A comprehensive study analyzing the health records of more than 10 million Americans indicates that birth order may be associated with an increased risk for a wide range of health conditions throughout a person's life. The research, published in Nature Health, identified links between family position and 150 different health outcomes, including neurodevelopmental disorders, allergies, digestive issues, and substance abuse.
Firstborn children, according to the findings, were more likely to be diagnosed with neurodevelopmental conditions 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 suggests these links are strongest for brain development and allergy-related conditions in firstborns.
Second-born children, in contrast, showed a 43 percent lower likelihood of developing certain autism-related disorders than their older siblings. However, they were found to have higher rates of digestive and musculoskeletal diseases, infectious diseases, migraines, stomach problems, kidney infections, and herpes zoster. Substance abuse disorders were also among the conditions with stronger associations for second-born children.
Researchers propose that the observed differences are likely not due to a single cause but rather a combination of evolving biological and environmental factors that change with each child. For instance, younger children are often exposed to more viruses and bacteria from older siblings, a phenomenon known as the hygiene hypothesis, which could potentially reduce the risk of allergies. Conversely, subtle changes in the prenatal environment between pregnancies may influence neurodevelopmental outcomes.
Family dynamics are also considered a contributing factor. Parents may interact differently with firstborns versus later-born children, and younger siblings grow up in a social environment shaped by older siblings and evolving parental expectations. Another study mentioned in the notes suggests parents may become more lenient with subsequent children, potentially leading to differences in activities and developmental focus.
Scientists caution against using these findings to predict individual health outcomes. Professor Andrey Rzhetsky, a study author from the University of Chicago, emphasized that birth order is a universal factor that is interesting for population-level analysis but should not dictate clinical decisions for any single child. The study's large scale provides a broad overview of potential associations rather than deterministic predictions for individuals.