Parents often notice differences between their first and second child and wonder whether birth order really matters.
A massive new study suggests it may be linked to health in unexpected ways, although the differences are much smaller than many people might imagine. The findings were published in Nature Health.
Researchers analysed health records from more than 10 million Americans belonging to over 5 million families with exactly two children. By following diagnoses recorded over two decades, they explored whether being born first or second was associated with different medical conditions.
Earlier studies mainly focused on allergies and asthma, reporting that younger siblings often develop these conditions less frequently. Scientists believe older children may bring more germs into the home, helping younger siblings build stronger immune tolerance during early childhood.
The new study greatly expanded this idea by examining hundreds of diseases across almost every body system. After careful analysis, the researchers found that 150 of 418 diseases showed some relationship with birth order.
First-born children were diagnosed more often with autism, ADHD, tic disorders, Tourette syndrome, anxiety, depression and several allergic conditions. Second-born children were more likely to be diagnosed with migraine, gastritis, shingles, certain joint disorders and substance abuse problems.
Why these patterns appear remains uncertain. One explanation is that parents often observe their first child more closely and seek specialist assessments sooner, increasing the chance of early diagnosis. Doctors may also pay closer attention when evaluating a family’s first child.
Another possibility involves the immune system. Younger siblings are exposed to viruses and bacteria brought home by older brothers or sisters, and this exposure may help reduce the likelihood of allergies and asthma. Social influences from older siblings may also affect some behaviours later in life.
Importantly, the researchers cautioned against overinterpreting the results. Birth order was linked with disease risk, but it was not proven to cause any illness. Most children, regardless of whether they are born first or second, will never develop many of these conditions.
The authors also noted that pregnancy differences, parenting styles, diagnosis timing and other hidden factors may all contribute to the observed patterns. No single explanation can account for every finding.
The enormous sample size and the comparison of siblings living in the same household make this an important contribution to birth-order research.
However, insurance claim data cannot capture every factor that influences health, and observational studies cannot establish cause and effect. The results are best viewed as clues for future research rather than predictions about an individual child’s future health.
Source: Nature Health.
