Younger siblings are often portrayed as reckless, scatterbrained and less put-together than their older siblings. Social science researchers have underpinned these portrayals with data, showing that younger siblings obtain, on average, slightly less education and earn slightly less in adulthood. The explanation for this has often been that firstborns get more attention and quality time from parents than their younger siblings, resulting in the latter to "act out." But a new study from Northwestern University suggests there may be a more immediate reason: germs.
Using Danish administrative data covering all firstborn and second-born siblings born in Denmark between 1981 and 2017, researchers found that younger siblings are two to three times more likely than older siblings to be hospitalized for a respiratory illness in the first months of life - largely because older siblings bring home viruses such as Respiratory Syncytial Virus (RSV).
"This early-life disease exposure has lasting consequences," said co-lead author Hannes Schwandt, health economist and associate professor of Human Development and Social Policy at Northwestern's School of Education and Social Policy. "It reduces earnings, educational attainment and adult mental health, while increasing rates of chronic respiratory conditions like asthma later in life."
Schwandt explained the implications of the findings, if there are plans to look at U.S. data and why people are so fascinated with birth order.
What are some of the long-term effects that early respiratory illness exposure has on younger siblings?
Beyond effects on earnings and educational attainment (lower high school and college graduation rates, lower test scores), we find that greater early-life respiratory disease exposure increases hospitalizations for chronic respiratory conditions like asthma and COPD (Chronic Obstructive Pulmonary Disease) later in life, and raises the likelihood of using mental health care services as young adults. Together, these suggest that illness during the first months of life can have cascading effects across health, human capital and economic domains.
How does this research differ from previous studies on birth order?
Prior birth order research has documented that later-born children tend to have worse long-run outcomes, but has largely attributed this to differences in parental time and resources. Our team identify a previously overlooked biological mechanism - intra-family disease transmission - and show that controlling for infancy disease exposure cuts the estimated birth-order earnings penalty roughly in half, suggesting germs, not just parenting, are part of the story.
Why should the general public care?
Almost every family with more than one child experiences this pattern, yet its long-term stakes have never been documented at this scale. Our research suggests that something as ordinary as a toddler's cold, passed to a new baby sibling, can have measurable effects on that child's future earnings and health decades later. That points to concrete, behavioral adjustments and actionable policy levers - paid family leave, breastfeeding support and RSV vaccination - that could meaningfully improve children's life trajectories.
Why did you use Danish datasets? Are there plans to analyze data in the U.S.?
Denmark's administrative registries let us link entire birth cohorts to hospitalization, education, earnings and mental health records over decades, which is not possible with U.S. data at this scale and completeness. We don't have specific current plans to replicate this in U.S. data, though comparable patterns (RSV is also the leading cause of infant hospitalization in the U.S.) suggest the mechanism and consequences likely generalize, and it's a natural direction for future work if suitable linked data become available.
Was there anything that surprised you about the study findings?
We were struck by how large and persistent the long-term effects were from what looks, in the moment, like a routine childhood illness rather than a dramatic health shock. We were also surprised that controlling for this disease channel explained roughly half of the classic birth-order earnings gap documented in earlier research, suggesting it's a much bigger part of the story than previously recognized.
Why do you think people are fascinated with studying birth order? Birth order is something almost everyone can relate to personally, and it offers an intuitive natural experiment: Siblings share genes and a household but arrive at different points in family life. It also taps into long-standing questions about how much of our life outcomes are shaped by circumstances outside our control versus effort or choice, which makes it endlessly compelling to a general audience.
How would you like to expand on this research?
We're interested in extending the disease index to study effects on other family members, such as whether parents experience career disruptions from caring for sick children, and whether disease spreads further through parents' workplaces. We also see this as relevant to understanding the long-term legacy of COVID-19-era disruptions to childhood illness patterns, since pandemic restrictions temporarily suppressed and then rebounded RSV spread, potentially creating differential effects across birth cohorts.
The study, "Germs in the Family: The Short- and Long-Term Consequences of Intrahousehold Disease Spread," was published July 13 in American Economic Review. In addition to Schwandt, co-authors include N. Meltem Daysal of the University of Copenhagen, Hui Ding of Fudan University and Maya Rossin-Slater of Stanford University.
