Whether someone is the older or younger sibling may be linked to certain health outcomes later in life, according to a massive new study.
Analyzing medical data from 10 million U.S. siblings over two decades, researchers from the University of Chicago and Columbia University found links suggesting that birth order may be connected to specific long-term health risks.
Researchers examined 569 disease categories across families and among siblings, identifying 150 conditions that were significantly associated with birth order.
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First-born children showed a higher likelihood of neurodevelopmental and psychiatric diagnoses, including autism, ADHD, Tourette's and OCD, as well as immune-allergic conditions like food allergies.
Second-born children were statistically more prone to migraines, shingles, substance use disorders and gastrointestinal issues like gastritis.
Discussing the findings on "Fox & Friends," Dr. Marc Siegel, senior medical analyst for Fox News, noted the value of such a wide-scale study.
"I love this study because it opens a lot of questions more than it gives answers, but I think I have some answers," the doctor said.
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Regarding the heightened prevalence of ADHD among first-borns, Siegel offered his own hypothesis about parental behavior.
"My theory, they get more stimuli. The parents are like hovering over them, helicoptering them ... and they don't know where to look," he said.
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Addressing the disparities in allergic conditions, Siegel pointed to early environmental exposure, stating, "I think it's because the firstborn's immune system hasn't seen the environment before. And if it's not set properly, you get an allergy."
Siegel also touched on biological and psychological factors when evaluating differences between siblings.
"I think the womb changes after you have your first child, and the second child gets a womb that's already been used. So therefore, it's more set for it," he said.
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Regarding the higher rate of substance use disorders among second-born children, Siegel suggested emotional drivers, saying, "it may be the same idea: 'I want attention. I'm looking for attention. I wanna do something to get a rise out of you.'"
While the research highlights patterns across a large population, the study comes with limitations.
The findings demonstrate correlations rather than direct causation, meaning the trends don't necessarily stem from physiological changes, early microbial exposures or social dynamics.
Additionally, the data relies on commercial health insurance claims, which can reflect differences in diagnosis, healthcare-seeking behavior and parental attention rather than true biological functions alone.
Factors such as the age of parents between births and varying age gaps between siblings also complicate these associations. Ultimately, researchers emphasized that birth order serves as a modest population-level risk marker rather than a definitive health predictor.
The research was published in the journal Nature Health.