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Birth Order and Personality: What the Research Actually Shows

The idea that birth order shapes personality is a popular, long-running belief β€” but as explained below, large recent studies have found little solid evidence behind it. Take it as an entertaining talking point rather than established science.

Firstborn

Popular belief: Firstborns are supposedly more responsible, natural leaders, and achievement-oriented, a pattern often explained by their role helping raise younger siblings and by receiving their parents' full attention and expectations first. What the research shows: firstborns likely do get more chances to practice responsibility by helping with younger siblings, but there is little evidence this translates into a measurable, lasting personality difference.

Middle Child

Popular belief: middle children are said to act as family peacemakers with strong negotiation skills, sociability, and flexibility, supposedly because splitting parental attention between older and younger siblings sharpens their social instincts. What the research shows: the phrase "middle child syndrome" is widely used in popular culture, but consistent empirical evidence to support it is thin.

Lastborn

Popular belief: lastborns are often described as outgoing, easygoing, sociable, and a bit spoiled. Psychologist Frank Sulloway argued that rather than competing with already-established older siblings, lastborns tend to pursue a different "niche" strategy, which he linked to more rebellious, risk-taking tendencies. What the research shows: Sulloway's theory drew major attention after his 1996 book Born to Rebel, but later large-scale studies have not clearly replicated it.

Only Child

Popular belief: only children are often assumed to be mature, perfectionistic, and unusually comfortable talking with adults, a pattern attributed to growing up receiving their parents' full attention without having to share resources with siblings. What the research shows: the old stereotype that only children are "spoiled and less social" has likewise been assessed as a weakly supported bias in multiple studies.

Why this idea became so widely believed

In 1996, psychologist Frank Sulloway popularized birth order theory in his book Born to Rebel, arguing that competition for parental resources between siblings evolutionarily drove different "ecological niche" strategies β€” firstborns adopting a conservative strategy to protect what they already had, and younger siblings pursuing more innovative, different paths.

But large-scale studies found something different

In 2015, psychologists Rodica Damian and Brent Roberts analyzed data from roughly 377,000 high school students and found that birth order's effect on the Big Five personality traits was either not statistically significant or extremely small. That same year, a research team led by Julia Rohrer analyzed large samples of about 20,000 people across the US, UK, and Germany and reached the same conclusion. A small correlation between birth order and IQ did show up in some data, possibly linked to shrinking per-child educational resources in larger families.

Frequently Asked Questions

So does birth order have zero effect on personality at all?

It's hard to rule out completely, but what large-scale studies consistently show is that even if some effect exists, it's far too small to notice in everyday life. Factors like individual temperament, home environment, and parenting style appear to matter far more.

For twins, which one counts as the "firstborn"?

The twin born first is conventionally treated as the firstborn and the other as the second-born. But since the underlying theory itself rests on weak evidence, this distinction is unlikely to carry any real practical meaning.