The argument that birth order shapes who you become has been around since 1874, when the British scientist Sir Francis Galton noticed that men in prominent public positions were firstborns far more often than chance would predict. For a long time, that observation stayed in the territory of dinner-party conversation, interesting but too fuzzy to act on. The research that has accumulated since then is a lot harder to wave away.
The clearest evidence of the firstborn advantage in birth order achievement came from Scandinavia, where researchers had something most psychologists can only dream about: data on virtually an entire country’s population. Over decades of studies using Norwegian and Swedish registries covering hundreds of thousands of families, the pattern kept repeating. Firstborn children consistently came out ahead on IQ scores, years of education, and adult earnings.
What makes that finding genuinely surprising isn’t the gap itself. It’s the explanation.
It’s Not About Biology
A study of more than 240,000 Norwegian men found that older siblings score higher on IQ tests than their younger brothers and sisters. In cases where the first child dies in infancy, however, the second-born child raised as the firstborn assumes the mantle, performing as well as the actual elder child on intelligence exams.
That last detail is the critical one. According to Scientific American, the effect is primarily social, not biological. When a firstborn child died in infancy and the second-born was raised as the functional firstborn, the second-born showed the cognitive profile typical of firstborns. This finding strongly supports environmental explanations – parental investment and family dynamics – over biological ones.
In other words, being born first doesn’t hardwire anything. What matters is the role you end up playing in the family. It’s a matter of what researchers call social rank in the family – the highest scores were racked up by the senior child, whether the firstborn or, if the firstborn had died in infancy, the next oldest.
On average, firstborn males in the Norwegian study had an IQ of roughly 103.2, whereas the second-born child scored about 100.4 and third-borns 99. Three IQ points doesn’t sound like much. But when you’re talking about a pattern that held across hundreds of thousands of families and reproduced itself within siblings raised in the same home, it adds up to something real.
The Parental Attention Explanation
Two frameworks dominate the research on why this happens. The first is the Resource Dilution Model. The basic idea is exactly what it sounds like: firstborn children benefit from greater parental investment and attention, which may account for their higher achievement levels. Larger families distribute parental attention and resources more thinly. Firstborns, benefiting from undivided parental attention early on, may gain a developmental advantage over their younger siblings.
The second framework is the Confluence Model, developed by psychologist Robert Zajonc. Zajonc’s Confluence Model proposes three reasons to explain why an expanding family is linked with lower intellectual and academic functioning. The first is that firstborn children receive the most parental support and attention, and this intensity depreciates with each subsequent child. Secondly, the family’s vocabulary and the way concepts are discussed becomes less sophisticated with each additional child.
It is likely that with the first child, parents make thoughtful efforts to ensure they answer questions like “why is the world round” with care, whereas the lastborn receives a less evocative response, offered either by a harried parent or even another child in the family.
There’s a third dynamic that doesn’t fit neatly into either model but is just as significant. Firstborns, by virtue of being older, end up tutoring their younger siblings. Explaining a concept to someone else, even a five-year-old asking about dinosaurs, solidifies understanding in ways that simply receiving instruction doesn’t. The older child reinforces their own learning every time they explain something to a younger sibling. The younger child receives a simplified version of the same lesson.
What This Looks Like Over a Lifetime
The IQ gap is just the start. Birth order has long been noted as a systematic source of environmental differentiation within families, with firstborn siblings showing higher intelligence, educational achievement, and income than those born later.
The career data is even more striking. A study by economist Sandra Black, a visiting fellow at the Brookings Institution, who collaborated with researchers from Sweden’s Institute for Evaluation of Labour Market and Education Policy, found that firstborn children are more likely than their siblings to eventually rise to become CEOs, government officials, and others in high-ranking positions. The trio analyzed Swedish government data on half a million men born between 1952 and 1982, examining results of psychological and intellectual tests administered before mandatory military service. More than half of US presidents were firstborn children or firstborn sons, and surveys of business leaders consistently show firstborns overrepresented at the top – among them Christine Lagarde, Elon Musk, Richard Branson, Jeff Bezos, and Marissa Mayer.
Black and her colleagues discovered that firstborn children were more likely than those with older siblings to have certain personality traits that breed career success. Firstborn kids tend to grow up to be emotionally stable, persistent, outgoing, and willing to take responsibility and initiative.
The Middle Child and the Youngest
None of this means younger siblings are destined to underperform. The research describes averages, not fates. But the patterns for middle and youngest children are worth understanding on their own terms.
Middle children may struggle with feelings of neglect and lower self-esteem due to divided parental attention, often leading them to develop distinct identities to avoid competition with their siblings. The flip side of that is real: middle children consistently turn up in research as more socially agile, more comfortable with negotiation, and more willing to challenge the status quo. They’ve had to be. Nobody was running the household around their needs.
Youngest children, raised in a more relaxed household, might exhibit carefree attitudes and stronger social skills but may also face challenges academically due to a lack of structured guidance. Parents on their third or fourth child are simply different parents than they were the first time around. Less anxious, looser with rules, less likely to spend forty minutes working through a phonics workbook on a Tuesday night. That looseness can be a gift – youngest children often grow up with a sense of ease and emotional fluency that their older siblings envy – but it rarely translates into the same kind of driven, structured achievement orientation.
Firstborn children show higher motivation, likely due to greater parental expectations, while only children exhibit the lowest motivation levels, potentially due to a lack of sibling competition. Only children are an interesting exception to the usual pattern: they receive the same focused parental attention as a firstborn but without any dilution. Studies generally find they score comparably to firstborns, sometimes slightly higher.
The Limits of the Research
No direct relationship has been identified between birth order and academic performance in every study – and that matters. The birth order effect is real but modest, and it competes with far stronger influences. Socioeconomic status, school quality, the particular personality of each parent, the spacing between siblings, whether a family goes through a divorce or a financial crisis – all of these carry more explanatory weight than birth order alone.
Modern meta-analyses suggest that while birth order has a measurable impact, its effects are smaller than initially hypothesized.
These differences are routinely interpreted as reflecting causal processes related to childhood experiences. A 2022 study published in PNAS Nexus confirmed that birth order differences in education have no genetic origin – there were no differences in polygenic scores for educational attainment between firstborns and later-borns – pointing instead to postnatal environments as the source of the gap. Some work suggests that parental resources are more diluted for later-born siblings as a result of competing demands for parental attention, while other work considers whether the presence of older siblings adversely affects the cognitive environment in which younger siblings are raised.
The honest answer is that both are probably true to some degree, and neither is the whole story. Birth order shapes the environment a child grows up in, and that environment shapes the child. But it does so alongside a hundred other variables, and the direction of the effect can shift depending on the family.
Read More: What Growing Up With Siblings Really Teaches You About People
What This Actually Means
The strongest takeaway from the research isn’t a ranking system for your children. It’s a clearer picture of what’s actually driving the gap. When firstborns do better on average, it’s not because they were born with a head start – it’s because of what happened after they arrived. Undivided parental time during those critical early years. Being the reference point against whom children are judged by their own progress rather than against siblings. The accidental tutoring effect that comes from explaining things to smaller people.
That means the advantage is, at least in principle, something that can be redistributed. Not perfectly, and not without effort – a family with three children under eight is not going to replicate the focused attention a couple gives a singleton. But knowing that the gap is environmental, not biological, changes the question entirely. It stops being “are later-born children at a disadvantage they can’t overcome?” and starts being “which specific conditions are driving the gap, and which of them can be shifted?”
The within-family finding is the hardest one to explain away. Same parents, same household income, same neighborhood school – and still a measurable gap. Some of these differences in sibling outcomes go back to the very first years of a child’s life, long before school, long before anyone was paying attention to birth order as a variable. Naming that isn’t a solution. But it’s usually where the real conversation starts.
AI Disclaimer: This article was created with the assistance of AI tools and reviewed by a human editor.