Younger siblings earnings gap may be partly tied to infant infections

A new Northwestern University study says the younger siblings earnings gap may be partly explained by respiratory infections in infancy, not only by differences in parental attention. The finding matters because it points to a health pathway that begins in the first months of life and may show up decades later in education, income and adult health.

The paper, published in the American Economic Review, used Danish administrative records for firstborn and second-born siblings born in Denmark from 1981 through 2017. Northwestern said the researchers found second-born children were two to three times more likely than firstborns to be hospitalized for a respiratory illness early in life.

The researchers attributed much of that excess exposure to viruses carried home by older siblings, including respiratory syncytial virus, or RSV. RSV is a common respiratory virus and, according to Northwestern, a leading cause of infant hospitalization in the United States as well.

Why might younger siblings earn less?

Birth-order research has long found that later-born children tend to end up with slightly less schooling and somewhat lower adult earnings, Northwestern said. Earlier explanations have focused on parental time and resources, with firstborns often receiving more one-on-one attention before another child arrives.

The Northwestern team argues that disease spread inside the household is another mechanism. According to co-lead author Hannes Schwandt, a health economist and associate professor at Northwestern's School of Education and Social Policy, exposure to respiratory illness early in life was associated with lower earnings, less education, poorer adult mental health and more chronic respiratory disease later on.

Schwandt said the study found links to lower high school and college graduation rates and lower test scores. The researchers also reported more later-life hospitalizations for chronic respiratory conditions such as asthma and COPD, along with a higher likelihood of using mental health care services as young adults.

One of the study's central findings is that accounting for respiratory illness in infancy reduced the estimated earnings penalty for later-born children by about half, according to Northwestern. The authors said that suggests infections carried among family members may explain a substantial share of the birth-order difference documented in earlier studies.

Why did the study use Danish records?

Northwestern said Denmark's national registries allowed researchers to connect birth records with hospitalizations, schooling, earnings and mental health data over long periods. Schwandt said comparable U.S. data are not available at the same scale and completeness, and the team does not currently have specific plans to repeat the work with U.S. records.

The researchers said the pattern may still be relevant beyond Denmark because RSV and other respiratory infections are common among infants in many countries. They framed U.S. replication as a possible future direction if suitable linked data become available.

Schwandt said the results point to practical ways to reduce exposure or limit its effects, including paid family leave, breastfeeding support and RSV vaccination. The study did not report testing those policies directly, but Northwestern said the findings identify them as possible levers for improving long-term child outcomes.

The paper is titled “Germs in the Family: The Short- and Long-Term Consequences of Intrahousehold Disease Spread.” Its listed authors include N. Meltem Daysal and colleagues, according to the American Economic Review publication details.

This story draws on original reporting from Medical Xpress.