A large-scale study has uncovered links between personality traits, birth order, and family size. Published in the Proceedings of the National Academy of Sciences, the research analyzed data from hundreds of thousands of participants and found that individuals from larger families tend to exhibit higher levels of traits related to honesty and agreeableness. Additionally, middle-born children scored highest on measures of cooperation and modesty compared to their older, younger, or only-child counterparts.
The notion that birth order influences personality has fascinated scientists and the public for decades. Traditional stereotypes suggest that firstborns are conscientious leaders, while youngest siblings are rebellious and attention-seeking. However, many earlier studies—often using limited personality measures—failed to find significant evidence supporting these ideas. This led some researchers to dismiss birth order as an influential factor in personality development.
In their new study, Michael Ashton of Brock University and Kibeom Lee of the University of Calgary revisited the question with two aims. First, they wanted to examine whether broader and more precise personality dimensions, such as those in the HEXACO model, could reveal patterns missed in earlier work. The HEXACO framework measures six key traits: Honesty-Humility, Emotionality, Extraversion, Agreeableness, Conscientiousness, and Openness to Experience. Second, the researchers sought to understand whether family size, a related but often overlooked variable, influences personality.
“Most previous research had found that adults’ personality trait levels were unrelated to their birth order,” Ashton and Lee told PsyPost. “When we started collecting online personality data about 10 years ago, we decided to include birth order in our survey, because we were measuring a wider array of personality traits than had been examined in previous research. We found—somewhat to our surprise—that birth order was related to some personality traits, and we then added a question to our survey about sibship size (i.e., number of siblings, including oneself).”
Ashton and Lee relied on two large-scale datasets collected through an online personality assessment platform, hexaco.org. The first sample included over 710,000 adults, while the second sample consisted of nearly 75,000 participants. Both samples were composed mainly of English-speaking participants from countries like the United States, Canada, the United Kingdom, and Australia.
In addition to completing the HEXACO personality inventory, participants provided demographic information, including their birth order (categorized as only child, oldest, youngest, or middle child) and their age and gender. In the second sample, participants also indicated the number of children in their family, referred to as sibship size.
To ensure the reliability of their data, the researchers implemented strict screening procedures. Participants had to complete all 100 HEXACO items, pass three attention-check questions, and provide consistent demographic responses. For the second sample, additional checks ensured that participants’ reported birth order aligned with their sibship size (e.g., an “only child” could not come from a family with more than one child).
The researchers also controlled for several confounding factors. Age and gender were statistically controlled in both samples, ensuring these variables did not unduly influence the results. In the second sample, the researchers also considered participants’ religious upbringing and current religiousness, as these factors are known to correlate with traits like agreeableness and honesty.
The analysis revealed several consistent patterns linking personality traits to both birth order and family size. Middle-born participants scored highest on Honesty-Humility and Agreeableness, traits that reflect modesty, fairness, and cooperative behavior. These findings held true across both samples, with middle children scoring approximately 0.20 standard deviations higher than only children on these traits. Following middle-borns, the rank order of scores for these traits was youngest children, oldest children, and only children, who scored the lowest.
Family size also emerged as a significant factor. Participants from larger families tended to exhibit higher levels of Honesty-Humility and Agreeableness. For example, in the second sample, participants from families with six or more children scored 0.30 to 0.36 standard deviations higher on these traits compared to those from one-child families. These findings suggest that growing up with more siblings may foster social and cooperative tendencies, potentially due to increased opportunities for interaction and the need to share resources and responsibilities.
“We found that for two personality dimensions—Honesty-Humility and Agreeableness—middle-borns averaged highest and only children averaged lowest, with ‘oldests’ and ‘youngests’ in between,” Ashton and Lee explained.
“However, these differences were mainly a reflection of how many children were in one’s family when growing up—what researchers call ‘sibship size.’ (Note that ‘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, and only children by definition are from families with one child.)”
“On average, people from larger families tended to be higher in Honesty-Humility and Agreeableness,” the researchers said. “When we compare people from families with the same sibship size, the birth order differences became rather small, although middles and youngests tended to be slightly higher in these traits than oldests did.”
Interestingly, the study found that only children scored higher on Openness to Experience, a trait associated with intellectual curiosity and creativity. Within families of the same size, oldest children also tended to score slightly higher on Openness compared to their younger siblings. These differences in openness may stem from the greater intellectual stimulation often experienced by only children or the added responsibilities and expectations placed on firstborns.
While differences in other personality traits were generally smaller, the study noted some additional patterns. For instance, middle children tended to score slightly higher on Extraversion compared to only children, and older siblings were marginally more conscientious than their younger counterparts. These findings, though less pronounced, further highlight the subtle ways in which family dynamics can shape personality.
The researchers also found that religious upbringing partially mediated the relationship between family size and cooperative traits. Participants from larger families were more likely to report a religious upbringing, which in turn correlated with higher levels of Honesty-Humility and Agreeableness. However, even after controlling for religious influences, the link between family size and these traits remained significant, suggesting that other factors, such as sibling interactions, play a role.
“Although we found significant effects of sibship size and birth order on prosocial personality traits, we emphasize that these effects are modest differences that occur on average—many people are exceptions to these trends,” Ashton and Lee told PsyPost.
“One way to think of it is like this: If you choose at random someone who was an only child and someone who grew up in a family of six or more kids, there’s a 60% chance that the more agreeable or cooperative person of these two will be the latter (as opposed to 50% if there were no difference). So, 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.”
Additionally, the study focused primarily on English-speaking participants, most of whom were from Western countries. Cultural differences in family dynamics and parenting practices might influence how birth order and family size affect personality in non-Western contexts.
“We are planning to investigate how robust these findings are in non-English-speaking countries,” the researchers said.
The study, “Personality differences between birth order categories and across sibship sizes,” was published December 23, 2024.
