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Decades of research yield mixed conclusions
For over a century, psychologists have debated whether a child's position in the family-eldest, youngest, or only-shapes their character. Yet despite widespread assumptions, scientific evidence remains inconclusive.
The challenge of measuring birth order effects
Rodica Damian, a psychology professor at the University of Houston, highlights key obstacles in studying birth order. Many earlier studies relied on small sample sizes, and self-reported personality tests may skew results due to personal bias.
The five-factor model-extraversion, agreeableness, conscientiousness, neuroticism, and openness-serves as the standard framework for assessing personality. However, researchers caution that variables like family size, socioeconomic status, and cultural norms complicate efforts to isolate birth order's influence.
Intelligence shows slight birth order link
While broad personality traits show no consistent pattern tied to birth order, studies have found a modest correlation with intelligence. Julia Rohrer of Leipzig University and her team analyzed large datasets from the UK, US, and Germany, confirming that firstborns tend to score higher on intelligence tests and self-reported intellect.
Damian suggests this may stem from early cognitive stimulation. Firstborns often receive more one-on-one attention from adults, exposing them to richer language and vocabulary. However, she notes that these differences are minor and can vary by day or context.
Cultural and family dynamics play a role
Though universal patterns remain elusive, birth order may still shape experiences within specific families or cultures. For example, eldest daughters in some households are expected to care for younger siblings, reinforcing traits like responsibility. Yet Rohrer warns against assuming such roles apply universally.
Historical practices like primogeniture-where the eldest child inherits wealth or titles-illustrate how birth order can influence life outcomes. Damian points out that when social structures prioritize birth order, its impact becomes tangible.
Age and environment may overshadow birth order
Researchers emphasize that age-related development often mimics birth order effects. Older siblings naturally appear more responsible simply because they are older. Similarly, a 10-year-old's carefree demeanor may contrast with a 14-year-old's self-consciousness, regardless of birth order.
External factors, such as peer groups, also play a role. Studies link delinquent behavior to social circles, suggesting that an older sibling's rule-breaking tendencies may stem from their environment rather than their position in the family.
Only children and middle siblings defy stereotypes
A 2025 study by Michael Ashton and Kibeom Lee, involving over 700,000 adults, found that middle- and last-born siblings scored slightly higher on honesty-humility and agreeableness than firstborns. However, these differences were minimal in families with the same number of children.
Only children, often stereotyped as selfish, showed no significant increase in narcissism or selfishness compared to those with siblings. Ashton and Lee's research also revealed larger personality differences between only children and those from very large families, suggesting family size may matter more than birth order.
Is birth order a 'zombie theory'?
Despite inconsistent findings, Rohrer rejects the idea that birth order research is a dead-end. She argues that the field is evolving, with studies gradually refining our understanding of how family dynamics interact with personality.
"From the scientific perspective, I think the literature is progressing quite productively."
Julia Rohrer, Leipzig University
Until clearer answers emerge, the debate over whether eldest daughters are inherently responsible-or whether younger siblings are truly more carefree-will likely persist.
