Study Finds Birth Order May Be Linked to Differences in Disease Risk

Birth order may be associated with differences in the risk of certain diseases, according to a large new study published in Nature Health. Researchers found that firstborn children were more frequently diagnosed with neurodevelopmental and allergic conditions, while second-born children were more likely to develop digestive disorders and substance use-related conditions.

The researchers analyzed medical records from more than 10 million individuals across approximately 5 million two-child families using the Merative MarketScan database, which contains information on hospitalizations, outpatient care, and prescription medications.

To reduce the influence of factors such as parental genetics, socioeconomic status, place of residence, and parenting practices, the team compared siblings within the same family as well as children from different families.

The most pronounced differences were observed among firstborn children, who were more likely to be diagnosed with:

• Autism spectrum disorder (ASD);

• Attention-deficit/hyperactivity disorder (ADHD);

• Tic disorders and Tourette syndrome;

• Food allergies;

• Allergic rhinitis (hay fever).

In contrast, second-born children showed a higher incidence of:

• Gastrointestinal disorders, including gastritis and duodenitis;

• Substance use disorders;

• Certain infectious diseases, including herpes zoster (shingles).

The researchers noted that previous studies have also suggested that younger siblings generally have a lower risk of allergic diseases. One possible explanation is that younger children are exposed to a greater variety of microbes through their older siblings early in life, which may help train the immune system and reduce susceptibility to allergies.

However, the authors emphasized that birth order itself does not cause disease. The study identifies statistical associations rather than cause-and-effect relationships. Differences in healthcare utilization, parental age, generational factors, and other unmeasured variables may also have influenced the findings.

The researchers also pointed out that the study included only families with two children, meaning further research involving larger and more diverse family structures will be necessary to confirm the results.