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By Alice Gibbs
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Whether you're the oldest child in the family or a younger sibling, your place in the birth order could be associated with the diseases you're most likely to develop, according to a major new study.
Researchers analyzed health records from more than 10 million people across 5.1 million U.S. families and found significant birth-order associations across dozens of conditions, including autism, ADHD, allergies, substance abuse and migraines.
The study, published in Nature Health, identified 150 diseases that showed statistically significant links to birth order, making it one of the largest investigations of the topic to date.
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Newsweek reached out to the study's authors via email for comment.
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Which Diseases Were More Common in First-Born Children?
The strongest links were observed among neurodevelopmental and allergy-related conditions.
Researchers found first-born children were more likely than their younger siblings to be diagnosed with autism, ADHD, Tourette's syndrome and obsessive-compulsive disorder. Higher risks were also observed for food allergies, allergic rhinitis, asthma and acne.
Among the strongest findings, second-born children had about 43 percent lower odds of being diagnosed with an "other or unspecified pervasive developmental disorder" than first-born children, while autism diagnoses were also significantly less common among younger siblings.
One of the study's authors, Andrey Rzhetsky of the University of Chicago, told Newsweek the autism findings remained notable even after researchers accounted for factors that could affect the results, including parental age and family structure.
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Which Diseases Were More Common in Younger Siblings?
Not all of the findings favored later-born children.
The researchers found second-born children were more likely to develop substance abuse disorders, migraines, gastritis, biliary tract disease, kidney infections and herpes zoster, the virus that causes shingles.
Substance abuse emerged as one of the clearest examples of a later-born excess, with second-born siblings showing higher odds of diagnosis than first-born children.
Other later-born associations appeared across digestive, musculoskeletal and infectious disease categories.
Some of these findings surprised the researchers themselves.
"We did not expect to see herpes zoster result coming," Rzhetsky said. He noted that the shingles finding was the largest effect identified in the study and remained consistent across multiple analyses.
Why Might Birth Order Affect Health?
The study does not prove that birth order causes any disease.
Instead, the authors argue that birth-order effects likely reflect a mixture of biological, environmental and social factors that differ between older and younger siblings.
For allergy-related conditions, the researchers point to the long-running "hygiene hypothesis," which suggests younger siblings may be exposed to a wider range of microbes early in life through their older brothers and sisters. That exposure may help shape immune-system development and reduce the risk of allergies.
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One of the study's most unexpected findings was how evenly the results were divided between first-born and second-born children.
"That 150 of the 418 adequately powered diseases cleared a Bonferroni threshold and the direction of effect split almost evenly: 79 with first-born excess, 71 with second-born excess," Rzhetsky said.
He said this was particularly surprising because researchers expected first-born children to have higher diagnosis rates simply because first-time parents may be more likely to seek medical care.
"The standing objection to any claims-based birth-order study is that first-time parents bring their children to the doctor more readily, which should have skewed nearly everything toward first-born excess," he said. "It did not."
The explanation for neurodevelopmental conditions is less clear. The authors said factors including parental age, pregnancy-related biology, healthcare utilization and differences in parental attention may all contribute to the patterns observed in the data.
Rzhetsky said the findings are likely the result of several overlapping factors rather than a single cause.
"The conservative reading is a mixture of pregnancy-order biology, differential surveillance, diagnostic timing, and residual parental-age confounding, in proportions this dataset cannot resolve," he said.
For substance abuse, researchers said the findings are broadly consistent with previous theories suggesting younger siblings may be more influenced by older siblings' behavior and social environments.
Study Limitations
Despite the findings, authors cautioned that birth order is only one factor among many influencing health outcomes and that most of the observed effects were relatively modest at the individual level.
Rzhetsky said one common mistake would be to treat the study as a direct measure of disease risk.
"That this is a map of disease. It is a map of diagnosis," he said. "Claims data record billable encounters, so every estimate here is a composite of underlying risk, care-seeking, provider behavior, and coding practice."
The research relied on insurance claims data, meaning it measured diagnosed conditions rather than all cases of disease. The study also focused on two-child families covered by employer-sponsored insurance, which may limit how well the findings apply to other populations.
Still, Rzhetsky emphasized that the findings should not be used to make decisions about an individual child's health.
"No one should change a clinical decision because a child is a first-born," he said. "Birth order is interesting here because it is a universal exposure applied to an entire population, not because it is informative about any one child."
Reference
Kramer, B., Kushner, S.A. & Rzhetsky, A. Birth order and disease risk across the human phenome. Nat. Health (2026). https://doi.org/10.1038/s44360-026-00177-z
Contact Newsweek editors on this story: Rebecca Flood and Emma Lee-Sang
Update 08/05/26 10:45 a.m. ET: This story was updated with comment from study author Andrey Rzhetsky.
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