A study spanning 10.3 million children with at least one sibling has found that birth order is statistically linked to differences in risk across 150 of the 569 health conditions researchers examined — including higher rates of autism and ADHD diagnoses among firstborns compared with their younger siblings. The research, from teams at the University of Chicago and Columbia University and published in Nature Health on August 7, 2026, is among the largest sibling-comparison studies of its kind, and its authors are explicit that what they found is a population-level pattern, not a prediction about any individual child.
A Data Set Built to Rule Out Coincidence
Ten million children is a large enough sample that researchers could compare siblings raised in the same household, by the same parents, at different birth positions — a design that helps separate birth-order effects from the countless other differences between unrelated families. Out of 569 distinct health conditions checked, 150 showed a measurable difference tied to birth order, according to coverage of the findings. That’s roughly one in four conditions studied — a wide enough footprint that the researchers describe birth order as associated with disease risk more broadly than previously appreciated.
What Firstborns Actually Carry More Risk Of
Firstborn children in the study showed higher rates of autism, ADHD, Tourette’s syndrome, obsessive-compulsive disorder, and allergies compared with their younger siblings. That list is worth reading carefully: it groups two neurodevelopmental conditions that get significant public attention with two conditions — Tourette’s and OCD — that rarely make headlines, plus a category, allergies, that has nothing to do with brain development at all. The breadth of that list is itself a clue that no single, simple mechanism explains it; researchers are looking at everything from parenting-style differences with a first child to biological factors tied to a first pregnancy.
Younger Siblings Have Their Own List
The pattern runs in both directions. Second-born and later children in the data showed higher rates of stomach problems, migraines, shingles, and substance-abuse-related issues. No condition in the study belonged exclusively to one birth position across the board — instead, the researchers found a set of trade-offs, with firstborns and later-borns each carrying somewhat elevated risk for different things, rather than one birth position simply being healthier than the other.
The Age-Gap Twist
One of the more specific findings involves how far apart siblings are in age. A wider age gap between a firstborn and their next sibling was associated with a higher autism-risk difference for the firstborn — but a smaller allergy-risk difference, running in the opposite direction. That interaction matters because it suggests the mechanism behind at least some of these associations isn’t fixed by birth order alone; something about the spacing between children is doing independent work, which is a more complicated story than “first kids are just wired differently.”
Why “Modest” Is the Operative Word
The researchers describe the effects they found as modest in size, and they’ve flagged that further work is needed to understand mechanisms and to separate out confounding factors such as parental age at each birth — older parents, for instance, tend to have different baseline health profiles than younger parents, and that alone could explain part of what looks like a birth-order effect if it isn’t carefully controlled for. None of this means a firstborn child is likely to be autistic or have ADHD, or that a younger sibling is destined toward migraines; population-level statistical associations, even ones found across 10 million children, describe patterns in large groups, not the odds facing any one child in a family of two, three, or four.
A Pattern Worth Watching, Not a Verdict
What makes this study notable isn’t that it proves birth order determines health outcomes — it doesn’t, and the researchers say so directly. What it does is give scientists a real, large-scale signal to chase, across a list of conditions wide enough that the old idea that firstborns are simply more anxious looks almost quaint by comparison. The next round of research will likely spend years trying to figure out which parts of this pattern trace back to biology, which trace back to how parents behave differently with a first child versus a third, and which trace back to something else entirely — and until that work is done, the honest takeaway for any individual family is that birth order is one small thread in a much larger, largely unpredictable tapestry.
