-Champaign, United States

The study controlled for potentially confounding factors - such as a family's economic status, the number of children and the relative age of the siblings at the time of the analysis - that might skew the results, Damian said.

For example, wealthier families tend to have fewer children than other families, and so have a higher proportion of first-borns who also have access to more resources that may influence their IQ or personality, she said. According to Damian, many previous studies of birth order suffered from small sample sizes. Many compared children with their siblings - a "within-family" design that some assert is better than comparing children from different families, as did the new analysis. "But such studies often don't measure the personality of each child individually," she said. "They just ask one child - usually the oldest, 'Are you more conscientious than your siblings?'"  And of course the results differ depending on whom you ask.

Roberts: "Another major problem with within-family studies is that the oldest child is always older. People say, 'But my oldest kid is more responsible than my youngest kid.' Yes, and they're also older."

An ideal within-family study would follow families over time, collecting IQ and personality data from each child when he or she reached a specific age, the researchers added. The team evaluated a subset of the children in the study - those with exactly two siblings and living with two parents. This allowed the researchers to look for specific differences between first- and second-borns, or second- and third-borns. And the findings confirm those seen in the larger study, with specific differences between the oldest and a second child, and between second and third children. But the magnitude of the differences was, again, "minuscule," Roberts said.

"The message of this study is that birth order probably should not influence your parenting, because it's not meaningfully related to your kid's personality or IQ."

Rodica Damian PhD, professor of psychology, University of Houston.

Abstract Highlights
•The average absolute association between birth order and personality was .02.
•The average absolute association between birth order and intelligence was .04.
•Controls were age, sex, sibship size, socio-economic status, and family structure.
•The effects did not differ across different social categories.
•The effects did not differ in sibships of three raised by two parents.

Abstract
We tested birth order associations with personality traits and intelligence using Project Talent, a representative sample (N = 377,000) of U.S. high school students. Using a between-family design and several background factors (i.e., age, sex, sibship size, parental socio-economic status, and family structure), we were able to control for potential confounds, and estimate the links between birth order and outcomes across several different social categories. In addition to differences between firstborns and laterborns across the entire sample, we also tested birth rank trends in a sub-sample of targets from sibships of three, raised by two parents. Overall, the average absolute association between birth order and personality traits was .02, whereas the one between birth order and intelligence was .04.

The paper, "The associations of birth order with personality and intelligence in a representative sample of U.S. high school students," is available online or from the U. of I. News Bureau.

The National Institute on Aging at the National Institutes of Health supported this research.

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