A comprehensive analysis involving millions of siblings has uncovered a correlation between birth order and the risk of more than 150 medical conditions. This extensive study, the largest of its kind, examined data from over 10 million individuals, identifying associations that range from autism and anxiety to hay fever.
For over a century, birth order has been a subject of scientific fascination, with researchers attempting to establish connections to personality traits and intelligence. However, many earlier investigations faced criticism for selective data usage or inadequate control of confounding variables.
A significant study conducted in 2015 by Julia Rohrer at Leipzig University in Germany addressed these issues. This research analyzed data from 20,000 children and concluded that birth order exerted minimal influence on personality. It did, however, note a slight association with intellect, indicating a difference of approximately 1 to 2.5 IQ points between the eldest and youngest siblings.
A New Approach to Birth Order Research
The recent analysis adopted a different focus, specifically investigating the risk profiles associated with various health conditions. Benjamin Kramer from the University of Chicago and his team initially compared 1.6 million pairs of siblings. They achieved this by matching firstborns from one family with second-borns from another, ensuring similarities in sex, birth year, parental age, and the age gap between siblings. This method aimed to mitigate confounding factors, such as variations in parental treatment of first and subsequent children.
In addition to this matched-pair approach, the researchers also analyzed data from 5.1 million families, focusing on genetically related siblings. The combined dataset revealed that out of 418 medical conditions examined, 150 showed a discernible association with birth order. Specifically, 79 conditions were more prevalent among firstborns, while 71 were more common in those born second.
Specific Health Associations by Birth Order
The study highlighted several strong associations for firstborns, particularly concerning neurodevelopmental conditions. These include autism, tics, Tourette syndrome, and childhood psychosis. Firstborns also demonstrated a higher propensity for acne, allergies, hay fever, and various anxiety disorders.
Conversely, individuals born second exhibited an increased risk for a different set of conditions. These included substance abuse, shingles, biliary tract diseases such as gallstones, gastritis (inflammation of the stomach lining), and migraines.
Interpreting the Findings and Potential Mechanisms
Rohrer, who led the 2015 study, described the new findings as rigorous. However, she cautioned that the observed associations are modest. For instance, firstborns show a 3.6 percent relative increase in the risk of depression. She emphasized that these results do not permit deterministic conclusions, noting that individual life experiences cannot be replicated in different birth order positions.
The research team explored several potential explanations for these observed patterns. One hypothesis, termed the “friendly foe” hypothesis, suggests that firstborns’ increased allergy and hay fever risk might stem from later-born children experiencing greater exposure to microbes from older siblings. This early exposure, according to the theory, could foster enhanced immune tolerance. The study noted that this association weakened as the age gap between siblings widened.
A similar trend was observed with substance abuse, where the elevated risk for second-borns diminished with larger age differences. While the authors linked this to research indicating higher risk-taking behavior in later-born children, Rohrer suggests alternative explanations. She posits that later-born individuals might be more represented in careers that could increase exposure to drug-related environments.
Regarding the higher prevalence of autism in firstborns, the researchers proposed a combination of biological and environmental factors. Some studies indicate that a mother’s immune system may react more intensely during the first pregnancy, potentially influencing fetal brain development. Additionally, families with an autistic child may be less likely to have subsequent children, introducing a potential bias in samples where a second child is born after an autism diagnosis.
Rohrer also introduced the concept of “diagnostic substitution.” She explained that diagnoses like ADHD and autism are influenced by cognitive assessments. The same behaviors might be classified differently depending on a person’s intelligence. For example, autistic traits combined with lower intelligence might be diagnosed as intellectual disability, whereas the same traits alongside average intelligence could be diagnosed as ADHD. Given that firstborns generally have slightly higher IQs, this differential diagnosis could contribute to observed patterns.
Future Research Directions
Ray Blanchard of the University of Toronto suggested that the study’s outcomes might be refined by considering sibling sex and the inclusion of only children. He cited his research, which shows that older brothers can slightly increase the likelihood of homosexuality in later-born boys. This phenomenon has been linked to maternal antibody development against male-specific proteins, which may subtly alter the immune system and influence sexual orientation in subsequent sons.
Blanchard contended that these nuanced distinctions are crucial in birth order studies, particularly concerning sexual orientation, and recommended their incorporation into future research, including Kramer’s work.