For years, people have assumed that firstborn children do better in life because they are raised with stricter discipline, greater responsibility, or higher parental expectations. But a new study suggests the real explanation may be far less sentimental and far more biological: infections.

Research highlighted by The Economist suggests that firstborn children, on average, tend to perform better academically and earn more later in life than their younger siblings. While this pattern has long been recognised, the latest findings point towards an unexpected factor behind it: germs.

Popular stereotypes often paint eldest siblings as organised and dependable, while younger siblings are seen as rebellious or carefree. Yet large-scale studies have found little convincing evidence linking birth order to personality traits. Instead, scientists are now looking at how early childhood exposure to illness may shape development.

According to the research, children born after the first sibling are exposed to significantly more viruses and bacteria from infancy. Older siblings frequently bring home infections from nurseries, schools, or playgrounds, exposing younger children to repeated illnesses at a very early age.

Researchers believe that frequent infections during those formative years, even relatively mild illnesses such as seasonal flu, could slightly affect neurological development and cognitive growth. The impact on any individual child may be almost impossible to notice, but across millions of families, the pattern begins to emerge statistically. On average, firstborns appear to gain a small developmental advantage.

One striking example involved a Dutch family whose second child had already suffered five serious colds before reaching the age of two, largely due to infections brought home by his older sister. The mother recalled that the child often appeared exhausted and less responsive during infancy. While anecdotal evidence does not prove the theory, it offers a vivid glimpse into how an older sibling can dramatically alter the biological environment in which a younger child grows up.

Perhaps the most intriguing detail is that this so-called “firstborn advantage” does not appear among only children. Researchers argue that the effect depends specifically on the cycle of infection passed between siblings.

Still, the picture is not entirely one-sided. Younger siblings may benefit in other ways. Greater exposure to germs during childhood often strengthens the immune system over time, while growing up with older siblings can also improve social adaptability and resilience. Those qualities may not always appear in examination results, but they can prove equally valuable later in life.

Ultimately, the study challenges the comforting myths families often create about birth order. Success may have less to do with personality or parenting style than with biology, chance, and the invisible world of microbes.

The unsettling implication is difficult to ignore: sometimes the difference between siblings is not shaped by parenting at all, but by the viruses they catch growing up.