How Birth Order Impacts Your Health and Mental Risks

According to a study published in Nature Health, birth order is significantly linked to distinct long-term health risks across millions of siblings. Analyzing medical data from 10 million U.S. siblings over two decades, researchers from the University of Chicago and Columbia University identified 150 conditions out of 569 disease categories that share a statistical association with whether a child is born first or second.

First-Born Sibling Health Risks and Neurodevelopmental Diagnoses

First-born children show a higher statistical likelihood of receiving neurodevelopmental and psychiatric diagnoses, according to the study data. These conditions include autism, attention-deficit/hyperactivity disorder (ADHD), Tourette’s syndrome, and obsessive-compulsive disorder (OCD). Additionally, first-borns exhibit an increased prevalence of immune-allergic conditions, such as food allergies.

Appearing on Fox & Friends, Fox News senior medical analyst Dr. Marc Siegel discussed these findings and offered hypotheses regarding parental behavior and immune system development. “My theory, they get more stimuli. The parents are like hovering over them, helicoptering them … and they don’t know where to look,” Siegel said regarding the heightened prevalence of ADHD among first-borns.

Addressing the disparities in allergic conditions, Siegel pointed to early environmental exposure. “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 stated.

Second-Born Health Risks and Physiological Factors

Second-born children face increased statistical risks for a different set of medical conditions. According to the research findings, younger siblings are more prone to migraines, shingles, substance use disorders, and gastrointestinal issues such as gastritis.

Siegel also pointed to 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,” Siegel said. Regarding the higher rate of substance use disorders among second-born children, Siegel suggested emotional drivers, stating it may stem from a desire for attention.

Study Design, Cohort Details, and Limitations

According to background data from Nature, the underlying research screened nearly 28 million families from Merative MarketScan data spanning 2003 to 2024. Researchers established analytical cohorts including a primary between-family matched cohort of 3.2 million individuals in 1.6 million matched pairs, a stricter clinically matched cohort of 1.1 million individuals, and a within-family sibling comparison cohort involving 5.1 million families.

The study highlights population-level statistical correlations rather than direct causation. Researchers noted that the trends do not necessarily stem strictly from physiological changes, early microbial exposures, or social dynamics. Furthermore, the data relies on commercial health insurance claims, which can reflect variations in diagnosis, healthcare-seeking behavior, and parental attention rather than true biological functions alone. Factors such as parental age between births and varying age gaps between siblings also complicate these associations, leading researchers to emphasize that birth order serves as a modest population-level risk marker rather than a definitive health predictor.

Frequently Asked Questions

Does birth order directly cause medical conditions like ADHD or migraines?

No. According to the researchers, the study demonstrates population-level statistical correlations rather than direct causation, serving as a modest risk marker rather than a definitive predictor.

What conditions are more common in first-born children?

First-born children show higher rates of neurodevelopmental and psychiatric diagnoses such as autism, ADHD, Tourette’s syndrome, and OCD, as well as immune-allergic conditions like food allergies.

What health risks face second-born children?

Second-born children face increased statistical risks for migraines, shingles, substance use disorders, and gastrointestinal issues such as gastritis.

How large was the study population?

Researchers analyzed medical data from 10 million U.S. siblings over two decades, screening millions of family records to build matched cohorts for comparison.