The question of whether siblings’ birth order fundamentally shapes their personality has captivated families and psychologists for decades. However, the evidence proves far more complex than you might expect.
As the eldest daughter of two, I frequently recognize myself in the traits stereotypically attributed to oldest siblings: responsible, conscientious, perfectionist. My mother, also an eldest daughter, embodies these same characteristics. My younger sister, conversely, maintains a considerably more carefree approach to life—despite the fact that we were raised in identical circumstances by the same parents and remain close, our personalities diverge dramatically.
This observation led me to wonder whether such differences might stem from our birth order—is there legitimate scientific foundation to the notion that being the oldest, youngest, or only child fundamentally influences who we become and how we navigate the world?
Despite captivating both the scientific community and general public for more than a century, the question of whether birth order among siblings shapes personality remains vigorously debated.
Historically, research has yielded inconsistent and often contradictory findings. Multiple factors contribute to this confusion, though the fundamental challenge can be summarized simply: it’s extraordinarily difficult to measure accurately. Rodica Damian, associate professor of psychology at the University of Houston, Texas, explains that previous studies frequently suffered from inadequate sample sizes. Additionally, since personality assessments typically rely on self-reporting, they remain vulnerable to various forms of bias.
Recent investigations highlight numerous confounding variables that complicate efforts to determine whether birth order effects are systematic—meaning they affect every individual identically. The total number of siblings represents one significant factor: family dynamics in a two-child household would presumably differ dramatically from those in a seven-child family. Being the eldest or youngest child in these differently sized families might constitute vastly different experiences, rendering direct comparisons problematic.
Family size and the experience of childhood within any given household may intertwine with countless other variables, such as socioeconomic status (wealthier families with higher socioeconomic standing tend to have fewer children, for instance). Furthermore, individual age and gender could profoundly influence experiences both within the family unit and beyond.
Within this complex context, researchers have failed to establish that birth order produces consistent, universal impacts on personality. However, this doesn’t render birth order entirely irrelevant—it may indeed play significant roles within specific families or cultures.
“I think people harbor numerous beliefs that are somewhat outdated, or that were never adequately supported initially,” observes Julia Rohrer, a personality researcher at Leipzig University in Germany. “For example, the ‘eldest daughter syndrome’ phenomenon represents a prominent example—naturally, women often continue to occupy different roles and face expectations to provide more care. Additionally, first-borns are expected to assume responsibility for younger siblings,” she explains. “For some women, this might perfectly capture their experience, but for others it doesn’t because every family operates differently.” In essence, not every eldest daughter will demonstrate responsibility and caring—but for some, the concept of “eldest daughter syndrome” may resonate because they genuinely experienced childhood responsibilities caring for younger siblings and believe this experience fundamentally shaped their character.
Rohrer and her colleagues discovered that birth order “does not exert lasting effects on broad personality traits” after analyzing three comprehensive datasets from surveys conducted in the UK, US, and Germany, each containing data from several thousand participants. However, the study did confirm previous findings regarding birth order’s impact on one specific characteristic: intelligence.
Intelligence represents a complex phenomenon, and the study exclusively measures it through performance on intelligence tests and self-reported intellectual capacity. “We confirmed the effect that firstborns score higher on objectively measured intelligence and additionally found a similar effect on self-reported intellect,” Rohrer and her colleagues documented in their study. Previous research had established that intelligence test performances “decline slightly from firstborns to later-borns.”
Regarding birth order and other personality traits, Rohrer suggests that personal reflection remains meaningful despite the absence of universal patterns: “It does provide a framework where you can connect with others who experienced similar circumstances, allowing for experience sharing and mutual understanding,” she says of terms like “eldest daughter syndrome.” Nothing problematic exists in framing your experience this way, “provided you don’t assume this experience is universal,” she emphasizes.
Damian echoes this sentiment: “While we don’t discover personality differences that are systematic, that doesn’t preclude social processes within individual families or cultures that can produce different outcomes based on birth order.”
Consider, for example, the UK’s historically male-preferring primogeniture tradition, where the eldest child would inherit family wealth, estates, or titles. Only in 2013, with passage of the Succession to the Crown Act, did primogeniture within the monarchy end, eliminating a male heir’s power to displace an elder daughter’s rightful claim to the Crown. The primogeniture concept proves surprisingly widespread and persistent: in Succession, the HBO satirical comedy-drama about a family’s battle for media empire control, one character declares “I’m the eldest boy!” in the finale. He believes his birth position should entitle him to assume his father’s CEO position. (He’s actually the second-eldest son, but that’s another matter entirely).
“If social practices are founded on birth order, then absolutely, birth order will impact your outcomes,” Damian confirms.
Age-related experiences can easily be mistaken for personality traits or behaviors influenced by birth order, researchers explain. Consider the older, “responsible” sibling as an illustration: “As people mature, they become more responsible, more self-controlled. So, the firstborn will always be chronologically older than later-born siblings, and as you observe your children’s development, the firstborn will invariably appear more responsible,” Damian notes.
“Another factor is that people become increasingly self-conscious with age,” she adds. “So the second-born might seem more sociable and less neurotic, because a 10-year-old is typically much happier and more confident… compared to the 14-year-old, who’s anxious about everything. That stems from facing different developmental challenges.”
Additional factors such as children’s social circles also matter significantly. Multiple studies demonstrate connections between delinquent peers and delinquent behavior, for instance, so an older child might actually be more rule-breaking depending on their chosen companions.
As previously mentioned, one consistent finding emerging from birth order research involves the relationship between birth order and intelligence, with firstborns averaging slightly higher scores on intellect-related assessments than younger siblings. “[The intelligence connection] primarily manifests in verbal intelligence test results, and produces a very modest effect,” Damian explains. Moreover, “if you took a test twice, you’d probably score differently depending on the day or mood, whatever you consumed that morning, or how long you slept.”
This phenomenon might also be explained by cognitive stimulation during early childhood years. Damian notes that the higher the adult-to-child ratio in a family, the greater their exposure to mature language and vocabulary. However, as more siblings enter a family, intellectual stimulation levels might decrease. “So it’s not necessarily that they’re genetically more intelligent or possess greater potential—it’s more that this translates into higher verbal IQ scores on tests, which could result from knowing more words because more adults versus children communicated with them,” she explains. “With two children, perhaps some reading time gets consumed managing sibling interactions where verbal input becomes less sophisticated.” Research also suggests that as older siblings tutor or explain concepts to younger siblings, they engage “more cognitive resources.”
Intriguingly, these intelligence patterns don’t replicate globally. Data from developing nations differs significantly from developed country findings. In Indonesia, later-born siblings are more likely to receive superior educational opportunities than older siblings, potentially due to financial constraints that ease only when older siblings begin contributing to family income.
According to Damian and her colleague, birth order produces “negligible effects” on career paths as well. Previously, scientists believed older siblings would pursue more academic or scientific careers, while younger ones would choose more creative paths. However, Damian discovered the opposite: in her longitudinal study examining US high school students from 1960 and the same participants 60 years later, first-borns ultimately entered more creative professions.
Only children frequently face stereotypes of being more selfish than children with siblings, supposedly because they don’t compete for parental attention. Recent studies, however, demonstrate this isn’t accurate, and that growing up without siblings doesn’t lead to increased selfishness or narcissism. Additional research suggests that social behavioral differences between only children and those with siblings “are not large or pervasive, and may grow smaller with age.”
Birth order research has traditionally excluded only children on grounds that they cannot be fairly compared to children raised with siblings. However, comparing personality traits of siblings and only children is possible, according to a 2025 paper by Michael Ashton, professor of psychology at Brock University, Canada, and Kibeom Lee, professor of psychology at the University of Calgary, Canada.
Their study presented remarkable new findings. It examined associations between personality, birth order, and sibling numbers in 700,000 adults online in one sample and more than 70,000 in another separate sample. Middle-born and last-born siblings averaged higher scores on “Honesty-Humility” and “Agreeableness” scales than first-born siblings. “Honesty-Humility” measures how honest and humble someone is, meaning high-scoring individuals are unlikely to manipulate others, break rules, or feel entitled. Low-scoring persons may be more inclined to rule-breaking and may experience strong self-importance. On the agreeableness scale, high-scoring individuals tend to be forgiving, lenient in judging others, even-tempered and willing to compromise, while low-scoring people may hold grudges, be stubborn, quick to anger, and critical of others.
“These differences were quite small in magnitude, particularly when comparisons involve people from families having identical numbers of children,” Ashton and Lee explain in an email. “In contrast, differences in these dimensions between persons from one-child families (i.e., only children) and persons from six-or-more-child families were considerably larger, somewhere between the sizes that social scientists would classify as ‘small’ and ‘medium’.”
So, I inquire, is birth order’s influence merely a zombie theory—a concept that is incorrect but refuses to disappear? Rohrer disagrees. “I’m uncertain whether I’d characterize it as a zombie theory,” she responds. “From the scientific perspective, I believe the literature is advancing quite productively.”
Therefore, we may eventually achieve clearer understanding of what being an eldest daughter truly means. Until then, I’ll continue allowing my younger sister to believe I’m inherently more intelligent than her.
Author: AI
Published: 21 June 2025