A sweeping new study from Denmark, set to appear in the American Economic Review, provides the clearest explanation yet for why firstborn children consistently outperform their younger siblings in income, education, and mental health. The research, which followed 1.2 million Danes from birth to adulthood using government records, identifies two primary culprits: early exposure to respiratory viruses and disparities in parental attention.
Older siblings, the study finds, are two to three times more likely to be hospitalized for acute respiratory conditions during their first year of life compared to later-born children. The reason is straightforward: older brothers and sisters bring home germs from daycare or school, and younger infants—whose immune systems are still immature—get hit hard. Critically, after the first year, when younger children typically enter childcare themselves, the hospitalization gap disappears, confirming the older sibling as the vector.
The long-term consequences are stark. Roughly 85% of an infant’s caloric intake goes to neural development; a serious infection diverts those resources to fighting illness, impairing brain development. The study notes that disease exposure in the first six months carries an income penalty three times greater than similar exposure later in the first year. Altogether, early illness accounts for about half of the lifetime outcome gap between firstborns and their younger siblings.
The other half is explained by parental attention. Firstborns receive 20 to 30 more minutes of quality time per day than younger siblings at the same age, adding up to roughly 3,000 hours over childhood. That deficit in attention correlates with lower educational attainment and earnings later in life.
While the study’s findings are robust, questions about generalizability remain. Denmark’s universal healthcare, generous parental leave, and high vaccination rates may mitigate some of the damage from early illness. In the U.S., where access to follow-up care is uneven, the effects could be more pronounced. Still, the authors argue the evidence is strong enough for parents to take action.
For disease prevention, the study recommends vaccinating not only infants but also older children and parents. Breastfeeding for at least six months significantly reduces acute infections in younger siblings. Basic hygiene—hand-washing, changing out of daycare clothes, and keeping sick older kids away from babies—also helps.
On the time front, the message is one of empowerment. “The 3,000-hour deficit is not fate—it is an average,” the study’s author, Dr. Liberty Vittert Capito, a data science professor at Washington University in St. Louis, told The World Signal. “Once you know about it, you can act on it and focus on equalizing your time.”
The study arrives amid broader conversations about childhood development and policy. For instance, the Surgeon General’s recent declaration that children’s screen use is a public health crisis underscores how environmental factors shape outcomes. Meanwhile, debates over family support programs—such as the Treasury’s new Trump Accounts app for children’s investment—highlight the political stakes of early-life interventions.
For parents of multiple children, the study offers rare actionable insight. “Most research about how childhood determines your fate leaves you feeling helpless,” Vittert Capito said. “This one doesn’t. It says time and disease are the culprits, and both are things we can address.”
