Oldest? Youngest? Birth order doesn't influence personality, study says.

Everyone knows the familiar stereotypes: the smart, responsible first-born sibling; the rebellious baby of the family; the easy-going middle child.

But those stereotypes bear little validity, according to a recent study that researchers say is “the biggest in history looking at birth order and personality.” 

A study of 377,000 high school students by University of Illinois psychologists found that firstborns have an IQ advantage, on average, of just one point over their younger siblings. 

The analysis also showed a pattern of differing personality traits between first-borns and younger siblings – first-borns were, on average, more extroverted, agreeable, and conscientious – but those differences were “infinitesimally small" as well, said Professor Brent Roberts, who led the study.

Statistically, the results correlate with the classic stereotypes developed by psychologist Alfred Adler in the early 20th century: the firstborn as a natural leader; the spoiled, lazier youngest child; and of course the oft-ignored middle child. Adler's theory has been debated among psychologists for years: some, like Frank Sulloway, maintain that Adler was largely correct. Others, such as Jules Angst and Cecile Ernst, claim the effects of birth order have been significantly exaggerated. 

In real life applications, Prof. Roberts says, the differences in personality and intelligence are so minor that they're ultimately insignificant. 

“[I]n terms of personality traits and how you rate them, a 0.02 correlation doesn’t get you anything of note," Roberts explained in a statement. "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,” he continued. “It’s not noticeable by anybody.”

Postdoctoral researcher Rodica Damian, now a psychology professor at the University of Houston, said the large sample size was the key to finding accurate results. This study compared children from different families, whereas most other analyses have only compared children within the same families.

“Such studies often don’t measure the personality of each child individually,” Damian said. “They just ask one child – usually the oldest, ‘Are you more conscientious than your siblings?’”

The problem with this, she explains, 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,” Damian said. 

Damian and Roberts also examined a subset of families within the study, all of whom had exactly three children and two parents living under the same roof, in the hopes of revealing significant differences between the oldest, middle, and youngest siblings. But these differences were also “miniscule,” according to Roberts. 

Ultimately, Damian says, “birth order probably should not influence your parenting, because it’s not meaningfully related to your kid’s personality or IQ.”