The debate over how birth order influences personality has persisted for years. But do selfish tendencies really occur more often in only children? New findings suggest they might.
A recent study conducted by Michael C. Ashton and Kibeom Lee from the University of Calgary has found that middle-born children with more siblings tend to score higher in traits such as honesty-humility and agreeableness. Historically, research on birth order and personality has often overlooked direct comparisons between only children and those raised with siblings.
This latest research distinguishes itself by examining whether personality traits correlate with birth order and the number of siblings, addressing gaps in earlier studies. Over 700,000 adults participated in the study, which used a personality inventory to evaluate key dimensions.
The findings revealed that traits linked to cooperativeness—specifically honesty-humility and agreeableness—were highest among middle-born participants, followed by last-born, first-born, and only children, in that order. Replication of the assessments confirmed the same pattern: individuals with more siblings exhibited higher levels of these prosocial traits.
“When we noticed that middle-borns averaged highest and only children averaged lowest in these personality dimensions—with ‘oldests’ and ‘youngests’ in between—we wondered whether the differences were mainly a reflection of sibship size. After all, ‘middles’ are always from families with at least three children, whereas oldest and youngest children are in many cases from families with exactly two children,” University of Calgary researcher Kibeom Lee said in an email to The Debrief.
To investigate further, the researchers included a question about sibship size. The results showed that, on average, prosocial traits increased slightly as the number of siblings grew, with modest gains observed across families with one to six or more children.
“We suspect that when people grow up with more siblings, there’s a greater need to cooperate, which might then lead people to develop a slightly more cooperative personality. But we emphasize that these are modest differences that occur on average—many people are exceptions to these trends,” the researchers explain.
Among participants with the same number of siblings, first-born individuals scored slightly lower on honesty-humility and agreeableness compared to middle-born and last-born individuals. The findings suggest a connection between growing up with more siblings and developing a more cooperative personality.
The study also accounted for variables such as socioeconomic status and cultural influences.
“We examined whether the religiousness of people’s upbringing was responsible for the association of sibship size with these personality traits. We found that when the religiousness of upbringing is controlled, the tendency for people from larger families to be higher in Agreeableness and Honesty-Humility mostly remains, but it does become somewhat weaker,” Lee told The Debrief.
Social changes play a significant factor in family sizes and the country they are living in. The decline of social class levels could or could not play a part in cooperative traits in future generations.
“Other things being equal, we might expect a slight decline in the average levels of these cooperative traits as families become smaller. But the differences would be small, and it’s possible that other societal changes would also influence these traits, whether in similar or in opposite ways.
“One other difference that might result from smaller families is that the average level of intellectual curiosity might increase—we found that people who grew up as only children were slightly more inquisitive than people who grew up with siblings,” Lee said.
The study, “Personality differences between birth order categories and across sibship sizes,” was published in PNAS on December 23, 2024.
Chrissy Newton is a PR professional and founder of VOCAB Communications. She currently appears on The Discovery Channel and Max and hosts the Rebelliously Curious podcast, which can be found on The Debrief’s YouTube Channel on all audio podcast streaming platforms. Follow her on X: @ChrissyNewton and at chrissynewton.com.