August 11, 2026, 4:06 pm | Read time: 6 minutes
Whether firstborn or secondborn, the position among siblings might be more closely linked to health than previously thought. A new study shows that birth order can be associated with the risk of various diseases. While older siblings seem more susceptible to certain illnesses, younger siblings are more prone to others.
Can Birth Order Really Affect Health?
The question of whether birth order is related to health has long intrigued scientists. Previous studies found links to allergies, asthma, autism, mental disorders, and metabolic diseases.1,2 However, they mostly focused on individual diseases or small groups of conditions.
Differences between firstborns and secondborns could be related to factors such as parental age, family environment, or early exposure to microorganisms. Therefore, the researchers examined the link between birth order and the risk of a wide range of diseases.
How Researchers Studied Millions of Siblings
The study was based on U.S. health insurance data from 2003 to 2024. Researchers identified 5,135,006 suitable two-child families, totaling 10,270,012 individuals. Birth years ranged from 1978 to 2013. They examined 569 diseases or diagnostic groups, finding clear associations in 150 cases.3
Comparing Firstborns and Secondborns from Different Families
Initially, researchers compared 1,616,881 firstborns with secondborns from other families. To ensure a fair comparison, they selected individuals with similar characteristics, such as gender, birth year, and living in similarly populated areas. They also considered the observation period in the insurance data, parental age at birth, and the age gap between siblings.
Then Siblings Were Compared Directly
Next, researchers compared the two siblings directly. This allowed for a better analysis of familial influences, as siblings share some of their genes and usually grow up under similar conditions.
Additional Tests to Validate the Results
Further analyses were conducted to determine if other factors could explain the results, such as location, frequency of doctor visits, or parental age.
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These Diseases Show the Greatest Differences
Autism and ADHD: More Common in Firstborns
The difference was particularly pronounced with autism: Secondborns had about 26 percent lower odds of an autism diagnosis compared to firstborns. For ADHD, the difference was much smaller, with secondborns having about 7 percent lower odds.
Differences Also Seen in Allergies and Acne
Similar differences were observed in allergies and skin issues. Secondborns had about 20 percent lower odds of a food allergy diagnosis compared to firstborns. The risks for acne were 13 percent lower, for allergic rhinitis 9 percent lower, and for asthma 3 percent lower.
The greatest difference was found in other or unspecified pervasive developmental disorders. Secondborns had about a 43 percent lower risk of such a diagnosis compared to firstborns. This group includes various developmental disorders, overlapping with but not identical to autism.
Secondborns More Often Affected by These Conditions
For other conditions, the pattern reversed. Secondborns had about 35 percent higher risks for a shingles diagnosis. The risks for substance abuse were 19 percent higher, for biliary tract diseases 18 percent, for stomach and duodenal inflammations 14 percent, and for migraines 13 percent higher.
Direct Sibling Comparison Confirms Key Findings
In the additional direct sibling comparison, significant differences persisted: Secondborns had about a 20 percent lower risk for an autism diagnosis and 6 percent lower for an ADHD diagnosis. However, the risks for a substance abuse diagnosis were about 14 percent higher.
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Why Firstborns and Secondborns Might Have Different Risks
The results do not show a simple pattern where firstborns are inherently healthier or sicker. Notably, certain groups of diseases are more common: Researchers found more frequent links with nervous system developmental disorders and allergic diseases in firstborns. In contrast, secondborns more often showed links with certain digestive and musculoskeletal disorders.
Could Older Siblings Protect Against Allergies?
The authors offer a possible explanation for the differences in allergies. Younger siblings might be exposed to a greater variety of microorganisms earlier through the older child, potentially influencing immune system development. Interestingly, the difference in allergic rhinitis decreased as the age gap between siblings increased. This fits the possible explanation but does not prove it.
Explaining Autism Is More Complex
Explaining autism and other developmental disorders is more challenging. The study does not point to a single cause. According to the authors, multiple factors could be at play, such as biological differences between pregnancies, parental age, parental attention, and the timing of a diagnosis.
What the Study Cannot Prove
A major strength of the study is its vast data base. Additionally, researchers tested their results in various ways. Of the 150 particularly well-supported associations in the main analysis, 127 also pointed in the same direction in the direct sibling comparison. This corresponds to 84.7 percent.
The most important limitation: The study shows associations, not cause and effect. It cannot prove that birth order itself is responsible for the differences. One cannot predict whether someone will develop a specific disease based on their position in the sibling order.
Parental Age Cannot Be Fully Accounted For
An important potential influence is parental age. Parents are inevitably older with the second child than with the first. Researchers tried to account for this difference as accurately as possible statistically. However, according to the authors, the influence of parental age cannot be completely separated from birth order.
Health Insurance Data Has Limitations
The data itself also has limitations. Health insurance data shows documented diagnoses and not necessarily every existing condition. Additionally, firstborns visited the doctor slightly more often and had more documented treatment days. When researchers accounted for this difference, the overall results remained very similar.
The Results Do Not Automatically Apply to Everyone
Only two-child families were studied. The data comes from people with employer-sponsored health insurance in the U.S. According to the study, this group disproportionately includes people with higher incomes and white individuals. Therefore, it is unclear if the results apply equally to other population groups or countries.
The study was funded by the National Institute of Mental Health and the National Institutes of Health. According to the publication, the funders had no influence on the study design, data collection and analysis, decision to publish, or manuscript preparation. The authors reported no conflicts of interest.