Virus and infection can explain why younger siblings earn less

Infants who suffer from respiratory illnesses may face long-term consequences, including lower educational attainment and reduced income in adulthood, according to a Danish study.

The study, titled "Germs in the Family," found that babies experiencing respiratory diseases in infancy tend to achieve lower levels of education and earn less as adults than their older siblings. Researchers from the University of Copenhagen observed a significantly higher risk of hospitalization for respiratory conditions in second-born children compared to firstborns, particularly during the vulnerable first six months of life.

"We see that second-born children have two to three times higher risk of being hospitalized with respiratory diseases in infancy compared to first-born children of the same age. This is especially clear in the first six months of life, when children are biologically most vulnerable," said N. Meltem Daysal, associate professor at the Department of Economics and CEBI at the University of Copenhagen.

The research, which analyzed register data for over 1.2 million Danish children, also revealed that the risk of severe illness in babies increases when the age gap between siblings is small. The study, published in the American Economic Review, suggests that early, severe illness can have lasting effects on a child's development and future prospects.

While it's known that older siblings often perform better academically and professionally, previous theories focused on family size. However, a 2005 study suggested birth order, not family size, was the key factor. This new research points to the transmission of viruses from older siblings, who are more exposed to germs from daycare and school, as a significant contributor to these disparities.

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We see that second-born children have two to three times higher risk of being hospitalized with respiratory diseases in infancy compared to first-born children of the same age. This is especially clear in the first six months of life, when children are biologically most vulnerable.