The study was published in the scientific journal “Nature Health.” Scientists analyzed health insurance data from 5.1 million families in the United States raising two children each. In total, more than 10 million people were included in the study, and their medical records were examined for associations between birth order and 569 diseases and health disorders.
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Out of 418 diseases with enough cases for reliable analysis, statistically significant associations with birth order were found for 150.
Firstborns – more neuropsychiatric and allergic disorders
The most prominent differences for firstborns were related to neurological development and mental health disorders. They were more often diagnosed with autism, attention deficit and hyperactivity disorder, tics and Tourette’s syndrome, obsessive-compulsive disorder, anxiety, eating disorders, and depression than second-born children.
The greatest difference was found when assessing other or unspecified diverse developmental disorders. The second child’s chance of having such a diagnosis was about 43% lower than that of the firstborn. The chance of an autism diagnosis for second-born children was about 26% lower, and for tics and Tourette’s syndrome about 31% lower.
However, these numbers do not mean that being born first causes a neurological development disorder. The study showed statistical associations but not their causes. Moreover, the authors calculated the ratio of diagnosis probabilities, not the direct likelihood of a specific child developing the condition.
Firstborns were also more often diagnosed with food allergies, allergic rhinitis, asthma, and some skin conditions, including acne and atopic dermatitis. The chance of a food allergy diagnosis for second-born children was about one-fifth lower.
Younger children may be protected by microbes
One possible explanation for the difference in allergic diseases is the different microbial environment of the children. The first child usually encounters a lower diversity of microorganisms in early childhood. Meanwhile, the younger child grows up alongside an older brother or sister who brings more diverse microbes home from kindergarten, school, and other environments.
It is believed that early contact with harmless environmental microorganisms can help the immune system learn to respond properly and reduce the tendency toward allergies. This assumption was strengthened in the study by evaluating the age difference between children: the smaller the age gap, the more pronounced the younger child’s advantage in terms of allergic rhinitis and food allergy diagnoses.
However, this is not about deliberately “hardening” children against disease-causing agents. What may be important is the natural diversity of environmental microorganisms, not more frequent dangerous infections.
Youngest children – more frequent migraines and digestive diseases
Not all identified differences were favorable for younger children. They were more often diagnosed with migraine, shingles, gastritis and duodenitis, bile duct, kidney, and some joint and connective tissue diseases.
The most pronounced difference was found for shingles – the second child’s chance of receiving this diagnosis was about 35% higher. The chance of diagnoses for psychoactive substance use disorders in second-born children was about 19% higher, bile duct diseases 18%, gastritis and duodenitis 14%, and migraines 13% higher.
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According to scientists, more frequent problematic use of psychoactive substances among younger children may be related to social factors. An older brother or sister may introduce them earlier to the circle of older teenagers’ friends and riskier behavior. This association weakened as the age difference between children increased – partly supporting the possible influence of an older sibling.
The reasons for differences in other diseases, such as migraines or digestive disorders, are not yet clear.
Is the firstborn’s health monitored more closely?
Researchers also considered the possibility that parents monitor the first child’s health more closely, thus more often visiting doctors and noticing various disorders earlier. Firstborns indeed had about 4.5% more recorded visits to healthcare facilities than second-born children.
However, after additionally accounting for visit frequency, the overall picture of identified associations changed very little. Moreover, out of 150 statistically significant associations, 79 diseases were more often diagnosed in firstborns, and 71 in younger children. If greater attention to the firstborn alone explained everything, it is likely that almost all disease diagnoses would have been more frequent in this group.
Another possible reason is parents’ decision not to have more children when a severe disorder is diagnosed in the firstborn. The study showed that families with a first child diagnosed with autism were somewhat less likely to have a second child. However, this factor explained only part of the observed difference and did not apply to many other neurological development disorders.
There is no single explanation
The study authors highlight several possible mechanisms. First and subsequent pregnancies are biologically not entirely the same. Differences may involve the mother’s immune system activity, hormonal environment, and other processes affecting the child before birth.
Parental age may also matter. At the birth of the second child, parents are naturally older, and their age is associated with the likelihood of some disorders. Researchers tried to account for this factor but acknowledge that it is impossible to completely separate the effects of parental age and birth order.
Different parental behavior may also contribute: how much attention is given to each child, when difficulties are noticed, how quickly help is sought, what social connections the child forms, and the influence of an older brother or sister.
Therefore, the most cautious conclusion would be this: the identified differences are most likely caused not by a single reason but by a combination of biological, immune, social, and diagnostic factors.
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