Your place in the family may shape more than personality. A new study indicates birth order could affect the risk of developing certain diseases. First-born children face higher odds of conditions like ADHD and food allergies, while second-born children show increased risks for substance abuse and gastrointestinal disorders.
First-borns and neurodevelopmental risks
The study, published in Nature Health, analyzed health data from over five million two-child families. It found first-born children had a 26% higher risk of autism and a 7% higher risk of ADHD compared to younger siblings. These children were also 20% more likely to develop food allergies and 9% more likely to have hay fever.
Andrey Rzhetsky, a study co-author and professor of medicine at the University of Chicago, said the results came from observing families where older and younger siblings had distinct health patterns. “We are talking about population-level effects,” he explained. “At the individual level, the effects are subtle.”
Related: Best Foot Massagers for Neuropathy, According to Podiatrists
Second-borns face different health challenges
For second-born children, the risks changed. The analysis revealed a 19% higher likelihood of substance abuse disorders and a 14% increased risk of gastrointestinal conditions. A shorter gap between siblings appeared to reduce the chances of neuropsychiatric and allergic conditions.
The causes of these differences are not yet understood. While the study did not explore why they occur, experts have proposed explanations. Mark Hicar, chief of pediatric infectious diseases at the University at Buffalo, noted biological changes between first and later pregnancies. “There are shifts in the mother’s body, including immune responses,” he said. “Each baby contains foreign material from the father, and the mother’s immune system becomes more tolerant with later pregnancies.”
Parenting behavior may also contribute. First-time parents often seek medical evaluations more frequently, while later-born children might experience fewer environmental restrictions. “Planned dietary exposures tend to be stricter with first-borns,” Hicar added.
Related: New study finds supplement eases knee arthritis pain
Rzhetsky points out that siblings also tend to share a microbiome. “This could explain signals in infectious diseases, allergies, and the potential protective effect of a small age gap for neurodevelopmental conditions,” he says.
Future research could explore whether these patterns apply to larger families or different cultures. The results provide an interesting look at how family structure might influence health, though they do not offer clear answers.
