Study details

For the study, researchers analyzed a health records database of more than 10 million individuals that were part of over 5 million two-child families in the United States. The data was collected over 20 years from commercial insurance claims.

The researchers screened for 569 disease categories utilizing two approaches. First, they compared 1.6 million firstborn children with secondborn children from different families who were matched by sex, birth year, location, and similar parental ages. The researchers also directly compared firstborn and secondborn siblings within the same household across all families to help account for shared genetics, parenting, and home environments.

Of the 418 diseases with enough cases to analyze, the researchers found that 150 showed significant associations with birth order. Firstborns were more likely to be diagnosed with neurodevelopmental and some mental health conditions, such as autism, ADHD, Tourette's, depression, and anxiety.

Firstborns also had higher rates of allergy-related conditions like asthma, allergic rhinitis, and food allergies. However, sibling age gaps of less than four years were associated with a lower rate of allergies and asthma.

Meanwhile, secondborns had high rates of substance abuse, migraines, gastrointestinal issues like gastritis, shingles, and some joint conditions.

"The standing objection to any claims-based birth-order study is that first-time parents bring their children to the doctor more readily, which should have skewed nearly everything toward firstborn excess," Rzhetsky added. "It did not."

Rzhetsky said the findings are likely the result of multiple overlapping factors rather than one single cause. "The conservative reading is a mixture of pregnancy-order biology, differential surveillance, diagnostic timing, and residual parental-age confounding, in proportions this dataset cannot resolve," he said.

Rodica Damian, an associate professor of social psychology at the University of Houston who was not involved in the study, said the study's strength is in its large sample size and design. "I'm confident that the results that [the researchers] see are probably there, but it's important to note that they didn't measure actual disease occurrence," Damian said.

In addition, how a person interacts with the healthcare system might not have anything to do with their likelihood of developing a condition. The study didn't include uninsured families or those on Medicaid, which means it skewed wealthier and healthier as poorer families may not have access to high-quality healthcare, according to Julie Rohrer, who studies birth order at Leipzig University in Germany and was not involved in the study.

Regardless, Rzhetsky noted that the study's findings should not be used to make decisions about an individual child's health.

"No one should change a clinical decision because a child is a firstborn," he said. "Birth order is interesting here because it is a universal exposure applied to an entire population, not because it is informative about any one child."