You read about it in popular media or see first-hand how birth order has a potent impact on personality. The first-born is the achievement-oriented leader. The youngest is the attention-seeking rebel, and so on.
But it may come as a surprise that there is no scientific basis behind the notion that birth order shapes personality. In fact, birth order appears to mean little or nothing in terms of what kind of people we are.
“The theory that birth order impacts personality development has been widely debunked,” said Kevin Triemstra, a pediatric psychologist at Akron Children’s Hospital.
The topic has been debated and studied for generations. Two large studies published in 2015 found no relationship between birth order and personality traits.
One of the studies – involving about 20,000 people in the United States, Britain and Germany – said specifically that birth order had no effect on outgoingness, emotional stability, agreeableness, conscientiousness or imagination. The researchers found, however, that birth order did have an impact on intelligence (earlier-born children had higher IQs than later-born kids).
“But there is still a lot of debate on whether that is true,” said Dr. Triemstra. “Several studies of national samples in the U.S. and U.K. showed minimal influence of birth order on intelligence. So there isn’t conclusive evidence either way.”
So why is the belief that family position affects personality so entrenched?
Dr. Triemstra said we see evidence that validates our beliefs, and we tend to ignore evidence to the contrary. Some families do fit the stereotype, but many do not.
“A lot of people still believe birth order has a significant impact on certain personality traits,” he said. “There’s a confirmation bias at work. If I believe birth order affects personality, I make mental notes when I see families that fit that pattern. I dismiss information that doesn’t fit that pattern.”
Assigning such significance to birth order is akin to using astrological signs to help explain personality traits, he said.
“We like to know why people do certain things, why they behave a certain way or why we do things we do. It helps us make sense of our own personality.”
Rethinking these assumptions can be a good thing.
“I think sometimes when we talk about the way we act, we think of it as something that can’t change,” he said. “People say, ‘That’s just how I am. I’m short-tempered because of my sign.’ Well, you don’t have to be.”