The Blubber

Older siblings tended to make more money than their younger siblings, per BI and NBER.


For years, researchers have observed a trend where the eldest child tends to earn more money and perform better on cognitive tests than their younger siblings. However, experts have debated the primary factors driving this phenomenon.

In a recently updated working paper published in February by the National Bureau of Economic Research, researchers identified a significant factor affecting younger siblings: their likelihood of getting sick as infants.

Analyzing data on first- and secondborn siblings born in Denmark between 1981 and 2017, the researchers discovered that younger siblings were two to three times more likely than their older counterparts to be hospitalized for respiratory conditions during their first year. This disparity was most pronounced when the secondborn child was born in the fall or winter, when respiratory illnesses are more prevalent, and when the siblings were closer in age.

"We believe that these patterns are explained by the fact that infants with older siblings have more virus exposure — due to their older sibling, who is often in group childcare and 'brings home' viruses," explained N. Meltem Daysal, an associate professor in the Department of Economics at the University of Copenhagen and one of the paper's authors.

The researchers also found potential long-term financial implications of early illness. Analyzing the earnings of children born between 1981 and 1989 in Denmark, they observed that among employed siblings, younger siblings earned 2.4% less on average than their older siblings at the same age. Daysal suggested that severe respiratory illness in infancy could impair brain development, affecting later mental health and, consequently, earnings.

Daysal highlighted that younger siblings living in areas with higher rates of infant respiratory illness were more likely to utilize mental-health resources in young adulthood.

Their research indicated that approximately 50% of the earnings gap between younger and older siblings could be explained by differences in early health outcomes.

While these findings shed light on one aspect of the firstborn advantage, Daysal noted that studies from various countries consistently show that older siblings tend to have more education and economic success than their younger counterparts. For instance, a study of Norwegian men from 2007 found a slight IQ difference between first- and secondborn children, translating to a 2% disparity in annual earnings. A 2011 Career Builder survey of US adults also indicated that firstborn children were more likely to have a six-figure salary.

Despite these trends, some researchers have challenged certain findings. A 2015 paper, for example, found minimal differences in IQ and personality among siblings.

While the higher likelihood of respiratory illness among younger siblings offers a compelling explanation, experts have pointed to various other factors contributing to the divergent outcomes between older and younger siblings.