New large-scale research suggests that birth order may have a measurable impact on the likelihood of developing certain diseases, even if it has little effect on personality.
The study, based on data from more than 10 million individuals in over five million two‑child families, found that sibling position is linked to differing patterns of health risk across a wide range of conditions.
Researchers examined 418 diseases and discovered that 150 showed statistically significant differences depending on whether a person was firstborn or second-born. About half of these conditions appeared more frequently in firstborns, while the rest were more common among later-born siblings.
Firstborns were more likely to develop conditions linked to brain development and the immune system, including autism, Tourette syndrome and allergies. They also showed higher rates of anxiety and skin conditions such as acne.
Second-born individuals, by contrast, were more prone to conditions that appear later in life, including substance abuse, migraines, shingles and digestive problems.
Although the differences are relatively small in individual terms, researchers say the patterns are consistent and detectable even among siblings raised in the same household, suggesting underlying biological mechanisms rather than environmental factors alone.
One explanation for the higher rate of allergies among firstborns is the so-called “hygiene hypothesis,” which proposes that early exposure to microbes helps strengthen the immune system. Younger siblings are more likely to encounter such exposure through older children, potentially reducing their risk.
Pregnancy itself may also play a role. During a first pregnancy, a mother’s immune system encounters foetal proteins for the first time, which may trigger stronger immune responses. Scientists believe this could influence early brain development, possibly explaining links to neurological conditions.
However, experts caution that diagnosis patterns may also contribute to some differences. Researchers note that firstborn children tend to score slightly higher on intelligence tests, which may influence how certain developmental conditions are classified.
The findings highlight the possibility that birth order, long dismissed as a key factor in personality, may still have important implications for health.
Researchers say the results could offer practical insights for medical screening and early intervention, particularly in cases where risk factors are marginal or unclear.
Source: earth.com
