MONDAY, Oct. 19, 2015 (HealthDay News) -- Forget what you're heard about birth order determining your adult personality, a new study suggests.
Birth order does not influence any of the "big five" personality traits -- extroversion, emotional stability, agreeableness, conscientiousness or openness to experience, said lead researcher Julia Rohrer, a graduate student at the University of Leipzig in Germany. She and her colleagues reviewed data on more than 20,000 adults from the United States, Great Britain and Germany to arrive at their conclusions.
Firstborns did score higher in intelligence, but Rohrer said she believes this stems more from social interactions within a family than from birth order.
"A firstborn can 'tutor' their younger siblings, explaining how the world works and so on," Rohrer said. "Teaching other people has high cognitive demands -- the children need to recall their own knowledge, structure it and think of a good way to explain it -- which could be a boost to intelligence for some firstborns."
Longstanding psychological theory holds that firstborns are privileged but also burdened with responsibility; middle children are attention-seekers who struggle to find their own identity; and the "baby" becomes spoiled from being showered with affection, Rohrer said.
To test this theory, Rohrer and colleagues reviewed personality and intelligence data on 5,240 Americans, 4,489 Britons, and 10,456 Germans, all gathered by national surveys conducted in each country.
The researchers report they were unable to find any pattern of personality traits based on where a person fell within the family. This finding held whether they lumped all the data together or examined each country separately.
"What is most striking about these findings is that they are in conflict with those of other large studies that have shown that birth order differences in personality definitely exist, even if they are rather modest in magnitude," said Frank Sulloway, an adjunct professor of psychology at the University of California, Berkeley, who specializes in birth order and personality.
Prior research may have mistaken the effect that kids' age has on their personality with some potential effect from the order of their birth, Rohrer said.
For example, Rohrer's own younger sister "often remarks that I act so grown up while she still considers herself immature," she said.
But is that because Rohrer is firstborn, or is it because she's grown out of a similar immaturity she had at that same age?
"We might wrongly confuse age effects with birth order effects," Rohrer said. "The relevant question for detecting a birth order effect would rather be: 'Will my younger sister be at my level of conscientiousness when she is as old as I am now?'"
However, the researchers did find that there are more sibling relationships in which the firstborn is smartest, rather than later-born children.
Rohrer said it's possible that later kids are affected by a lessening of the parent's focus.
"A first-born will enjoy full parental resources, including attention, the second-born will have to share with the firstborn from the start, and it gets even worse for the third-born," she said.
Sulloway believes that the study's reliance on survey data may have diluted the birth order effects that might otherwise have been observed.
The study also did not consider how birth order differences in personality might change with age, he added.
"The nature of these effects in 10-year olds is different from the nature of these effects among people in their 20, 30s, and 40s," Sulloway said. "Thus, laterborns, who are expected to be more peer-oriented than firstborns, may be less conscientious than firstborns when they are relatively young, but as they become older and as they relate to people in peer relationships, they may actually be more conscientious than firstborns."
But Toni Falbo, a professor of educational psychology at the University of Texas at Austin, said the new study is "extremely well done." He added that "the results are consistent with those found by many, many other researchers."
Will psychologists and others give up the idea that birth order matters, in view of these results? "I don't think so," Falbo said. "Birth order theories make intuitive sense, and since most of us on planet Earth today grew up with siblings, some psychologists and the general public will continue to think that birth order really matters."
The study was published Oct. 19 in the Proceedings of the National Academy of Sciences.
For more on child development, visit the U.S. Centers for Disease Control and Prevention.