A study of more than 10 million Americans has revealed that a person's position in their family influences their lifelong health risks, linking firstborn children to higher rates of autism and ADHD.
Published in the medical journal Nature Health, the research analyzed health records from 5.1 million US families across 150 medical conditions, uncovering distinct health profiles for oldest and younger siblings.
Firstborn children showed the strongest links to neurodevelopmental conditions affecting brain development, as well as allergy-related issues. Beyond autism spectrum disorder and attention deficit hyperactivity disorder (ADHD), oldest children had higher rates of Tourette's syndrome, obsessive-compulsive disorder (OCD), food allergies, hay fever, asthma, and acne compared to their younger siblings.
Second-born health risks and substance abuse
In contrast, second-born children were less likely to be diagnosed with many neurodevelopmental issues, showing a 43 percent lower likelihood than firstborns of developing certain autism-related disorders.
However, younger siblings faced heightened risks for a wide variety of physical illnesses and behavioral health conditions. Researchers found that second-born children experienced higher rates of digestive, musculoskeletal, and infectious diseases, as well as migraines, stomach problems, kidney infections, and herpes zoster, the virus responsible for shingles.
Substance abuse disorders also emerged as one of the strongest associations identified in the study among younger siblings.
Biological and environmental factors
The study's authors explained that no single mechanism accounts for the findings across all 150 conditions. Instead, they believe different health outcomes are driven by distinct biological and environmental factors that evolve with each child born into a household.
Under the hygiene hypothesis, younger siblings are exposed to more bacteria and viruses transmitted by older brothers and sisters. Scientists believe this early immune system stimulation may help explain why later-born children appear less prone to developing food allergies and hay fever.
Conversely, differences in neurodevelopmental conditions such as autism and ADHD may stem from subtle biological shifts in the prenatal environment between successive pregnancies.
Family social dynamics also play a role as household environments change. Parents frequently alter their parenting style between first-born and later-born children, while younger siblings grow up in a social setting shaped by older brothers and sisters alongside changing expectations at home.
Despite the wide-ranging population findings, researchers warned against applying the results to individual medical cases. Study co-author Professor Andrey Rzhetsky, of the University of Chicago, emphasized that birth order should not guide healthcare choices.
"No one should change a clinical decision because a child is first-born," Professor Rzhetsky told Newsweek.
"Birth order is interesting because it is a universal exposure across an entire population, not because it predicts the health of any individual child," Rzhetsky added, warning that assuming the study acts as a direct prediction of a person's future health would be a major mistake.
Parenting styles and family dynamics
The research arrives several months after a separate study from Australia's Monash University indicated that younger siblings experience different upbringing environments. That research found parents tend to become more lenient with later-born children, relaxing household rules regarding screen time, homework, and device usage.
As a result of these shifting parental expectations, younger siblings spent more time using digital devices and less time engaging in enrichment activities such as reading, music, and homework compared to firstborns.
Researchers from Monash University noted that these changing family dynamics demonstrate how household environments evolve as families grow, offering potential insights into why birth order has long been connected to differences in child behavior and long-term health risks.
