Academic studies can be fascinating ... and totally confusing. So we decided to strip away all of the scientific jargon and break them down for you.
The Background
Parents and researchers have long speculated that birth order could determine kids' personalities. Stereotypes say that first-born children are high-achieving and bossy, while younger siblings are adaptable and mischievous. These anecdotal claims, however, aren't exactly backed up by consistent studies. Past research consistently suggests that birth order could have an effect on intelligence -- children's performances on intelligence tests tend to decline slightly from firstborns to later-borns -- but researchers have yet to find too much compelling evidence to suggest that older and younger children develop key personality traits according to their birth order.
To further investigate, researchers from the University of Leipzig and the University of Mainz in Germany conducted a study using longitudinal data from three prior studies on international children surveyed over the course of several decades.
The Setup
Researchers analyzed survey data collected throughout the participating children's lives from the National Child Development Study (4,489 participants from Great Britain), the National Longitudinal Survey of Youth 1997 Cohort (5,240 participants from the U.S.) and the Socio-Economic Panel (10,457 participants from Germany). Each of these surveys included self-reported personality inventories, which measured the Big Five personality traits: extraversion, emotional stability, agreeableness, conscientiousness, and openness to experience. All of the surveys also included IQ tests.
Since people's personalities can change as they get older -- one might become more conscientious with age, for example -- the researchers made sure to control for the children's ages in their analysis. They also controlled for number of children in the family to rule out socioeconomic factors that might influence personality, since past research suggests that families who have a lower socioeconomic status tend to have more children.
The Findings
After looking at three data samples of over 20,000 participants, the researchers found that birth order had no significant effect on four out of the Big Five personality traits: extraversion, emotional stability, agreeableness and conscientiousness. They did find a small, but significant, decline in self-reported intellect, which applies to openness to experience, the fifth of the Big Five personality traits. The researchers hypothesize that younger siblings may underestimate their intellect because they're comparing themselves to their older siblings, who have a "developmental advantage."
That said, the researchers also saw a small decline in IQ scores from first-borns to younger siblings, as previous studies have found.
The Takeaway
For now, it seems safe to assume that birth order doesn't impact personality. And it probably doesn't have much of an effect on intelligence, either -- the differences in intelligence and self-reported intellect the researchers observed, while statistically significant, were pretty small.
It might be fun to stereotype ("Look at how bossy he is -- such a first-born"), but these tropes shouldn't limit anyone's perceived potential to be intelligent, extroverted or any other nuanced trait people can possess.
Moral of the story? Your sibling rank doesn't have to be your destiny.
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