Research involving over 10 million siblings indicates that eldest children face higher chances of autism and allergies, whereas younger ones are more prone to migraines and shingles.
Common beliefs about family dynamics often lack evidence, yet a person’s position as the first or second child could influence health outcomes.
In families with two kids, birth sequence might affect susceptibility to over 150 medical issues, based on the most extensive research to date. Scientists examined records from more than 10 million siblings, identifying connections to disorders ranging from autism and anxiety to seasonal allergies.
For over 100 years, experts have explored how birth order relates to traits like personality and intelligence. Earlier investigations drew criticism for selective data use or ignoring external influences.
A significant 2015 study by Julia Rohrer at Leipzig University in Germany reviewed data from 20,000 children, addressing these flaws. It revealed minimal impact on personality and only a slight IQ difference—about 1 to 2.5 points lower for younger siblings.
This latest work shifted focus to disease risks. Benjamin Kramer from the University of Chicago and his team compared 1.6 million sibling pairs by matching first-borns from various families with second-borns, aligning factors like gender, birth year, parents’ ages, and age differences. They also studied genetically linked siblings from 5.1 million households.
Among 418 health conditions, 150 correlated with birth position. First-borns had higher rates for 79, while second-borns showed elevated risks for 71.
Key links for eldest children included neurodevelopmental issues like autism and Tourette syndrome, plus childhood psychosis. They also faced greater odds of acne, allergies, hay fever, and anxiety. Second-borns had increased risks for substance misuse, shingles, gallstone-related diseases, stomach inflammation, and migraines.
Julia Rohrer described the research as thorough but noted the links are small—for instance, a 3.6 percent higher relative risk of depression for first-borns. She emphasized that results are not predictive, as individuals experience only one birth position.
The researchers investigated possible explanations. For example, eldest children’s higher allergy risks might stem from reduced early exposure to microbes compared to younger siblings, who benefit from interactions with older ones. This was supported by data showing allergy differences lessened with larger age gaps.
A similar trend appeared for substance abuse in second-borns, with risks decreasing as age differences grew. The team connected this to studies on risk-taking in later-borns, though Rohrer noted such findings are often unreliable and could relate to career paths exposing them to drug use.
For autism’s prevalence in first-borns, the study suggested biological and social factors, such as stronger maternal immune responses in initial pregnancies affecting fetal brain development. Some families may halt childbearing after an autism diagnosis, potentially skewing data.
Rohrer mentioned ‘diagnostic substitution,’ where behaviors might be labeled as intellectual disability in lower-IQ cases but as ADHD or autism in others, influenced by first-borns’ slight IQ advantage.
