Your Birth Order May Predict Everything From Allergies to Shingles Risk

If you've ever wondered whether your place in the family lineup shaped your health, a new study puts a real number on it. A phenome-wide analysis of more than 10 million people finds that whether you were the first or second child born into your family tracks with your risk for dozens of conditions people commonly research for themselves, from autism spectrum diagnoses and food allergy to migraine and shingles. The researchers even invoke the "old-friends hypothesis" (the idea that early exposure to microbes helps train a developing immune system) to explain part of the pattern. But the study itself never measured a single microbiome sample. Here's what the data actually shows, and where the microbiome talk gets ahead of the evidence.

Key takeaways:

• A study of 10,270,012 people from 5.1 million two-child families tested 569 diseases against birth order; 150 of 418 comparisons that met statistical thresholds (about 36%) held up after strict correction for multiple testing.
• Firstborns showed higher rates of autism, food allergy, and related conditions; second-borns showed higher rates of shingles, migraine, and a handful of other conditions.
• The authors reference the "old-friends hypothesis" as a possible explanation for the allergy-related pattern, but this study did not directly measure anyone's microbiome, so that link is a proposed explanation, not a tested finding.
• This is correlational claims-data epidemiology in a US employer-insured population; it cannot show that birth order causes any of these conditions.

Birth Order Tracks With Risk Across 569 Diseases

Researchers Benjamin Kramer, Steven A. Kushner, and Andrey Rzhetsky, publishing in Nature Health on August 3, 2026, tested birth order against 569 diseases in a dataset of 10,270,012 people drawn from 5,135,006 two-child families, making it one of the largest birth-order studies ever run. Of the 569 conditions, 418 met the between-family statistical threshold for testing, and 150 of those (roughly 36%) remained significant after a strict statistical correction (Bonferroni correction, a standard method for avoiding false positives when testing hundreds of outcomes at once). The split was close to even: 79 conditions showed a firstborn excess and 71 showed a second-born excess.

Autism, Allergies, Migraine, and Shingles: Who's Affected

Firstborns showed higher rates than their second-born siblings for a cluster of neurodevelopmental and allergic conditions: autism (odds ratio, or OR, of 0.74 for the second-born relative to the firstborn), tics or Tourette syndrome (OR 0.69), food allergy (OR 0.80), allergic rhinitis (OR 0.91), acne (OR 0.87), and ADHD (OR 0.93). An odds ratio below 1.0 here means the second-born sibling in a family had lower odds of that diagnosis than the firstborn did. For context, CDC surveillance data put current autism prevalence at about 1 in 31 US children, so even a modest relative shift touches a condition many families already track closely.

Second-borns, meanwhile, ran higher than firstborns for a different set of conditions: herpes zoster, commonly known as shingles (OR 1.35), substance abuse (OR 1.19), biliary tract disease (OR 1.18), gastritis and duodenitis (OR 1.14), and migraine (OR 1.13). Shingles is caused by reactivation of the chickenpox virus and typically strikes later in life or during immune stress, per the CDC's overview of shingles, which makes its appearance in a birth-order study genuinely surprising and not something the "old-friends" story obviously explains.

How Researchers Tracked Ten Million Siblings

The team used two complementary designs, on purpose, so one could check the other. The first was a between-family matched cohort of 1.6 million individuals from different families, matched on sex, birth year, and urbanization. The second was a within-family sibling comparison across the full 5.1 million two-child families, using conditional logistic regression (a statistical method that compares siblings against each other, which cancels out shared family and genetic background). Both designs drew on Merative MarketScan Commercial Claims and Encounters data spanning 2003 to 2024, covering roughly 200 million insured lives in the US employer-sponsored insurance system. The two designs agreed with each other reasonably well: cross-design concordance came out to r = 0.66, with 74.2% of associations pointing in the same direction in both analyses, which is what gives this pattern more weight than a single-design study would carry.

What This Means If You're a Firstborn or Second-Born

None of this is a reason to worry about a diagnosis your family already has, or to expect one because of where you fall in the birth order. Birth order is not something you can change, and the study does not identify a mechanism, let alone anything actionable a doctor could test or treat differently based on it. If you're a parent of a firstborn managing food allergy or a possible autism evaluation, or an adult second-born dealing with recurring migraine (per MedlinePlus's migraine overview, migraine already affects about 12% of Americans) or a shingles diagnosis, this study doesn't change what's worth asking a doctor about. It's a population-level statistical pattern, not an individual risk calculator, and it's not a reason to seek out microbiome testing or supplements the study never tested.

The Limits of Claims-Data Epidemiology

Several limitations keep this from being more than a starting point. The data comes from billable insurance diagnoses, not confirmed clinical incidence, so some of what's being counted is how often a condition gets coded, not just how often it occurs. The design is correlational epidemiology, not a mechanistic study, so it cannot establish that birth order causes any of these outcomes. The sample is a US employer-insured population, which skews toward people with steady jobs and employer coverage and is not fully generalizable to the broader population. And the microbiome connection specifically is hypothesis-referenced, not directly measured: the authors invoke the old-friends hypothesis, the idea that later-born children get more early exposure to older siblings' germs and household microbes, which may shape immune development, to help explain the allergic-disease pattern, but no stool sample, gut-microbiome sequencing, or immune marker was collected in this study. Add to that structural correlation between birth order and parental age, the standard age-period-cohort confounding that affects birth-order research generally, and the possibility that two-child families differ systematically from larger or single-child families, and the honest read is: a strong statistical pattern, not a proven mechanism.

What Comes Next for Birth-Order Research

The obvious next step, one the current study can't take on its own, is a design that actually measures the proposed mechanism instead of inferring it. That would mean tracking early microbial exposure, immune markers, or household environment directly in birth-order cohorts, rather than relying on insurance codes and sibling order as a stand-in for biology. Until studies do that, the old-friends explanation for the allergy-related half of these findings remains a plausible story layered on top of solid epidemiology, not a demonstrated cause.

Common Questions About Birth Order and Disease Risk

Does birth order cause disease? No. This is an observational, correlational study; it cannot establish that being a firstborn or second-born causes any of the conditions it examined.

What is the old-friends hypothesis? It's the idea that early-life exposure to a broader range of microbes, often from older siblings or a less sanitized household, helps train the immune system and may lower risk of allergic disease later on. This study references that idea to help explain its allergy-related findings but did not measure anyone's microbiome directly.

Which conditions showed the strongest birth-order pattern? Firstborns showed higher rates of autism, tics or Tourette syndrome, and food allergy. Second-borns showed higher rates of shingles, substance abuse, and migraine.

Is this study representative of everyone? Not fully. It draws on a US employer-insured population, which skews toward higher income and steadier employment than the general population, so the findings may not generalize to uninsured, underinsured, or non-US groups.

References

1. Helen Lewis, Michael Goodman (2015). Aaron Isaac Goodman. BMJ. https://doi.org/10.1136/bmj.h4561
2. Benjamin Kramer, Steven A. Kushner, Andrey Rzhetsky (2026). Birth order and disease risk across the human phenome: evidence from 10 million siblings. https://doi.org/10.64898/2026.03.26.26349438
3. CDC Autism Data and Research. cdc.gov. https://www.cdc.gov/autism/data-research/index.html
4. CDC: About Shingles. cdc.gov. https://www.cdc.gov/shingles/about/index.html
5. MedlinePlus: Migraine. medlineplus.gov. https://medlineplus.gov/migraine.html