For decades, research has linked the success of firstborn children to personality traits. The assumption has been that eldest children develop into responsible, conscientious individuals—traits that may contribute to their later advantages in education and career. However, large-scale studies have found little evidence that birth order significantly shapes personality. Instead, the advantages observed in firstborns appear tied not to character but to early environmental exposures during critical developmental stages.
This shift in focus comes from examining health data rather than psychological assessments. Researchers have increasingly turned to biological factors to explain the firstborn advantage, particularly how early-life health conditions might influence long-term outcomes.
The biological cost of having older siblings
The new understanding emerges from analyzing how older siblings influence the health of younger children. A team of researchers, including economists and health scientists from the USA, China, and Denmark, examined early health records to determine whether differences in infant health correlate with later success. Using comprehensive Danish administrative data—including hospital, tax, and education records—they tracked health outcomes for firstborn and second-born children born between 1980 and 2015.
The findings reveal a notable disparity: second-born children were hospitalized for severe respiratory illnesses two to three times more frequently than firstborns at the same age. The explanation lies in the role of older siblings as carriers of pathogens. When children attend daycare or school, they bring viruses home, exposing infants who lack the immune maturity to combat them effectively. During this critical period of lung and brain development, severe infections—such as those caused by influenza or Respiratory Syncytial Virus (RSV)—pose significant risks.
How early infection impacts the brain
The connection between early respiratory infections and later cognitive and professional outcomes lies in how the body responds to illness. Severe infections trigger inflammation, which can interfere with brain maturation in two key ways. First, the inflammatory response may directly damage developing brain tissue. Second, the body’s energy allocation shifts during illness, potentially diverting resources from neurological growth to immune function.
Since the first year of life is a period of rapid brain development, disruptions during this time can have lasting effects. While firstborns are generally spared the highest levels of early viral exposure, later-born children may face measurable cognitive disadvantages. These biological differences could contribute to the observed patterns in education and earnings, where firstborns tend to outperform their siblings.
This biological framework aligns with broader economic observations. For instance, earlier studies using Norwegian population data confirmed that firstborns typically earn more and achieve higher education levels than their younger siblings. The current Danish research builds on these findings by exploring potential health-related mechanisms behind the birth-order effect.
A thin foundation for a bold conclusion
The proposed biological explanation remains speculative and requires further validation. While the research presents compelling correlations, it has not yet undergone peer review, meaning its methodology and conclusions have not been independently verified. Critical questions remain unanswered, such as how the study controlled for other influential factors like socioeconomic status or parental health. Additionally, the precise criteria for defining severe respiratory illnesses and the exact sample size used in the analysis are not fully disclosed, making it difficult to assess the generalizability of the findings.
Relying on administrative data offers both strengths and limitations. While Danish registers provide extensive population-level information, observational studies can only establish correlations, not causation. The fact that second-born children experience more severe illnesses and firstborns achieve greater success does not definitively prove that early infections cause the observed disparities. Other factors, such as the distribution of parental attention or resources, may still play a role.
Furthermore, it is unclear whether these patterns would hold in different cultural or geographic contexts. In societies with varying childcare practices, vaccination rates, or living conditions, the dynamics of pathogen transmission between siblings could differ significantly. If the firstborn advantage persists in settings where older siblings do not frequently bring home high levels of pathogens, the viral transmission theory may not fully explain the phenomenon.
The current evidence suggests a plausible biological mechanism for birth-order effects, but it remains an unproven hypothesis. The data highlights a trend, though it does not establish a definitive causal relationship. If this explanation is confirmed, it could reshape our understanding of how early-life health influences long-term development—raising questions about whether other birth-order advantages, such as differences in entrepreneurship or political preferences, might also stem from early biological factors rather than personality traits.
