But the correlation on those personality differences is so tiny that it really doesn't speak to any noticeable effect between individuals born first and those born later.
"In terms of personality traits and how you rate them, a 0.02 correlation doesn't get you anything of note," University of Illinois psychology professor Brent Roberts, the study's lead author, said 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. It’s not noticeable by anybody."
Added co-author Rodica Damian: "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."
The researchers say their study is the biggest ever conducted on the subject of birth order, IQ and personality. It's also unusual in another way: Instead of comparing siblings from within the same family, as many smaller studies on the subject have done, it compared each individual participant to the rest of the sample. The study's authors also controlled for a number of factors that could have influenced the results: Family size, parental socioeconomic status, family structure, age and gender.
"[Within-family] studies often don't measure the personality of each child individually," said Damian, who was a postdoctoral researcher at the University of Illinois when the study was undertaken and is now a professor of psychology at the University of Houston. "They just ask one child, usually the oldest, 'Are you more conscientious than your siblings?'"
According to 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."
The authors suggested that a within-family study with a large sample size and a longitudinal approach -- in other words, one in which the researchers measured the oldest kid, and then waited for their younger siblings to grow up and reach the same age before being measured -- could also be an effective approach to looking at the same question.
The notion that birth order can determine a lot about your personality stems from the work of Alfred W. Adler, a onetime colleague of Sigmund Freud.
As the study notes, Adler's ideas about birth order were at the center of a huge disagreement that led to Adler's resignation from the Psychoanalytic Society. He eventually branched off on his own and founded the discipline of individual psychology.
The fact that Adler, like Freud, characterized first-born children as neurotic messes and middle children (of which Adler was one) as being relatively healthier probably had something to do with it.
Since then, the basis for those explanations has changed, but familiar breakdowns of personality traits that supposedly apply to each sibling by order of birth have been popularized by an entire genre of parenting books and pop-psychology resources.
That popularity often seems to have outpaced the actual evidence supporting the theory, something Damian and Roberts's work seems to underlines.