Birth Order: Does It Really Shape Personality?

What the research says and why the belief still persists

How do personality traits form? Growing up in the same household, why do siblings often develop different attitudes and behaviors? What shapes these differences? One explanation that is commonly offered is birth order. But does birth order really influence personality?

Birth order theory was first introduced by Alfred Adler, an Austrian psychologist and founder of individual psychology, who proposed that an individual’s position in the family influences patterns of behavior and personality development. According to Adler, firstborn, middle, and youngest children experience the family environment differently, leading to distinct psychological adaptations.

My family had three sisters. I grew up as the middle child, with my older sister two years older than me and my younger sister one year younger. We had a relatively stable household, with both parents working and emotionally steady. I don’t recall ever hearing arguments between my parents. If disagreements existed, they were kept private.

Despite growing up in the same environment, the three of us are markedly different from one another. This raises an obvious question: is birth order responsible for these differences? Interestingly, many of our differences seem to align with commonly described birth order patterns. But this does not rule out other explanations. Genetics may play a larger role than birth order alone, or family dynamics may shape behavior in ways that are difficult to isolate. Still, this does not fully explain why certain sibling patterns appear so consistently across families.

In this piece, I focus on birth order and draw from my own family as an example.

Birth order theory suggests that where a child falls in the family can shape behavior and personality, with oldest children often seen as more responsible, middle children as peacemakers, and youngest children as charming and social. But how accurate are these ideas?

Why Birth Order Makes Sense to Families

Birth order is often used as a quick way to explain personality. Within families, it becomes an easy explanation for who is responsible, who is visible, or who seems more socially at ease. These ideas are repeated casually and passed down over time. They feel natural and obvious. But obvious does not always mean accurate.

Families tend to notice differences between siblings, not the reasons behind them. Roles often emerge early in childhood and are remembered selectively, especially when they fit an existing story. Over time, these stories harden into identity. A child becomes “the responsible one,” “the diplomatic one,” or “the social one,” and those labels can follow them long after the family dynamics that shaped them have changed. Let me give you examples of how this played out in our family.

Oldest child

In my family, my oldest sister was always seen as the most responsible and balanced of the three of us. From a very early age, she carried not only her own lunchbox but mine and my younger sister’s as well, and walked us to school since she was two years older. She later became an engineer and was also the first to move to the U.S. for her master’s degree in 1994. She was consistently viewed as the most responsible in our family, especially by my father. She was by far his favorite.

Middle child

As the middle child, I often felt a bit lost. According to my mother, I was cared for by my grandmother at a very young age. My younger sister and I are only 13 months apart, which meant she needed more attention, and for much of my first year, my mother was pregnant. I’m not suggesting neglect, only that my mother was understandably focused on my youngest sister, while my grandmother took care of me.

I think this early experience may have played a role in my avoidant attachment tendencies. Growing up, I also found myself trying to avoid conflict with my sisters. I often stepped into the role of a mediator, between my sisters who fought frequently as children. I was considered my mother’s favorite child, though I believe this attention had more to do with her trying to make up for not being as present during my early years. She also saw me as the most intelligent of her children, largely because I did well in school with relatively little effort.

Youngest child

My youngest sister was the baby of the family. That doesn’t mean she was spoiled. She was treated as the youngest and, at times, pushed aside or even bullied. As a result, she developed a strong sense of what is right and wrong and learned how to stand up for herself, something she still does very firmly.

She also grew up to be charming and extremely social. She has a natural ease with people, makes friends easily, and has a wide circle of friends wherever she goes.

As you can see, many of the traits we exhibited appear consistent with common birth order descriptions: my oldest sister as the responsible one, me as the forgotten and diplomatic one, and my youngest sister as the charming and social one.

But Does Research Actually Support This?

Over the past several decades, researchers have tested birth order theory using large samples and within-family comparisons. Across studies, the findings are remarkably consistent.

Large studies by Stefan C. Schmukle, Marco Del Giudice, and colleagues, published in Psychological Science, examined siblings within the same families and found little evidence that birth order reliably shapes core personality traits.

Research over many years does suggest that firstborns tend to score slightly higher on measures of intelligence than their younger siblings, possibly because they receive more cognitive stimulation from parents early on.

So if research doesn’t support birth order theory, why do these traits feel so widely accepted?

Why Birth Order Traits Persist

Family roles are often confused with personality traits. In many families, children slowly take on roles that make sense within the household. One child may be expected to be responsible, another may receive less attention, and another may learn to be social or adaptable. These expectations are not usually stated out loud, but they are felt and reinforced over time.

As children grow, their behavior adapts to these roles. Expectations often reinforce the behaviors that are expected, even when those behaviors do not reflect core personality traits. A child who is expected to be responsible may act that way more often. A child who feels less visible may become more independent or avoid conflict. Another may learn to connect easily with others because that brings attention or ease. These behaviors help children navigate family life, even if they do not reflect deep or fixed personality traits.

Families naturally prefer simple explanations over complex ones. Saying “this is just how they are” feels more comfortable than sitting with uncertainty. Birth order provides a clear and familiar story that ties everything together. It offers meaning and coherence in situations that are otherwise complicated, emotional, and hard to explain.

Birth order may not shape personality the way we think, but the stories families tell still shape how people see themselves.