Scientists have long debated whether birth order shapes personality.
Now, a study of more than 10 million Americans suggests your place in the family could also influence your lifelong health.
Researchers found links between family position and the risk of 150 health conditions, including autism, ADHD, allergies and digestive disorders.
Firstborn children were more likely to develop a range of neurodevelopmental conditions, including autism, ADHD, Tourette’s syndrome and obsessive-compulsive disorder (OCD).
They also had higher rates of food allergies, hay fever, asthma and acne than their younger siblings.
Second-born children, meanwhile, were less likely to be diagnosed with many neurodevelopmental conditions.
The researchers found they were 43 percent less likely than firstborns to develop certain autism-related disorders.
However, they had higher rates of digestive, musculoskeletal and infectious diseases, as well as substance abuse disorders, migraines, stomach problems, kidney infections and herpes zoster – the virus that causes shingles.
Firstborn children showed the strongest links to brain development and allergy–related conditions, while second–born children had higher rates of digestive, musculoskeletal, and infectious diseases, according to the study’s chart
Substance abuse was among the strongest associations identified in the study.
The findings, published in Nature Health, were based on the health records of more than 10 million people from 5.1 million US families.
The researchers said there is unlikely to be a single explanation for the findings. Instead, they believe different conditions are probably driven by different biological and environmental factors that change with each child.
For example, younger siblings are exposed to more viruses and bacteria through their older brothers and sisters – something known as the hygiene hypothesis, which may help explain why they appear less likely to develop allergies.
By contrast, differences in neurodevelopmental conditions such as autism and ADHD may stem from subtle changes in the prenatal environment between pregnancies, the researchers suggested.
They also said family dynamics could play a role.
Parents often behave differently with first- and later-born children, while younger siblings grow up in a different social environment, shaped by older brothers and sisters as well as changing expectations at home.
One of the study’s authors, Professor Andrey Rzhetsky, of the University of Chicago, told Newsweek: ‘No one should change a clinical decision because a child is first-born.’
A chart from the research published in Nature Health, this shows the conditions first–born children are more likely to develop compared to second–born kids
Researchers found that 150 conditions – including autism and ADHD – showed significant links to birth order, suggesting where someone falls in the family lineup may influence their health risks
‘Birth order is interesting because it is a universal exposure across an entire population, not because it predicts the health of any individual child.’
He warned that one of the biggest mistakes would be to view the findings as a direct prediction of someone’s future health.
The findings come just months after another study suggested younger siblings really do have it easier.
Researchers at Australia’s Monash University found parents tend to become more lenient with later-born children, relaxing rules around everything from screen time to homework.
As a result, younger siblings spent more time using digital devices and less time on enrichment activities such as reading, music and homework than first-borns.
The researchers said the findings suggested family dynamics change as families grow – something that could help explain why birth order has long been linked to differences in behavior, and perhaps even health.
