A massive study of health records from more than 10 million American siblings suggests that birth order may be associated with the risk of developing certain diseases and medical conditions later in life.
Researchers from the University of Chicago and Columbia University analyzed two decades of U.S. health insurance data on more than 10.3 million children with one sibling. The study examined 569 disease categories and found statistically significant differences between first-born and second-born children in 150 conditions.
The findings, published in Nature Health, do not show that being born first or second causes a particular disease. Instead, researchers say birth order appears to be a modest population-level marker associated with differences in disease risk.
What the Study Found
The researchers compared siblings within the same families as well as first-born and second-born children from different families. The approach was designed to reduce the influence of factors such as household income and other family characteristics.
Among the 569 disease categories examined, 150 showed different associations with birth order.
First-born children were more likely to have several neurodevelopmental and psychiatric conditions, including autism, attention-deficit/hyperactivity disorder, Tourette’s syndrome and obsessive-compulsive disorder.
They also had higher rates of immune-allergic conditions. The study found that first-born children had a 20% higher risk of food allergies and a 9% higher risk of hay fever compared with their second-born siblings. Autism was associated with a 26% higher risk among first-born children, while ADHD was associated with a 7% higher risk.
Second-born children, meanwhile, showed higher rates of several other conditions. Researchers found a 19% higher risk of substance use disorders and a 14% higher risk of gastrointestinal conditions. Migraines and shingles were also among the conditions statistically more common among second-born children.
The researchers described the overall differences as widespread but modest.
“Birth order is associated with disease risk more broadly than previously appreciated,” the researchers wrote.
Researchers Say Birth Order Is Not a Diagnosis
Despite the large number of records included in the analysis, the findings do not mean that a child’s position in the family determines his or her future health.
The researchers emphasized that the study demonstrates correlation rather than causation.
In other words, the analysis cannot establish that being a first-born child causes autism, allergies, or any of the other conditions associated with first-born children. Likewise, being second-born does not mean a person is destined to develop substance use problems or gastrointestinal disease.
The researchers also noted that health insurance claims can reflect differences in how often people seek medical care or receive diagnoses.
Parents may be more likely to seek medical evaluations for their first child, particularly when they are experiencing a condition for the first time. That difference in health care behavior could contribute to some of the observed patterns.
Other variables can also be connected to birth order. First-born children necessarily have younger parents at the time of their birth than their younger siblings, while the length of time between pregnancies can vary considerably from family to family.
Why Might Birth Order Matter?
The study did not attempt to determine why the associations exist, leaving several possible explanations for future research.
One possibility involves biological differences between pregnancies.
Dr. Mark Hicar, chief of pediatric infectious diseases at the University at Buffalo Jacobs School of Medicine and Biomedical Sciences, said the mother’s age, changes to her body and differences in immune responses could potentially contribute to differences between pregnancies.
Researchers have also pointed to the microbiome and environmental exposure. Younger siblings may encounter microorganisms brought home by older brothers or sisters, potentially influencing immune development.
The age gap between siblings appeared to matter for some findings as well. Researchers reported that a larger age gap was associated with a higher likelihood of autism among first-born children, while a larger gap was associated with a lower likelihood of first-born children having allergies that their siblings did not have.
Parental behavior could also play a role.
First-time parents may be more likely to closely monitor a child’s behavior and seek medical attention when something appears unusual. By the time a second child arrives, parents may respond differently to similar symptoms.
That possibility was also raised by Dr. Marc Siegel, senior medical analyst for Fox News, who suggested that first-born children may receive more parental attention and stimulation.
“My theory, they get more stimuli,” Siegel said while discussing the study on “Fox & Friends.”
He also suggested that differences between pregnancies and early environmental exposures could potentially help explain some of the allergy findings.
Those explanations, however, remain theories rather than conclusions established by the study.
What the Findings Mean for Parents
The researchers stressed that birth order should not be viewed as a reliable predictor of an individual’s health.
A higher relative risk does not mean a condition is likely to occur in a particular child. It simply means the condition occurred at a different rate in one birth-order group than another in the population studied.
The researchers also said birth order is a weaker health marker than many other factors and cannot be changed, meaning the findings do not suggest any specific action parents should take based solely on whether a child is first-born or second-born.
“At the individual level, the effects are subtle,” study co-author Andrey Rzhetsky, a professor of medicine at the University of Chicago, said of the findings, Prevention reported.
Genetics, environmental exposures, lifestyle, parental characteristics and other factors can all influence disease risk, often much more substantially than birth order.
A Large Study That Raises More Questions
The size of the analysis gives researchers an unusually broad look at the possible relationship between family position and health. Rather than examining one condition at a time, the researchers created what they described as a “disease atlas” covering hundreds of medical conditions.
The results suggest that birth-order associations may extend beyond the commonly discussed effects involving personality, development or childhood behavior.
But the study also leaves an important question unanswered: why do these patterns exist?
Researchers say additional work will be needed to distinguish potential biological, immunological and social explanations from differences in health care use, parental behavior and other factors that naturally accompany birth order.
For now, the findings offer evidence that being a first-born or second-born child is associated with differences in disease patterns across large populations, but they do not provide a blueprint for predicting an individual’s health.
As the researchers noted, birth order appears to be associated with widespread but relatively modest differences in disease risk — not a definitive explanation for why any particular person develops a particular condition.
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