Older Siblings Are Smarter and Richer Due to Early Exposure to Illnesses

Older siblings tend to have higher IQs, perform better in school, and earn more as adults compared to their younger brothers and sisters, according to multiple studies examining birth order effects.

This pattern has been observed across various research efforts, with data showing that firstborn children are more likely to achieve six-figure salaries while middle children often remain in entry-level positions earning $35,000 or less annually.

Recent research from health economists analyzing Danish birth records between 1981 and 2017 suggests that differences in early childhood illness exposure may help explain these long-term disparities.

In the first year of life, second-born children face two to three times higher odds of being hospitalized for respiratory conditions compared to their older siblings, according to findings from Stanford University researchers.

This increased vulnerability to illness during critical developmental periods may divert energy away from brain development, potentially affecting cognitive and academic outcomes later in life.

Additional analysis from the University of Copenhagen indicates that early-life illnesses could account for approximately 1.9 percent of the wage gap commonly observed between siblings.

Researchers note that firstborn children often experience fewer infections in infancy due to less exposure to pathogens brought home from group settings like daycare, while younger siblings face higher germ exposure when older brothers and sisters return from school or other activities.

Variations in parental attention during early childhood may also contribute, as firstborns typically receive more one-on-one care before the arrival of subsequent children.

These findings align with earlier studies from the University of Edinburgh, which documented IQ differences between firstborn and later-born siblings emerging within the first year of life.

While birth order patterns show consistent trends across populations, researchers emphasize that individual outcomes vary widely and are influenced by numerous factors beyond family position.