Scientists have long debated whether birth order determines personality.
Now a survey of more than 10 million Americans suggests that your place in the family can also affect your lifelong health.
Researchers found links between family position and the risk of 150 health problems, including autism, ADHDallergies and digestive disorders.
Firstborn children were more likely to develop a range of neurological conditions, including autism, ADHD, Tourette’s syndrome and obsessive-compulsive disorder (OCD).
They were also more likely to suffer from food allergies, hay fever, asthma and acne than their younger siblings.
Second-born children were less likely to be diagnosed with many neurodevelopmental disorders.
The researchers found that they were 43 percent less likely than firstborns to develop certain autism-related disorders.
However, they were more likely to suffer from digestive, musculoskeletal and infectious diseases, as well as substance abuse disorders, migraines, stomach problems, kidney infections and herpes zoster – the virus that causes shingles.
According to the study’s graph, first-born children showed the strongest association with brain development and allergy-related conditions, while second-born children had higher rates of digestive, musculoskeletal and infectious diseases.
Substance abuse was one of 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 American families.
The researchers said it is unlikely there is a single explanation for the findings. Instead, they believe that different conditions are likely caused by different biological and environmental factors that change in each child.
For example, younger siblings are exposed to more viruses and bacteria through their older siblings – something known as the hygiene hypothesis, which may help explain why they seem less likely to develop allergies.
In contrast, differences in neurodevelopmental disorders such as autism and ADHD may arise 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 toward first-born and later-born children, while younger siblings grow up in a different social environment, shaped by older siblings and by 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 a firstborn.”
A graph from the study published in Nature Health shows under which conditions first-born children develop more often than second-born children
Researchers found that 150 conditions – including autism and ADHD – showed significant links with birth order, suggesting that where someone falls in the family lineup can influence their health risks
‘Birth order is interesting because it is a universal exposure for an entire population, not because it predicts the health of an 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 that younger siblings actually have it easier.
Researchers from Australia’s Monash University found that parents tend to become more lenient with later-born children, relaxing the 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 firstborns.
The researchers said the findings suggested that 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.
