A new study suggests that firstborns may have an edge in life due to their older sibling being a 'vector' of germs. The study, published in a working paper by the National Bureau of Economic Research, found that infants exposed to higher levels of respiratory disease in their first year go on to earn about 0.8% less as adults, are slightly less likely to finish school, and have higher rates of chronic respiratory illness and psychiatric care.
The study used Danish population registers covering births from 1981 to 2017 to examine the relationship between infant respiratory disease and adult outcomes. The findings are small but meaningful, and the authors argue that the effects are likely due to the older sibling passing on viruses to their younger sibling during the most vulnerable months of their life.
However, the study's findings have not been widely covered in the scientific community, with many experts expressing skepticism about the claim that a chest infection in infancy can have a lasting impact on adult earnings.
While the study's design is elegant, the authors' claim that the findings can explain the birth-order gap in adult earnings is not supported by the data. The study's proxy for local viral pressure is a municipality-level count of hospital admissions, which is several steps removed from the biological event of a severe respiratory infection.
Experts argue that the study's findings are biologically plausible, but the mechanism by which a severe respiratory infection in infancy can affect adult earnings is not yet understood. Further research is needed to determine whether the effect is due to small biological effects or small social ones.
