Are first-borns and second-borns prone to different illnesses?
Yes. In a study of more than 10 million American siblings, birth order was linked to 150 different diagnoses. First-borns were diagnosed more often with autism, allergies and anxiety. Second-borns were diagnosed more often with shingles, migraines and substance use problems.
Researchers at the University of Chicago pulled two decades of insurance claims and built a cohort of 5,135,006 two-child families, or 10,270,012 children total. They then asked a simple question about 569 different diseases: does the older child or the younger child get this diagnosis more often?
The answer turned out to be “it depends on the disease,” and the pattern was not random. Certain whole categories of illness leaned one way, and others leaned the other way.
What the data show
Throughout the study, the numbers compare second-borns with first-borns. A figure below 1 means the second child was diagnosed less often, which is the same as saying the first child was diagnosed more often.
The biggest gaps showed up in brain and behavior conditions. Second-borns were about 26% less likely to be diagnosed with autism (odds ratio 0.737), and the range is tight enough that the true gap is very likely between 24% and 29% (95% confidence interval 0.714 to 0.760). Tics and Tourette syndrome ran about 31% lower in second-borns (0.693), and a broader group of developmental disorder codes ran about 43% lower (0.569). ADHD, which is far more common than any of those, ran about 7% lower in second-borns (0.93), and anxiety and phobic disorders about 11% lower (0.889). With cohorts this size, the chance that any of these gaps is a coincidence is vanishingly small.
Allergy followed the same direction. Food allergy was about 20% less common in second-borns (0.797), allergic rhinitis about 9% less common (0.910) and asthma about 3% less common (0.97). Acne was about 13% less common in second-borns (0.866).
Second-borns carried their own load. Shingles was the single strongest finding in that direction, about 35% more common in second-borns (1.348), followed by substance abuse at about 19% more common (1.192), gallbladder and bile duct disease at about 18% (1.179), gastritis and duodenitis at about 14% (1.142) and migraine at about 13% (1.128).
Of the 418 diseases with enough cases to test, 150 crossed the study’s strict statistical bar. The split was almost even: 79 favored the first-born and 71 favored the second-born.
Dr. Kumar’s Take
This is the kind of finding that gets turned into a personality quiz by the time it reaches social media. What the study actually shows is a small shift in odds across a population, not a prediction about your child. That is a nudge, not a verdict.
The design is what makes it credible. The team ran the same question two ways. They matched a first-born from one family against a second-born from a different family, and separately they compared siblings inside the same family, where parents, genetics, income and neighborhood are held constant by definition. The two approaches agreed on direction for 84.7% of the significant findings. Autism, for example, held up inside families at about 20% lower odds in the second-born (0.804).
The allergy signal is the most convincing piece here, because it fits a mechanism we have suspected for forty years. Younger siblings grow up sharing germs with an older child from day one, and that early microbial exposure appears to train the immune system toward tolerance. The study supports this in an elegant way: the allergic rhinitis effect faded as the age gap between siblings widened, which is exactly what you would expect if the older sibling is the source of the exposure.
How confident should we be?
Claims data record diagnoses that generated a bill, not disease itself. If parents simply take the first child to the doctor more often, first-borns would look sicker without being sicker. The researchers checked this directly and found first-borns had a mean of 24.0 doctor visit days compared with 22.9 for second-borns, a gap of about 4.5%. Rerunning the models with visit counts included barely moved the results.
There is a second argument against pure attention bias. If eager first-time parents were driving everything, nearly all the findings would favor first-borns. Instead the split was close to even, and several second-born conditions like shingles and kidney infection are driven by objective illness rather than parental watchfulness.
Real limits remain. Parental age is tangled up with birth order by definition, since first-borns always have younger parents. The data cover employer-insured families, who skew higher income, so the findings may not carry over to uninsured or Medicaid populations. And the study only looked at families with exactly two children.
Practical Takeaways
- Treat birth order as a faint background signal, not a diagnosis. A 10% shift in odds across millions of children says almost nothing about any one child in your house.
- If your older child has severe allergies or asthma, do not assume the younger one is protected. The younger sibling’s odds were lower by single digits for asthma, which is far from immunity.
- Bring your family history to appointments rather than your birth order. A sibling already diagnosed with autism, ADHD or food allergy is a far stronger reason for early evaluation than being born first or second.
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FAQs
Does birth order still matter once you are an adult?
Some of these diagnoses are childhood conditions, but several are not. Shingles, migraine, gallbladder disease and gastritis all showed second-born excess, and those are conditions that mostly surface in the teens and beyond. Children in this cohort were born between 1978 and 2013 and had to be at least 12 years old at their last observation, so the follow-up extends well past childhood for many of them. The honest read is that the pattern does not vanish at 18, though nobody has shown it drives outcomes in later life.
Why would being the first child raise allergy risk?
The leading explanation is microbial. A second child is exposed from birth to the germs an older sibling brings home from daycare and school, and that early, varied exposure appears to teach the immune system what to ignore. A first child has no older sibling doing that job. The strongest supporting clue in this study is timing: the first-born allergy disadvantage shrank as the gap between siblings grew, which is what you would predict if proximity to an older child is the active ingredient.
Could the autism finding just be because parents stop having kids after a diagnosis?
That concern is real and it has a name, reproductive stoppage. If parents who have a child diagnosed with autism are less likely to have a second child, autism would pile up among first-borns for reasons that have nothing to do with biology. The researchers tested it and found a first-born autism diagnosis was linked to about a 13% relative reduction in having a second child (odds ratio 0.870). That is a real effect, but it is too small to explain the roughly 26% gap they measured, and the within-family comparison, which is immune to this problem by design, still showed the same pattern.
Bottom Line
Birth order shows up across the human disease map far more broadly than anyone had documented, touching about 150 conditions in a cohort of 10 million siblings. First-borns lean toward allergic, immune and neurodevelopmental diagnoses, while second-borns lean toward infections, digestive problems, migraine and substance use. The effects are small for any individual child, but they are consistent across two independent study designs, and the allergy pattern in particular points at something biological rather than statistical noise.
