Exploring the Impact of Birth Order on Autism, Migraines, and More

A recent study involving over 10 million siblings reveals that birth order may significantly influence the risk of developing more than 150 health conditions, ranging from autism and anxiety to hay fever.

Birth order has intrigued researchers for over a century, igniting debates about its correlation with personality traits and IQ. However, many prior studies faced criticism for lacking robustness in data collection and analysis.

A groundbreaking study conducted by Julia Rohrer in 2015 examined data from 20,000 children, determining that birth order had minimal impact on personality, resulting in only a slight decrease in IQ — about 1 to 2.5 points for the youngest siblings.

The recent analysis took a comprehensive approach, evaluating the likelihood of various health outcomes. Researchers like Benjamin Kramer at the University of Chicago meticulously compared 1.6 million sibling pairs, accounting for gender, birth year, parental age, and age difference, thereby mitigating potential confounding factors that may arise from parental treatment differences.

Out of 418 medical conditions studied, 150 were associated with birth order, with 79 more prevalent among firstborns and 71 among second-borns.

Notably, firstborns displayed heightened risks for several neurodevelopmental disorders, including autism and Tourette syndrome, along with an increased tendency for anxiety, allergies, and acne. Conversely, second-borns exhibited greater susceptibility to conditions such as drug abuse, shingles, and migraines.

“This study provides a rigorous examination of the topic,” states Lawler, urging caution as the relationships observed are modest. For instance, firstborns have a 3.6% elevated risk of depression, emphasizing that individual life trajectories may differ significantly across birth order.

The research team explored several potential explanations for these findings. For example, the increased incidence of allergies among firstborns may align with the “friendly enemy” hypothesis, suggesting that younger siblings encounter more microorganisms from their older counterparts, fostering immune tolerance. Indeed, wider age gaps were linked to lesser allergy occurrences in firstborns.

A parallel trend was noted for substance abuse, with risk diminishing for second-borns as age differences increased. The authors connected this to enhanced risk-taking tendencies often observed in later-born children. However, Lawler emphasizes that much of this evidence remains contentious and may imply that later-borns often pursue environments that heighten exposure to substance-use opportunities.

Furthermore, the substantial prevalence of autism among firstborns might stem from both biological and environmental factors. The mother’s immune response in the first trimester is hypothesized to potentially impact the developing brain. Research indicates that families with one autistic child may choose not to have additional children, suggesting possible biases in families who do have a second child following an autism diagnosis in the first.

Another perspective from Lawler pertains to “diagnostic substitution.” Diagnoses of ADHD and autism often rely on cognitive assessments, where slight IQ variations may lead to different labels. Firstborns, possessing marginally higher IQs, might be diagnosed with autism, while their younger siblings may receive an ADHD diagnosis despite sharing similar symptoms.

As noted by Ray Blanchard from the University of Toronto, results may vary when considering sibling gender and birth order dynamics. His research suggests older brothers might increase the likelihood of later-born boys identifying as homosexual, potentially due to maternal antibodies affecting subsequent pregnancies. “These distinctions are pivotal in understanding birth order effects on sexual orientation,” concludes Blanchard, advocating for further studies that incorporate sibling gender hierarchies.