Study of 10 Million Siblings Links Birth Order to 150 Health Conditions. Here’s What Baby No. 2 Is More Likely to Have

Where you land in the birth order might do more than shape your personality. Researchers digging through two decades of medical claims on more than 10 million Americans found something that surprised even them: your spot in the family lineup is statistically tied to which diseases show up on your chart later in life. And the split between oldest and younger children isn’t small.

Researchers Scanned 569 Diseases Before Landing on the Real Number

A team from the University of Chicago and Columbia University pulled insurance claims covering 5.1 million two-child families and more than 10.3 million people between 2003 and 2024. They checked 569 disease categories against birth order, hunting for patterns strong enough to survive rigorous statistical scrutiny rather than coincidence. Of those, only a fraction held up. But that fraction told a striking story.

150 Conditions Came Back With a Clear Birth-Order Signature

When the numbers settled, 150 diseases showed a statistically significant link to whether a person was born first or second. First-borns leaned toward one cluster of conditions entirely. Second-borns leaned toward another, almost as if two different biological or environmental scripts had been running in the same household the whole time.

Oldest Kids Are Turning Up With Autism, ADHD and OCD More Often

First-born children showed higher odds of a specific bundle of neurodevelopmental and psychiatric diagnoses: autism, ADHD, Tourette’s and OCD among them. Food allergies and other immune-related conditions clustered here too. Doctors expected some of this might just reflect nervous first-time parents rushing to appointments. The data told a different story entirely.

Study Co-Author Shoots Down the Obvious Explanation

Andrey Rzhetsky, a professor of medicine and genetics at the University of Chicago, said researchers expected firstborns to rack up more diagnoses simply because first-time parents seek medical care more readily. “It did not,” he said of that theory, after checking the direction of the effects against it. Something else was driving the split. 

The “Hygiene Hypothesis” Might Explain the Allergy Gap

One theory researchers floated involves early germ exposure. Rzhetsky pointed to the hygiene hypothesis, the idea that lower exposure to allergens early in life raises the odds a child develops allergies or asthma later. Younger siblings often pick up more germs from an older brother or sister, especially if that older child didn’t attend daycare. First-borns, by contrast, may be encountering a cleaner environment when it matters most. advisory

Second-Borns Face a Completely Different Set of Risks

Flip to the younger sibling, and the pattern reverses. Second-born children showed statistically higher odds of migraines, shingles, gastrointestinal issues like gastritis, and substance use disorders. Fox News senior medical analyst Dr. Marc Siegel offered a psychological read, suggesting some second-born kids act out physically or emotionally to capture attention that already has a claim on it.

The Split Was Almost a Perfect Coin Flip

Of the 150 conditions that cleared the study’s statistical threshold, 79 skewed toward first-borns and 71 skewed toward second-borns, according to Rzhetsky. That near-even divide surprised researchers who expected one birth position to dominate. Instead, it looked like two distinct biological or environmental pathways, each carrying its own set of consequences. 

Even the Researchers Admit They Can’t Fully Explain It

Rzhetsky described the likely explanation as a mixture of pregnancy-order biology, differences in how closely families are monitored for symptoms, diagnostic timing, and leftover effects tied to parental age, in proportions the dataset can’t fully untangle. In other words, this isn’t one clean mechanism. It’s several forces tangled together, and untangling them will take more research than this study alone can offer. 

Birth Order Isn’t Destiny, But It’s Not Nothing Either

Researchers were careful to frame birth order as a modest population-level marker, not a diagnosis waiting to happen. Being a second child doesn’t guarantee migraines any more than being first guarantees ADHD. But across 10 million people, the pattern was real enough to publish, strong enough to survive scrutiny, and specific enough to reshape how scientists think about where kids fall in the family line.