No link between order of birth and personality or IQ, study finds


The age old myth that birth order influences traits like personality and intelligence doesn’t really hold true, a new research has established.

In a bid to check whether the age old myth holds true or not, researchers studied 377,000 high school students and found that though first-borns do have higher IQs and consistently different personality traits than those born later in the family chronology, the differences between first-borns and “later-borns” are so small that they have no practical relevance to people’s lives.

Researchers found that first-borns enjoy a one-IQ-point advantage over later-borns, but researchers say that even though the difference is statistically significant it is also meaningless.

Further, the researchers also observed consistent differences in personality traits between first-borns and later-borns with the first-borns being more extroverted, agreeable and conscientious, and had less anxiety than later-borns; however, the differences were “infinitesimally small” amounting to a correlation of just 0.02.

University of Illinois psychology professor Brent Roberts, who led the analysis with postdoctoral researcher Rodica Damian (now a professor of psychology at the University of Houston), said “It’s the biggest in history looking at birth order and personality.”

Roberts said that in terms of personality traits and how you rate them, a 0.02 correlation doesn’t get you anything of note.

The study controlled for potentially confounding factors – such as a family’s economic status, the number of children and the relative age of the siblings at the time of the analysis – that might skew the results, Damian said.

For example, wealthier families tend to have fewer children than other families, and so have a higher proportion of first-borns who also have access to more resources that may influence their IQ or personality, she said.

Many previous studies of birth order suffered from small sample sizes, Damian said. Many compared children with their siblings – a “within-family” design that some assert is better than comparing children from different families, as the new analysis did.

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

The results differ depending on whom you ask, she said.

“Another major problem with within-family studies is that the oldest child is always older,” Roberts said. “People say, ‘But my oldest kid is more responsible than my youngest kid.’ Yes, and they’re also older.”

An ideal within-family study would follow the families over time, collecting IQ and personality data from each child when he or she reached a specific age, the researchers said.

The team also evaluated a subset of the children in the study – those with exactly two siblings and living with two parents. This allowed the researchers to look for specific differences between first- and second-borns, or second- and third-borns.

The findings confirmed those seen in the larger study, with specific differences between the oldest and a second child, and between second and third children. But the magnitude of the differences was, again, “minuscule,” Roberts said.

“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,” Damian said.

The findings of the study have been published in the Journal of Research in Personality.