First-Borns Face Higher Autism and Allergy Risks
A massive study published in Nature Health by researchers from the University of Chicago and Columbia University has mapped how birth order tracks disease risk across more than 10 million US siblings.
According to the study data, first-born children face higher statistical odds of neurodevelopmental and immune-allergic conditions like autism and food allergies, while second-born siblings show increased risk for gastrointestinal disorders, migraines, and substance abuse.
Analyzing a Disease Atlas Across 10.3 Million Individuals
To examine where birth-order patterns diverge within families, researchers analyzed records from the Merative MarketScan commercial claims database. The dataset comprised inpatient, outpatient, and pharmacy claims covering more than 10.3 million individuals from approximately 5 million two-child families.
The study evaluated associations between birth order and 569 distinct diseases categorized into a comprehensive “disease atlas.” Of those 569 conditions, 150 showed statistically significant differences in risk depending on whether a child was born first or second.
Neuropsychiatric Diagnoses Versus Digestive Complaints
First-born children exhibited a higher likelihood of recording claims for allergies, autism, attention deficit hyperactivity disorder (ADHD), Tourette’s syndrome, and obsessive-compulsive disorder (OCD).
Conversely, second-born children showed higher odds of diagnoses for stomach problems, shingles, migraines, and substance abuse issues. The research team noted that these disease-order effects pervade nearly every clinical domain tested, pointing to a distinct concentration of neuropsychiatric diagnoses among first-borns and digestive or musculoskeletal complaints among later-borns.
Conditional Logistic Regression and Sibling Pair Analysis
To ensure the findings were robust and not skewed by external variables like family income or household pets, the research team utilized two distinct study designs. The primary analysis compared 5.1 million sibling pairs within the same family using conditional logistic regression. This within-family approach successfully minimized confounding factors shared by siblings, including parental genetics, socioeconomic status, and shared geographical exposures.
Additionally, the researchers constructed a between-family analysis featuring 1.6 million matched pairs from different families, controlling for variables such as birth year, sex, geographical region, and parental age. The consistency of results across both design methods confirms that birth order correlates with widespread, albeit modest, differences in individual disease risk.
Microbial Exposure and Parental Investment Dynamics
While the exact biological and social drivers require further study, researchers point to established environmental and immunological theories. Younger siblings traditionally experience greater microbial exposure in early life due to the presence of older brothers or sisters, a dynamic that scientific literature suggests may support the development of immune tolerance and lower the risk of certain allergic conditions.
Early-life theorist models also suggest that first-born children typically receive greater parental investment and face higher expectations, whereas later-born children develop in the social wake of older siblings. Furthermore, the study found that family dynamics like age gaps mattered; a wider age gap increased the likelihood of autism in first-borns while decreasing their relative likelihood of developing unique allergies not shared by their siblings.
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