Study Suggests Birth Order Is Associated With Differences in Childhood Health

Parents often notice that their children have very different personalities, despite growing up in the same household. Now, a large new study suggests that birth order may also be associated with subtle differences in health. Researchers found that first- and second-born children showed different risks for hundreds of medical conditions throughout childhood and adolescence.

The findings, published in Nature Health, come from one of the largest birth-order studies ever conducted. Scientists from the University of Chicago and Columbia University analyzed health insurance records from more than 10.3 million children, providing new insight into how family structure may influence health beyond genetics and environment.

How the researchers built a disease atlas

To investigate birth-order differences, the researchers examined medical records from children who had exactly one sibling. This allowed them to compare first-born and second-born children while minimizing the influence of larger family dynamics.

The team created what they describe as a disease atlas, analyzing associations between birth order and 569 different medical conditions. They compared siblings within the same family and also compared first- and second-born children across unrelated families to reduce the influence of factors such as income, home environment, and pet ownership.

Researchers also examined whether the age gap between siblings affected the results, since parental age, lifestyle, and family circumstances often change between pregnancies and may influence children's health.

Different health patterns emerged

Among the 569 conditions studied, 150 showed statistically meaningful differences between first- and second-born children.

First-born children were more frequently diagnosed with several neurodevelopmental and psychiatric conditions, including autism spectrum disorder, attention-deficit/hyperactivity disorder (ADHD), Tourette syndrome, obsessive-compulsive disorder (OCD), and various allergic conditions.

Second-born children, meanwhile, showed higher rates of several digestive disorders, migraines, shingles, and certain substance-use disorders.

The researchers emphasize that these differences represent relatively small changes in risk for any individual child. The patterns become noticeable only when examining very large populations.

Sibling age gaps may also matter

The distance in age between siblings appeared to influence some of the findings.

According to the study, larger age gaps were associated with a greater likelihood that only the first-born child would receive an autism diagnosis compared with the younger sibling.

On the other hand, wider spacing between siblings appeared to reduce the probability that allergies would occur only in the first-born child. This finding suggests that shared environmental exposures during childhood may influence immune system development in both siblings.

Why might birth order influence health?

The researchers do not believe a single explanation accounts for all of the observed differences. Instead, they propose that multiple biological and environmental factors probably work together.

Possible contributors include differences in prenatal conditions, immune development, parental age, early-life infections, stress levels, parenting experience, and changing family circumstances between pregnancies.

First-born children experience life with younger, first-time parents, whose lifestyles, medical care, and parenting approaches may differ from those during later pregnancies. Younger siblings, meanwhile, grow up alongside older children, increasing their exposure to infections, microbes, and different social interactions during early childhood.

The findings should be interpreted cautiously

The authors stress that birth order does not determine a child's health or future.

Because the study relied on insurance records, some medical conditions may have gone undiagnosed or been recorded differently between families. The data also cannot fully account for genetics, parenting style, household relationships, or other factors that may influence health.

Future studies will aim to separate the effects of birth order from influences such as parental age, pregnancy-related factors, and early-life infections. Researchers also hope to determine whether similar patterns appear in other countries with different healthcare systems and family structures.

A new perspective on birth order

Birth order has long been debated in psychology and medicine. Previous studies have suggested small associations with intelligence, personality, and social behavior, while finding little consistent evidence that birth order affects career success or overall life satisfaction.

By expanding the investigation to hundreds of childhood diseases, this new research suggests birth order may have subtle effects on health that begin very early in life. Even so, the authors emphasize that these influences are far smaller than established factors such as prenatal care, vaccination, nutrition, access to healthcare, and a stable family environment.