Does Birth Order Affect Personality? What Research Says
Birth order has modest, largely inconsistent effects on personality in modern large-scale research. Frank Sulloway’s influential theory of firstborns as conscientious conformists and laterborns as rebellious has not replicated reliably when family-level confounds are controlled. The IQ effect is slightly more robust.
The Birth Order Hypothesis
The idea that birth order shapes personality dates to Alfred Adler in the early 20th century and was given renewed scientific credibility by Frank Sulloway’s 1996 book Born to Rebel. Sulloway argued firstborns — most invested in the parental status quo — become more conscientious and conformist, while laterborns — competing for a different family niche — become more open and rebellious.
What Large-Scale Research Shows
- A 2015 study by Damian and Roberts (n=377,000) found birth order had essentially no consistent effect on Big Five personality traits when between-family confounds were controlled
- Studies across Germany, UK, and US national samples found similarly small, inconsistent effects
- Within-family effects are detectable but very small and do not generalize to between-family comparisons
- Intelligence shows a slightly more reliable birth order effect: firstborns score ~1–3 IQ points higher on average in some studies — possibly due to tutoring effects
Why the Myth Persists
Birth order feels intuitively true because we readily find confirming examples in our own families. The sibling experience is vivid and emotionally significant — it is easy to attribute personality differences to birth order rather than to temperament, peer groups, school experience, or formative events that are statistically more powerful.
What Actually Shapes Personality More
- Genetics: ~40–60% of personality variance is heritable
- Peer relationships: peer group effects on personality are larger than parenting effects in adolescence
- Non-shared environment: unique experiences not shared with siblings
- Major life events: trauma, bereavement, significant relationships
Frequently Asked Questions
Are firstborns really more conscientious?
The evidence is weak. Large studies controlling for confounds find small or no consistent birth order effects on Conscientiousness. The pattern may reflect selection bias in smaller studies.
Is there a birth order effect on IQ?
A small IQ effect is more reliably documented. Firstborns score ~1–3 IQ points higher in some large studies, possibly because teaching younger siblings reinforces their own learning.