Many families wonder whether birth order shapes personality, intelligence or success. A new study suggests it may also be linked to certain health conditions.
The research, published in Nature Health, found that first-born and second-born children showed small but measurable differences in their chances of being diagnosed with a range of diseases later in life.
Scientists have previously reported that younger siblings often have lower rates of allergies and asthma. One possible explanation is that older children bring home bacteria and viruses from school or childcare, exposing younger siblings to more microbes early in life. This early exposure may help train the immune system.
To see whether birth order affects health more broadly, researchers in the United States examined medical records from more than 10 million people belonging to over 5 million families with two children. The information came from commercial health insurance claims collected over about 20 years.
The team investigated 569 categories of disease. They compared first-born and second-born children from different families who were carefully matched by sex, birth year, location and parental age. They also compared siblings within the same family, helping reduce the influence of genetics and home environment.
Among 418 diseases with enough cases to study, 150 showed a statistical link with birth order. First-born children were more likely to receive diagnoses of autism, attention-deficit hyperactivity disorder, Tourette syndrome and tic disorders. They also had higher rates of depression, anxiety, asthma, hay fever and food allergies.
Second-born children showed higher rates of several different conditions. These included substance abuse disorders, migraine, shingles, gastritis and some joint problems. The researchers said these findings suggest birth order may influence health in more ways than previously recognized.
There are several possible explanations. First-time parents may pay closer attention to their oldest child and seek medical assessment earlier, making conditions such as autism or ADHD more likely to be diagnosed. Younger siblings may benefit from earlier exposure to everyday germs carried home by older brothers and sisters, which could lower allergy risk.
For behaviours such as substance misuse, younger children may spend time with older siblings and their friends, exposing them to different social influences at an earlier age. Pregnancy-related biological differences between first and later pregnancies may also contribute, although scientists do not yet know how important these factors are.
The researchers stressed that the study does not show that birth order directly causes any disease. It only found statistical associations. For any individual child, the differences in risk were generally small, and many other factors such as genes, lifestyle, healthcare access and environment are likely to have a much greater influence.
This is one of the largest studies ever conducted on birth order and health, giving the findings considerable statistical strength. Comparing siblings within the same family also improved reliability.
However, because the research relied on insurance records, differences in healthcare use or diagnosis rates may have affected the results. More research is needed before birth order can be considered an independent health risk factor.
Source: Nature Health.
