Personality, IQ Not Determined By Birth Order

September 10, 2015 | Reece Alvarez

Young children, siblings

Researchers at the University of Illinois conducted the largest study yet of birth order and personality. They found no meaningful relationship between birth order and personality or IQ.

Do you fit the stereotype of the responsible, smart firstborn child or are you the carefree “baby” at the bottom of the family — maybe a rebellious middle-child?

Well those “classic traits” may be nothing more than stories we tell ourselves about birth order and its impact on personality and intelligence, according to a “massive study” of 377,000 U.S. high school students by researchers at the University of Illinois at Urbana-Champaign.

"This is a conspicuously large sample size," said University of Illinois psychology professor Brent Roberts, who led the analysis with postdoctoral researcher Rodica Damian (now a professor of psychology at the University of Houston).

"It's the biggest in history looking at birth order and personality," Roberts said.

“Birth order is often invoked as an important variable to explain the development of personality and intelligence within and across families,” the researchers said in their study published in the Journal of Research in Personality. “We would have to say that, to the extent that these effect sizes are accurate estimates of the true effect, birth order does not seem to be an important consideration for understanding either the development of personality traits or the development of intelligence in the between-family context.”

University of Illinois psychology professor Brent Roberts and University of Houston psychology professor Rodica Damian. (Photo: L. Brian Stauffer)

However, because of the large size of the sample the researchers were able to “detect extremely small effects,” such as slight differences in both personality and intelligence.

According to Damian, firstborns enjoy a one-IQ-point advantage over later-borns — a difference that is statistically significant, but meaningless, she said.

The finding was in line with past research on birth order and intelligence that generally found a benefit for firstborns, with decreasing intelligence for each subsequent birth rank, according to the study.

The researchers also found consistent differences in personality traits between firstborns and their siblings. Firstborns tended to be more extroverted, agreeable and conscientious, and had less anxiety, but those differences were "infinitesimally small," amounting to a correlation of 0.02, Roberts said.

"In some cases, if a drug saves 10 out of 10,000 lives, for example, small effects can be profound," he said. "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."

The researchers accounted for confounding factors including the trend that wealthier families tend to have fewer children and thus have a higher proportion of firstborns who also have access to more resources that may influence their IQ or personality, Damian said.

Gender and socioeconomic factors have a much greater impact on personality and IQ and “warrant much more attention” than birth order, the researchers said in their study.

Brent and Damian also studied a subset of families with only two children to look for specific differences between first- and second-borns.

The findings confirmed those seen in the larger study, with specific differences between the oldest and a second child, 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,” Damian said.

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