Family matters: Birth order doesn’t affect personality, IQ among siblings, study suggests

WATCH: It turns out birth order has less to do with your personality and intelligence than originally thought. Dr. Rodica Damian explains the results of a new study, believed to be the largest of its type.

It’s been drilled into your head: eldest children are ambitious and responsible, the youngest is reckless and fancy-free while middle kids are sandwiched in between and hiding in the shadows.

A new study is debunking these common stereotypes. In what they say is the largest study conducted on sibling order, American scientists suggest that birth order doesn’t affect your personality and intelligence.

Researchers out of the University of Illinois say that while first-borns tend to have a slightly higher IQ, the differences between siblings are so small they offer no “practical relevance” to people’s lives.

Their findings are based on studying 377,000 high school students to zero in on their intelligence and personality traits.

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“This is a conspicuously large sample size. It’s the biggest in history looking at birth order and personality,” psychologist and lead researcher, Dr. Brent Roberts, said in a statement.

Turns out, there were only subtle differences in personality traits between first-borns and youngest-borns.

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First-borns tend to be more extroverted, agreeable, and conscientious. They have less anxiety than later-borns, too, the study suggests. But the differences were “infinitesimally small” – a correlation of about 0.02. It wasn’t enough to make any blanket statements about birth order, the scientists say.

“In some cases, if a drug saves 10 out of 10,000 lives, for example, small effects can be profound. But in terms of personality traits and how you rate them, a 0.02 correlation doesn’t get you anything of note. 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,” Roberts said.

The study controlled for economic status, number of children, relative age of the siblings and any other factors that might skew the results. Previous studies pull on small sample sizes, the researchers note. Others compare siblings within a family instead of comparing kids from different families.

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“But such studies often don’t measure the personality of each child individually. People say, ‘But my oldest kid is more responsible than my youngest kid.’ Yes, and they’re also older,” Roberts said.

To avoid this, the researchers collected IQ and personality data from the kids when they reached specific ages. Even then, the disparities were “minuscule.”

“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,” co-author, Dr. Rodica Damian, said.

The team’s full findings were published this weekend in the Journal of Research in Personality.

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The findings follow research out last month that suggested that parents’ perceptions of their kids may influence their life trajectories.

“Parents’ beliefs about their children, not just their actual parenting, may influence who their children become,” Alex Jensen, a Brigham Young University professor, said.

“It’s hard for parents to not notice or think about differences between their children, it’s only natural. But to help all children succeed, parents should focus on recognizing the strengths of each of their children and be careful about vocally making comparisons in front of them,” Jensen said.

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It’s subtle, but the differences in perception shape who your children become, they warned. If parents believed their child was the smarter one, he or she tended to get better grades. The less capable child, in his or her parents’ eyes, didn’t fare as well.

They also found a difference in grades. There was a disparity of about 0.21 in GPA.

“That may not sound like much. But over time those small effects have the potential to turn into siblings who are quite different from one another,” Jensen said.

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The only instance in the study that was an exception was when the eldest was a brother and the second-born was a sister – the daughters were always more competent in the classroom from their parents’ perspective.

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