Could Your Personality Be Determined by Your Birth Order?

When you think about your childhood, how much do relationships with your siblings come to mind? Or, what if you had no siblings, and could therefore claim the status (and the throne?) as the only child?

If you’re a parent now, do you ever compare your children to see who’s nicest, smartest, most cooperative, or most competitive? Conversely, as the parent of an “only,” do you worry that you’ve raised, or are raising, someone who’s destined to be selfish and entitled?

The History, and Frustrations, of Birth Order Research

The idea that birth order serves to influence, or maybe determine, an individual’s personality can be traced in psychology to the work of Alfred Adler, a disciple of Sigmund Freud, who proposed that there were stark differences among only, middle, and youngest children. His work later became the foundation for what would become the empirical quest to validate these ideas. However, those working in the field eventually became stymied by the endless statistical problems that such research inevitably involves (comparing different combinations of boys/girls, ages, etc.).

To make things worse, there is the cause-and-effect problem of comparing families where parents decided to have children one after the other, to have small families, or to be unable or unwilling to have more than one child. When you add situations such as adoption, blended families, and the role of extended family members, it’s no wonder that this work ran into insurmountable roadblocks.

Now, though, there are more advanced statistical ways to address the problem. These involve new measures of personality as well as the availability of ways to impose statistical controls on the many potentially confounding variables. Taking advantage of these methods, the latest entry into the fray provides a new twist intended to resolve the issue, if not once and for all, then at least with a bit more confidence.

The Latest Birth Order Study

Brock University’s Michael Ashton and University of Calgary’s Kibeom Lee (2025) decided to tackle the birth order question with a measure that they have developed and continue to publish known as the HEXACO Personality Inventory (Revised). Their online data collection tool continues to gather data and, by the time of their 2025 publication, the authors had at their disposal more than 770,000 adult participants.

HEXACO stands for the personality traits of honesty-humility, emotionality (the opposite of neuroticism), extraversion, agreeableness, conscientiousness, and openness to experience. The most recent study on birth order using another, and shorter, personality trait measure failed to produce significant findings. The Canadian authors believed that their HEXACO measure might prove more sensitive given that it’s longer and provides a “broader sampling” of personality traits.

Start imagining now the task that faced Ashton and Lee as they delved through their extensive data set. First, you have to separate the only children from children with siblings. Then you have to look at actual birth order (first, middle, last) and, from there, consider family size. Other issues might come to mind as ones you’d want to consider, such as the religiosity of the parents who are producing these children, given that family sizes can be affected by whether parents believe in birth control and/or the value of a large family.

Because massive samples can produce significant differences when the effects, in reality, are teeny-tiny, the authors used a statistic known as “d,” which measures effect size independently of sample size. A value of .2 is small, .5 is medium, and .8 is large. Ashton and Lee report that most differences were in the neighborhood of .2, Eventually, after taking religiosity of parents into account, the difference whittled down to .1 on the traits of agreeableness and honesty-humility. Even then, the largest effects emerged for families with two children and those with six.

Perhaps the most reliable finding from this investigation could be boiled down to greater cooperativeness in the children from larger families. As the Canadian research team concluded, “a commonsense possibility is that when one has more siblings, one must more frequently cooperate rather than act on selfish preferences.” This still begs the question of birth order effects within larger families. Sadly for the research team, but maybe predictably, any differences between sibs based on position did not pan out.

Returning to the family size question, aside from religiosity, the authors point to another possible complicating factor. Parents who have cooperative and agreeable children might decide that they should continue to grow their family; those whose children turn out to be difficult may cut themselves off at one. Although the Canadian team dismisses this possibility within their dataset, they do note that “There is some evidence that parents are more likely to have another child when their current child tends to be prosocial.”

One other intriguing finding not related to these prosocial qualities involves openness to experience, or intellectual curiosity. Oldest and only children were higher on this quality than younger and middle children, again, though, by a small degree. However, oddly enough, there were more oldest than younger siblings who completed the survey: “this ratio reflects a tendency for oldests to be more inquisitive than nonoldests, because persons who voluntarily complete an online personality inventory tend to be more intellectually curious.”

Is There a Future for Birth Order Research?

All of these qualifications if not less than eye-popping results may leave you feeling that perhaps this field of research will never produce simple answers. And why should it? As you reflect back on your early experiences, or perhaps those with your own children now, the position you occupy by virtue of the accident of your time of birth tells only part of the story. Nice people may come from larger families, where everyone had to share, but life presents many opportunities beyond the earliest family environments to shape personality. As shown in prior research on change in adulthood, people have many small and large experiences that can stimulate growth.

To sum up, it may be fun to entertain the idea that only children, oldest children, youngest children, and the easily overlooked middle children differ in fundamental ways. However, your own fate and that of those in your family, reflect a far more complex and—even more entertaining—set of possibilities.