Middle children are more cooperative than their siblings, study suggests

The debate has raged for more than a century: does birth order help to shape personality, or are conscientious firstborns and creative youngest children flawed stereotypes based on flimsy evidence?

After decades of contested claims, a handful of recent studies found there was little evidence for meaningful differences. But in a study published on Monday, psychologists have pushed back and claim there is an effect after all.

In one of the largest studies ever conducted on birth order, family size and personality, Canadian researchers gathered data from more than 700,000 volunteers and found that on average, middle children scored higher than their siblings on traits seen as important for cooperation.

Scores for the same traits were also higher in families with more children, suggesting that people may be more likely to develop a cooperative personality when they are raised as part of a bigger group.

The effects are not large, but Michael Ashton and Kibeom Lee, psychology professors at Brock University in Ontario and the University of Calgary in Alberta respectively, believe they challenge the idea that birth order and the number of children raised together have no meaningful impact on personality.

“The weight of that evidence now indicates that personality trait levels do differ as a function of birth order and sibship size,” they write in Proceedings of the National Academy of Sciences.

Researchers have speculated on the impact of birth order for more than a century. In 1874, the polymath Francis Galton, the youngest of nine siblings, gathered histories on a group of English scientists and found a large proportion were firstborns. He suspected the eldest received more attention from their parents, propelling them to greater intellectual heights.

Decades later, the Austrian psychologist Alfred Adler claimed firstborns were often conscientious and responsible, while the youngest might become independent and creative as they looked for ways to stand out. Middle children he saw as peacemakers, though others regarded them as “forgotten children”: the Lisa Simpsons who are often overlooked.

Ashton and Lee analysed personality traits reported by more than 700,000 English speakers who also gave details on whether they were a firstborn sibling, a middle child, the youngest or an only child. A separate group of 75,000 volunteers completed the same questions, along with the number of children they were raised with.

Previous studies have found evidence for firstborns being slightly smarter than late-born children, and the Canadian study saw this, too. But the researchers spotted other differences. People with more siblings tended to score higher on two traits linked to cooperation, namely agreeableness and what the scientists call honesty-humility, or the tendency to be fair and genuine with others. Middle children seemed to receive a further boost, scoring a little higher than the firstborns and youngest siblings.

The findings suggest that if an only child and a person from a family of six were chosen at random, there is a 60% chance that the more agreeable would be from the family of six. “You can’t tell much about the personality of a given individual from their birth order or family size, even though there are clear differences when averaging across many people,” Lee said.

While the number of siblings was the main factor shaping the personality traits, birth order mattered, too. “These differences were largely accounted for by sibship size effects,” Ashton said. “However, the birth order differences could not be entirely explained by sibship size, which indicates that there is also a small birth order effect on cooperative personality traits, with middles and youngests averaging slightly higher than oldests.”

If the effects are real, some drivers could be intuitive, the authors write: that having more siblings fosters a more cooperative personality, while being a middle child calls for good bonds with younger and older siblings.