The Germ Theory Unpacked
A curious notion suggests that firstborns might benefit from a cleaner initial environment. Parents, often more cautious with their first child, might maintain
stricter hygiene. This could theoretically lead to a stronger immune system in later siblings who encounter more microbes from their older siblings. However, this idea remains largely speculative, lacking substantial scientific backing. While early exposure to diverse microbes is vital for immune development, a direct, significant link between birth order and this process conferring an intellectual edge to later-borns is not firmly established in scientific literature.
Established Birth Order Theories
Beyond speculative biological factors like germ exposure, more widely accepted theories center on environmental and social influences. Parental investment is a key factor; firstborns frequently receive more concentrated attention and resources from parents who are often younger, more energetic, and less experienced. This intensified focus can translate into greater educational stimulation and support. Furthermore, older siblings may naturally adopt roles as mentors or role models for their younger siblings, fostering leadership and responsibility. Conversely, as families grow, parental resources like time and finances become diluted, potentially leading to less individual attention for later-borns. The confluence model posits that the intellectual ambiance of a home is an average among all children, meaning firstborns, in smaller family units, experience a higher average intellectual environment.
Research Insights on Success
The scientific investigation into birth order yields nuanced, sometimes conflicting, results. Some studies do indicate a minor average IQ advantage for firstborns, but these differences are typically small and influenced by numerous other variables. For instance, a comprehensive meta-analysis involving over 370,000 individuals identified a subtle but statistically significant tendency for firstborns to possess slightly higher IQ scores than their younger siblings. Crucially, the study highlighted that these disparities were so marginal as to be virtually unnoticeable in everyday life. Similarly, while birth order has been linked to personality traits and career paths, the evidence remains inconclusive. Some research suggests firstborns exhibit more conscientiousness and ambition, whereas later-borns might lean towards being more agreeable or even rebellious.
Germs in Perspective
While the idea of germ exposure is an interesting hypothetical, it is not considered a primary or significant factor in scientific discussions about birth order effects. The intricate development of the immune system is shaped by a multitude of elements, including genetic predispositions, dietary habits, and exposure to a broad spectrum of microorganisms throughout one's life, not solely during early infancy. The complexities involved mean that attributing any perceived firstborn advantage specifically to early germ exposure is an oversimplification not supported by current scientific understanding.
New Economic Angle on Germs
A recent working paper, slated for publication in the American Economic Review, proposes an unexpected explanation for the observed birth order advantage: the role of early respiratory illness. The study uses extensive Danish population data from 1981 to 2017, linking hospital, tax, and education records. It finds that infants in families with a higher local respiratory disease burden experience worse outcomes in adulthood. Specifically, children exposed to increased respiratory disease in their first year tend to earn approximately 0.8% less as adults, are slightly less likely to complete their education, and have higher rates of chronic respiratory issues and psychiatric care. These effects, though small, are argued to be meaningful. Younger siblings are hospitalized for acute respiratory illness at significantly higher rates than their older siblings during their first year of life, with the gap most pronounced in the early months, during winter, with short birth intervals, and when an older sibling attends group childcare. This aligns with clinical expectations, as toddlers are often 'viral chimneys' and infant immune systems are still developing.
Bridging Biology and Economics
The researchers constructed a municipality-level index of respiratory hospitalizations among slightly older children to gauge local viral pressure, assessing whether this pressure disproportionately harms younger siblings. This design effectively controls for shared family backgrounds, local environmental conditions, and seasonality, utilizing local epidemic waves as a source of variation. However, the study's readout is a proxy – hospital admission counts – rather than direct biological measurements like viral load, immune activation, or microbiome disruption. Hospitalization represents only the most visible aspect of respiratory infection, and the index is several steps removed from the actual biological event. While severe infant respiratory infection has been linked to later wheeze, asthma, and potentially cognitive impacts due to inflammation during critical brain development periods, or even long-term gut microbiome issues from early antibiotic use, these specific mechanisms remain untested in this study. The adult earnings gap might, therefore, be a combination of subtle biological and social factors, a more complex narrative that is harder to convey concisely.
