If you’re the little one in the house, you probably inherited your older brother’s clothes, toys, and textbooks. But, according to science, you also inherited something much more harmful, which is its viruses. And what might seem like a simple childhood anecdote about boogers and daycare has a real, measurable and profound impact in the health, educational level and even the economic future of minors.
Something common. For decades, sociology and economics have documented that first-born children tend to do better academically and economically. Traditionally, this was explained by theory of “parental care” pointing out that first-time parents dedicate more time, energy and resources to the first child, which is more complicated to offer later with a second child.
However, a new wave of macro studies with millions of data is showing that biology and epidemiology have as much or more weight than parenting.
The nursery. A recent massive study based on Danish administrative records, which has followed 1.2 million individuals for forty yearshas put figures to this phenomenon. And their conclusion suggests that younger siblings are two to three times more likely to be hospitalized for respiratory infections during their first year of life compared to older siblings.
The reason is obvious to any parent, since the older brother acts as a “vector of contagion”, bringing home countless pathogens from daycare or school just when the immune system of his younger brother, who in this case is a simple baby, is most vulnerable.
The impact on payroll. What is truly revolutionary about the Danish study is not that the babies get sick, but the long-term consequences. Here researchers have directly linked this severe exposure to early respiratory diseases with lower income, lower educational level and worse mental health outcomes, both in adolescence and adulthood.
In fact, data suggest that this early exposure to disease explains about half of the economic and vital gap between firstborns and younger siblings. The other 50% would still be attributable to the difference in parental care.
It had already been studied. These data confirm classic studies, such as the one carried out in 2005 with population registries in Norway, which already warned that the higher the order of birth, the worse the socioeconomic results. Norwegian data showed that the fourth or fifth child in a family has almost a year less of formal education than the first, and that women born last have higher rates of teenage pregnancy.
The disease. But birth order not only punishes the little ones, it simply distribute risks differently. A colossal study by the University of Chicago carried out in 2026 on 10.3 million people and a previous Swedish macroanalysis have drawn the definitive map of how our position in the family shapes our medical history.
In this case, although first-born children enjoy better overall health throughout their adult lives, first-born children have a significantly increased risk of developing autism, Tourette syndrome, childhood psychosis, anxiety and depression. Furthermore, they bear the brunt of autoimmune and dermatological diseases: more allergies, rhinitis and acne.
It’s just statistics. Faced with this avalanche of data from Denmark, Norway, Sweden and the US, it is easy to fall into determinism. However, the researchers behind all of these works emphasize a golden rule of statistical science by pointing out that all of this refers to population trends, not individual convictions.
Images | Annie Spratt
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