Almost everyone knows their place in the family lineup, which is exactly why a new analysis of more than 10 million American siblings is likely to travel further than most epidemiology. Researchers at the University of Chicago and Columbia University screened 569 diseases in commercial insurance claims and found that birth order was significantly associated with 150 of the 418 conditions with sufficient cases to test.
The study, published in Nature Health, used Merative MarketScan claims data covering 2003 through 2024. The headline number is striking. The single largest effect was stranger still and caught the authors off guard.
Shingles, of All Things, Topped the List
Firstborns showed higher recorded rates of neurodevelopmental and psychiatric diagnoses, including autism, tics and Tourette syndrome, obsessive-compulsive disorder and ADHD, along with allergy-related conditions such as food allergy, allergic rhinitis and asthma. Second-borns showed higher rates of substance abuse, migraine, gastritis, and biliary tract disease.
Then there is herpes zoster, the virus that causes shingles, which second-borns were about 35 percent more likely to have. "We did not expect to see herpes zoster result coming," senior author Andrey Rzhetsky of the University of Chicago told Newsweek, noting that it was the largest effect in the study and held up across multiple analyses.
All odds ratios in the paper compare second-borns with firstborns, so a value below 1.0 indicates firstborn excess. Autism came in at 0.74, food allergy at 0.80, allergic rhinitis at 0.91, and ADHD at 0.93. On the other side, substance abuse reached 1.19, biliary tract disease 1.18, and gastritis or duodenitis 1.14.
Two Study Designs Built to Cancel Out Family Effects
The team used two complementary approaches. The first matched 1.6 million firstborn and second-born pairs drawn from different families, aligned on sex, birth year, location, parental ages, and sibling spacing. The second compared siblings inside the same household, drawing on 5.1 million two-child families and 10.3 million individuals.
That second design does specific work. Siblings raised together share parental genetics, neighborhood, socioeconomic position, and family attitudes toward seeking care, so comparing them within a household removes confounders that plague between-family studies. Among the 150 significant associations, 85 percent pointed in the same direction in both designs.
Rzhetsky said the standing objection to any claims-based birth order study is that first-time parents take their children to the doctor more readily, which should push nearly everything toward firstborn excess. It did not. The directions split almost evenly, with 79 showing first-born excess and 71 showing second-born excess.
Association, Not Destiny, and the Numbers Are Small
The most important thing to understand about this study is what it does not show. It does not establish that being born first causes autism, or that being born second causes migraine.
Rzhetsky put the limitation bluntly. "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." Commercially insured Americans are also not a representative national sample, and uninsured and Medicaid populations are largely absent.
The effect sizes are modest. Asthma, for instance, came in at 0.97, a difference of roughly 3 percent in relative terms. For an individual family, that is not a meaningful basis for any decision.
Effect sizes did shift with the age gap between siblings, and the preprint version of the analysis lays out those patterns. The firstborn excess for autism was strongest when siblings were four to six years apart and weakened at both narrower and wider gaps. The firstborn excess for allergic rhinitis faded steadily as spacing widened, which fits the hygiene hypothesis: younger siblings are exposed to a broader range of microbes early because older siblings bring them home, and closer spacing means more of that exposure.
The authors also tested whether parents simply stop having children after a first child is diagnosed with autism, which could manufacture a false firstborn pattern. That effect was real but far too small to account for the association, and the within-family design is immune to it by construction.
Why It Still Matters Beyond a Social Media Talking Point
Birth order has a rocky scientific reputation, largely because personality research repeatedly failed to find much. A rigorous analysis of more than 20,000 adults in three countries found no effect on extraversion, emotional stability, agreeableness, conscientiousness or imagination, leaving only a slight firstborn edge on intelligence and self-reported intellect. A companion commentary called the relation to personality little or nothing at all.
Health records tell a different story, and the reason is statistical power. Screening the full range of diagnoses across millions of sibling pairs, rather than checking one condition at a time in a few thousand people, can detect small effects that smaller studies miss.
The practical value is for researchers rather than families, and Rzhetsky said as much. "No one should change a clinical decision because a child is a first-born," he said. Anyone worried about symptoms in themselves or a child should raise them with a clinician, who will work from that person's actual history rather than their position in the sibling order.
Key Questions Answered
What did the study find?
Birth order was significantly associated with 150 of 418 disease categories analyzed in US insurance claims from two-child families, with roughly half favoring each birth position.
Which conditions were more common in firstborns?
Neurodevelopmental and psychiatric diagnoses, including autism, ADHD, tics, and Tourette syndrome, plus allergy-related conditions such as food allergy, allergic rhinitis, and asthma.
Which were more common in second-borns?
Shingles showed the largest gap among the conditions studied. Substance abuse, migraine, gastritis, and biliary tract disease were also recorded more often among second-born siblings.
Does birth order cause these conditions?
No. The study identifies statistical associations in billing records. The senior author described it as a map of diagnosis rather than a map of disease.
How large are the differences?
Mostly small. Asthma, for example, differed by about 3 percent in relative terms, which is negligible at the level of an individual family.
Should parents change anything based on this?
No. The findings are relevant to researchers studying early-life exposures. Concerns about a specific child should go to a pediatrician.
