Many people have expressed a belief that birth order within a family can have an effect on a person's intelligence or personality, but researchers at the University of Illinois say their large-scale study suggests any differences are so small as to be meaningless.
Psychology professor Brent Roberts along with former postdoctoral researcher Rodica Damian conducted an analysis of 377,000 high school-age students to test the assumption.
They say their study showed first-born children do display a higher IQ and often have differing personality traits than their siblings that come later, but that the differences between first-borns and the "later-borns" are so slight as to have no significant impact on their lives.
Their analysis determined first-borns had a one-point IQ advantage over their following siblings, statistically significant in scientific terms but meaningless in suggesting any practical effects on a person's life.
There have been previous studies of birth order but most had small sample sizes, which made the latest study significant, says Roberts.
"This is a conspicuously large sample size," he says. "It's the biggest in history looking at birth order and personality."
Looking at personality differences, the study found first-borns tended to be slightly more extroverted, conscientious, agreeable and less anxious that later-borns, but that those differences were on a scale of 0.02, or "infinitesimally small," Roberts says.
"In some cases, if a drug saves 10 out of 10,000 lives, for example, small [statistical] effects can be profound," Roberts said. However, he noted, when it comes to personality traits a 0.02 difference is so small as to be invisible, something that wouldn't be apparent to 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 says. "It's not noticeable by anybody."
Damien, who is now a now a professor of psychology at the University of Houston, says she and Roberts controlled for factors that might skew results, including a family's economic level, the number of siblings and their relative ages.
"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," she says.
The study has been published in the Journal of Research in Personality.