A comprehensive analysis of over 10 million health records uncovers links between a child’s birth order and their risk of developing various mental health and neurological conditions, suggesting complex biological and social influences.
A large-scale study has found an association between a child’s birth order and the likelihood of developing a range of conditions later in life, with the strongest signals seen in some neurological and mental health disorders.
The research drew on health records from more than 10 million people in about 5 million two-child families, using insurance claims collected over 20 years. Researchers compared first-born children with younger siblings within the same family and also examined differences across households, adjusting for sex, year of birth, geography and parental age.
The study found birth-order differences across roughly 150 diseases. First-born children were more often linked with autism spectrum disorder, attention deficit hyperactivity disorder, Tourette syndrome, depression and anxiety. Younger siblings, by contrast, were more often associated with substance use disorders, gastritis, shingles, migraine and some joint conditions.
Researchers said the findings do not show that birth order directly causes disease. Instead, they suggest a pattern shaped by several overlapping influences, including prenatal biology, parental behaviour, the timing of diagnosis and wider social factors. One possibility is that first-born children receive closer parental monitoring, making developmental or psychiatric conditions more likely to be detected earlier. Younger children may grow up with older siblings who influence behaviour and social exposure, potentially increasing risk-taking in adolescence.
The results fit with earlier research showing that birth order can leave measurable traces in health across the life course. A meta-analysis by the Pregnancy and Childhood Epigenetics Consortium found differences in DNA methylation linked to birth order in newborn blood samples, suggesting a possible biological pathway. Other studies, including work using Norwegian data, have reported that first-born adults are more likely to be overweight or have high blood pressure, while later-born adults are more likely to smoke and report poorer physical and mental health. Researchers say the next step is to test whether the same patterns appear in families with more than two children and to work out which mechanisms matter most.
Disclaimer: This content is for informational purposes only and is not intended to be a substitute for professional medical judgment, advice, diagnosis, or treatment.
