A comprehensive study involving over 10 million Americans has revealed potential links between birth order and the risk of developing 150 different health conditions. The research, published in Nature Health, indicates that a child's placement within the family structure might influence their lifelong health, with firstborns showing a higher propensity for certain neurodevelopmental and allergy-related conditions, while later-born children may face increased risks for substance abuse and digestive issues.
Firstborn children were found to be more likely to be diagnosed with neurodevelopmental disorders such as autism, ADHD, Tourette's syndrome, and obsessive-compulsive disorder. They also exhibited higher rates of food allergies, hay fever, asthma, and acne compared to their younger siblings. The study's data suggests that firstborns have the strongest associations with brain development and allergy-related conditions.
In contrast, second-born children were less likely to be diagnosed with many neurodevelopmental conditions, with the study indicating they were 43 percent less likely than firstborns to develop certain autism-related disorders. However, they demonstrated higher rates of digestive, musculoskeletal, and infectious diseases. Substance abuse disorders, migraines, stomach problems, kidney infections, and shingles were also more prevalent among second-born children. The research highlights increased risks of digestive, musculoskeletal, and infectious diseases for second-born children.
Researchers suggest that no single factor explains these findings, but rather a combination of biological and environmental elements that vary with each child. For instance, the hygiene hypothesis posits that younger siblings are exposed to more pathogens through older siblings, potentially bolstering their immune systems against allergies. Conversely, subtle differences in the prenatal environment between pregnancies could influence neurodevelopmental outcomes.
Family dynamics may also play a role. Parental behavior often shifts between firstborn and subsequent children, and younger siblings grow up in a different social context shaped by older siblings and evolving family expectations. A separate study mentioned in the notes indicated that parents tend to become more lenient with later-born children, potentially impacting their activities and developmental pathways.
One of the study's authors, Professor Andrey Rzhetsky of the University of Chicago, emphasized that these findings should not be used to make individual clinical decisions. He noted that birth order is a universal factor, but it does not predict the health of any single child, and misinterpreting the results as direct predictions of future health would be a significant error.