Why Are Older Children Smarter Than Younger Ones? Scientists Explain
Elder children in families, on average, study better, more often receive higher education, and earn more in adulthood. This pattern has been known to scientists for a long time, but its causes remained a subject of debate.
Illustrative image. Photo: Nasha Niva
Usually, everything was explained by psychology: they said, firstborns are more responsible and disciplined, while younger ones are natural rebels and adventurers. However, large studies have not found convincing evidence that birth order significantly affects personality type.
A new study, highlighted by The Economist, offers a different explanation. A group of scientists from the USA, China, and Denmark noticed that children get sick very often, especially during the first years of life. The authors wondered: do older children bring infections home from school, kindergarten, or other contacts, infecting their parents and younger siblings?
Using Danish administrative data for analysis, researchers found that younger children are two to three times more likely than older children to be hospitalized with severe respiratory illnesses in their first year of life. And this is not just about temporary weakness of the body.
Scientists suggest that early severe illnesses can affect brain development both directly (through inflammatory processes) and indirectly (by redirecting the body's energy from organ development to fighting infection). As a result, the consequences of such illnesses can be felt even decades later.
The authors found a direct causal link between early illnesses and lower earnings in adulthood. Other scientific works also confirm that high fever and respiratory infections in the mother during pregnancy can negatively affect fetal brain development.
According to researchers' estimates, the factor of illness may explain approximately half of the difference in earnings between the first and second child in a family.
The remaining part is possibly related to parental behavior. Older children usually receive more attention, especially in the first years, while they are the only child in the family.
American time-use data shows that throughout childhood, firstborns receive an average of 20-30 minutes more of "quality time" with parents daily than second children of the same age.
Researchers explain this by parents trying to divide their attention equally among all children at any given moment. But since a new infant requires a lot of effort and care, the older child receives less time in that moment.
However, cumulatively over all years, older children still receive more intellectual stimulation during the crucial period of brain formation. Thus, younger siblings' complaints about lack of attention or unfairness have a completely real biological and social basis.
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