Welcoming Insights: Why Firstborns Earn More – and How Illness Plays a Role

A recent study from the University of Copenhagen suggests that early childhood illnesses may help explain why firstborn children tend to earn more than their younger siblings later in life. The research, which analyzed data on siblings born in Denmark between 1981 and 2017, found that younger siblings were two to three times more likely to be hospitalized for respiratory conditions during their first year, particularly when born in the fall or winter months.

According to N. Meltem Daysal, an associate professor at the University of Copenhagen, this increased susceptibility to illness among younger siblings is linked to older siblings bringing home viruses from group childcare settings. The study revealed that this early exposure to illness has lasting financial effects, with younger siblings earning an average of 2.4% less than their older siblings when they enter the workforce.

Similar findings were reported by researchers at the University of Edinburgh, who found that firstborn children tend to score higher on IQ tests, with the cognitive gap often appearing within the first year of life. The Copenhagen study, conducted in collaboration with Sydney University, examined data from 5,000 children who underwent reading and picture vocabulary tests every two years.

The results showed that children who suffered from illnesses during early childhood achieved lower academic scores, potentially because infections divert energy away from brain development. Researchers noted that nurseries and childcare environments are often prone to common colds and bugs, making it likely that firstborn children pick up viruses and transmit them to their younger siblings.

Another contributing factor identified in the research is the difference in parental attention during early childhood. Firstborn children typically receive more one-to-one time with parents before the arrival of additional siblings, while younger siblings often get less individualized care as parents manage multiple children.

While the study highlights a correlation between early health, cognitive development, and later earnings, researchers emphasize that birth order outcomes are influenced by a combination of biological, environmental, and social factors. The findings do not suggest that younger siblings are destined to earn less, but rather that early-life health disparities may contribute to measurable differences in socioeconomic outcomes over time.