To what degree does birth order influence one's personality and odds of success? Are firstborns really more conscientious and perfectionistic, while the babies of the family are more free-spirited and sociable?
A recent study from the University of Illinois at Urbana-Champaign confirmed there are indeed differences in personality and IQ associated with birth order, but found that the differences are so small as to have no practical impact. The findings appeared in the Journal of Research in Personality.
Popular books have made much of a so-called birth-order effect, but research findings have been inconclusive. Some studies have shown sizable differences by birth order in personality traits such as extraversion, while others have found no such effects. Research has more consistently shown a slight IQ advantage for older siblings, the theory being that the more adults and fewer children in a household, the richer the intellectual environment. Still, not all studies of birth order and IQ have found a link.
The Illinois study examined data on 377,000 US high school students. The data, collected as part of a separate longitudinal study, was detailed enough to allow researchers to control for factors such as number of siblings, socioeconomic status, family structure, age, and gender, which some researchers believe may have skewed the results of past studies. For instance, firstborns are more often found in smaller families — the odds of being a firstborn in a two-child family are higher than those of being a firstborn in a five-child family. Wealthier families tend to be smaller, so it's possible that a greater share of firstborns also have access to resources that could affect their IQ or personality.
Among the slight personality differences the researchers found, firstborn children were more conscientious and agreeable, and less sociable and neurotic, than later-born kids. Yet though these differences were statistically significant, they were so small as to be meaningless, the researchers wrote. Firstborns also had slightly higher IQs, but this difference — about one point — was also not enough to be perceptible.
"You wouldn't notice these differences by talking to a firstborn and a later-born who are the same age," said Rodica Damian, lead author of the study, an assistant professor of social psychology at the University of Houston.
To zero in on differences between birth ranks (i.e., first- versus second-born, second- versus third-born, etc.), the researchers also tested a subsample of kids with two siblings who live in two-parent households. While they found greater differences between first and last borns than between neighboring ranks, the differences were still minuscule.
The idea that birth order somehow marks a child is unlikely to disappear. To those who insist they see birth order-based differences among their siblings or kids, Damian suggests age can be a powerful biasing factor.
"Parents will often say their firstborn is more responsible," she said. "But unless you have a video camera and can go back to when the firstborn was the age of the second-born or lastborn, you can't fairly compare. Your personality changes as you age."