I first heard this question on The Economist, and it’s about a well-known conundrum. First-borns tend to do a little better in school and earn a little more as adults. As a firstborn, I relish this fact more than I should, even though the scientist in me knows it only applies at the population level. But I still have to ask why.
A new study offers an unexpected answer: an older sibling is a carrier of the disease. She comes home from school with her nose full of viruses and passes them on to her baby brother during the most vulnerable months of his life. Decades later, the baby brother earns a little less and leaves school a little earlier, and the gap in birth order we’ve been puzzling over becomes, in this saga, the long shadow of an infant chest infection.
This hypothesis is contained in a National Bureau of Economic Research working paper and is now listed as a forthcoming hypothesis in the American Economic Review. It is a study that economists have mastered, and that biologists are rarely bold enough to attempt.
The study uses Danish population records covering births from 1981 to 2017, linked to hospital, tax and education records, to ask whether infants in families with a heavy local respiratory disease burden fare worse in adulthood than their older siblings. The answer is yes.
Children who experience higher levels of respiratory illness in their first year earn, on average, 0.8 percent less adult income, are slightly less likely to complete school, and have somewhat higher rates of chronic respiratory illness and psychiatric care. The effects are small, but, the authors say, meaningful.
The title circulated well in the general and trade press. Business Insider has written about this, as has The Economist.
What I find amazing is who hasn’t. I looked for news coverage or substantive commentary in Nature and Science, the two magazines I read every week, and found nothing. I’ve looked for contributions to clinical infectious disease journals, and haven’t gotten there yet.
The paper pointing out the lifelong consequences of early exposure to respiratory diseases is being explained by economists and explained to the general public by business journals, while the biologists and doctors who will have to investigate how these diseases are related have barely entered the conversation.
It is worth looking closely at what the paper actually shows. In essence, the results are elegant. Younger siblings in Denmark are hospitalized for acute respiratory illness in the first year of life at two to three times the rate of their older siblings of the same age. The gap is largest in the first trimester, in winter, with short birth spacing, and when the older sibling is in group childcare. Each of these patterns is what a pediatric infectious disease doctor would expect. Young children are viral hosts, infants’ immune systems are still calibrating, and respiratory syncytial virus is more dangerous in a three-month-old than in a three-year-old. We’ve known this for a long time.
What’s new, and where the biologist has to slow down and catch his breath, is the claim that this might help explain why an adult earns more after 25 years.
The researchers constructed a municipal-level index of respiratory hospital admissions among slightly older children as a proxy for local viral pressure, asking whether severe pressure harms a younger sibling more than an older sibling. The design goes a long way toward controlling for shared family background, local conditions, and seasonality, and uses local epidemic waves as a source of variation.
But reading is not an infection. It is a municipal-level count of hospital admissions, compiled from administrative codes, and serves as a proxy for something a biologist might actually want to measure. It’s not the viral load, it’s not the immune activation, it’s not the cytokine profile, it’s not microbiome disruption, it’s not airway epithelial damage. Hospitalization is only the visible tip of a much larger iceberg than respiratory infection, as the authors themselves note, and the indicator is several steps removed from the biological event.
If childhood acute respiratory infections can affect adult incomes a quarter century later, we want to know how. Severe viral lower respiratory tract infection in the first months of life has long been associated with later wheezing and asthma, which is consistent with findings from the study of chronic respiratory disease in adulthood. It is also biologically plausible that inflammation during a period of rapid brain development leaves some imprint on later cognition. Severe illnesses in infants can lead to early exposure to antibiotics, which may compromise the gut microbiome, affecting immunity and metabolism in the long term.
None of these mechanisms have been tested yet.
Or there may not be one mechanism at all. The gap in adult income may be a combination of small biological influences and small social influences. This story may be closer to the truth, but it is more complex and difficult to tell on the cover of a magazine.
So, do firstborn babies really do better because of germs? Younger siblings get more serious respiratory infections in childhood, and on average their condition is slightly worse as adults. But the effect is small, the mechanism is undetermined, and the distance between a child’s chest injury and a 30-year-old’s salary is full of biology that has not yet been measured. Until that is done, I won’t go much further.
Anirban Mahapatra is a scientist and author. His latest book is When Medications Don’t Work. The opinions expressed are personal.
