A study of 10 million siblings: First-born children have a higher risk of ADHD and autism

A new study analyzing health data from over 10.3 million individuals across 5.1 million families has found that firstborn children have a higher risk of developing attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorders compared to their younger siblings. The research, which examined the health outcomes of firstborn and later-born children, covered 569 different diseases, with reliable statistical analysis conducted on 418 of them. This marks the largest investigation to date exploring whether birth order influences long-term health outcomes. The findings reveal statistically significant associations between birth order and 150 distinct conditions. Neurological and developmental disorders showed the most pronounced differences, with firstborns being more likely to receive diagnoses of autism, ADHD, and certain allergic conditions such as allergic rhinitis and food allergies. Conversely, second- and later-born children were more frequently diagnosed with specific gastrointestinal issues and disorders related to substance abuse in later life. Researchers emphasized that the study does not establish a cause-and-effect relationship but rather identifies statistical correlations. One possible explanation cited by the authors is the so-called "hygiene hypothesis," which suggests that younger children, due to older siblings, are exposed earlier to various microorganisms, potentially influencing the development of their immune systems. However, scientists caution that numerous other factors could influence the results, including differences in parental attention, frequency of medical check-ups, and the likelihood that parents might notice developmental challenges sooner with their first child. Despite the large sample size and methodologically rigorous analysis, the study has been published as a preprint, meaning it has yet to undergo peer review. The researchers noted that the observed differences are generally small at the individual level but could be significant when examining the health patterns of large populations. In other words, being a firstborn or later-born child does not guarantee the development of a particular illness, but broader trends in risk may emerge when looking at larger groups. The study's scope included comparisons across a wide range of health conditions, highlighting how birth order might correlate with diverse health outcomes. For instance, while firstborns showed increased risks for neurological and allergic conditions, later-born children faced higher rates of gastrointestinal problems and substance-related disorders. These findings add to the growing body of evidence suggesting that family dynamics and environmental exposures can play a role in shaping health trajectories. Experts stress that while the study provides valuable insights into potential population-level health trends, it should not be interpreted as deterministic for individuals. Factors such as genetics, environment, and lifestyle choices remain critical determinants of health. Additionally, the study underscores the importance of considering multiple variables when assessing health risks associated with birth order. Further research will be necessary to confirm these findings and explore the underlying mechanisms linking birth order to health outcomes. Researchers plan to conduct follow-up studies that incorporate additional data sources and consider interactions between genetic and environmental factors. Until then, the study serves as a foundation for understanding how early-life experiences within a family setting might influence long-term health.