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Research with young Norwegians shows a small but relevant difference between siblings of different positions in the family, and helps explain why home environment, parental attention, and roles assumed in childhood enter the debate about IQ.
The older children usually appear with a slight advantage in intelligence tests when compared to younger siblings, according to research conducted with young Norwegians of military enlistment age, although the difference is small in individual terms.
This average advantage does not allow us to state that birth order determines a person’s intellectual potential, as cognitive development depends on a wide range of family, social, school, and emotional factors.
Among the most cited studies on the subject is the work of Norwegian researchers Petter Kristensen and Tor Bjerkedal, who analyzed the relationship between birth order and intelligence in an article published in 2007 in the journal Science.
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According to the authors, the most likely explanation for the observed difference was in family dynamics and the roles occupied by children at home, rather than a genetic advantage linked to being born first.
The study gained attention because it used a large base of young men from Norway, evaluated in a military service context, which gave statistical strength to the comparison between firstborns, second children, and siblings in later positions.
In a related survey, published around the same period, more than a quarter of a million intellectual performance tests of Norwegian recruits were gathered, applied between 1984 and 2004, with participants mostly aged 18 to 19 years.
What the research found about IQ and birth order
When comparing siblings in different positions in the family, the researchers identified an average drop in scores as the birth order increased, with firstborns recording slightly superior performance to second children and younger siblings.
This difference, however, does not make older children naturally more intelligent, as the study published in Science itself favors an explanation based on family interaction, especially the social role occupied by the child.
The interpretation changes when it is observed that the average advantage would not simply be in the birth itself, but in the conditions experienced in the early years, such as attention received, responsibilities assumed, and practical position within family relationships.
Another relevant point is that the study indicated a greater weight of the social order than the biological order, especially in situations where a younger sibling came to occupy, in practice, the role of the eldest child.
This finding reinforced the hypothesis that the home environment, the distribution of responsibilities, and the way each child is treated by adults can influence small average differences in cognitive performance.
Why the Eldest Child May Have an Advantage in IQ
One of the explanations discussed by researchers involves the exclusive attention that the firstborn usually receives before the arrival of siblings, a period in which interactions with parents can be more concentrated and frequent.
This phase can favor language stimuli, initial school monitoring, and more direct exchanges with adults, elements that help compose the intellectual development environment during the first years of life.
With the arrival of other children, the family routine undergoes changes, and the attention of adults tends to be divided among more children, altering the type of stimulus received by each sibling.
In this new configuration, the eldest child often assumes roles of reference, help, or supervision in relation to the younger ones, which can reinforce skills related to organization, communication, and autonomy.
Even so, interpreting the results requires caution, because the average of a large group does not allow predicting the performance of a specific child nor reducing intelligence to a single family variable.
Factors such as school environment, family income, health, emotional stability, quality of care, and learning opportunities continue to have significant weight in the cognitive development of each person.
There are also limits in the analyzed base itself, formed by young men evaluated in the context of military service, which prevents transforming the results into a universal rule for all families, cultures, countries, age groups, and genders.
Middle Child and Youngest Do Not Have a Defined Destiny
The research does not allow concluding that middle children or youngest are at an inevitable disadvantage, as the data shows an average difference observed in large groups, and not a sentence about the intelligence of each individual.
In family life, younger siblings can receive different stimuli, live early with older children, and grow up in a home where parents already have more accumulated experience in daily care.
These elements can also influence behavior, language, and learning, without eliminating the importance of school, encouragement, emotional bonds, emotional stability, and access to development opportunities throughout childhood and adolescence.
Therefore, turning birth order into a personality diagnosis is an oversimplification, even though ideas about firstborn leaders, middle children as conciliators, and spontaneous youngest children are common in family conversations.
What studies more reliably indicate is that, in certain population bases, the position occupied by the child within the family appears associated with small average differences in cognitive performance.
Outside of this context, any generalization needs to be made with caution, because families have very different structures, routines, economic contexts, caregiving styles, and sibling relationships.
IQ difference between siblings is small and does not define the future
The advantage observed between firstborns and younger siblings draws attention due to the sample size, but it needs to be interpreted on a population scale, without turning a statistical average into a direct comparison between people from the same family.
Within a household, individual differences can be much greater than the average effect attributed to birth order, especially when school, health, personality, family support, and concrete learning opportunities come into play.
Being born first, therefore, may be associated with a slight statistical advantage in some surveys, but it does not guarantee better academic performance, professional success, or greater intellectual capacity throughout life.
The safest answer is that the oldest child usually has a slight average advantage in studies on IQ, especially in the cited Norwegian surveys, without this representing individual superiority.
In families with several children, science suggests that birth order may influence opportunities, expectations, and domestic roles, but the outcome for each child depends on a broader set of factors.
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