Birth order links to 150 health conditions

TL;DR

A study of over 10 million siblings links birth order to risks for more than 150 medical conditions.
Firstborns face higher chances of autism, allergies, and anxiety; second-borns of migraines, shingles, and substance abuse.
Associations are modest and not deterministic, with possible explanations like immune exposure and maternal factors.

The story at a glance

Researchers led by Benjamin Kramer at the University of Chicago analysed data from more than 10 million siblings, mainly pairs of first- and second-borns, and found birth order ties to 150 out of 418 conditions studied. Firstborns showed elevated risks for neurodevelopmental issues like autism and Tourette syndrome, plus allergies and anxiety, while second-borns had higher odds of migraine, shingles, and substance abuse. The article covers this new preprint study, published this week on medRxiv, building on past work that downplayed birth order's role in personality or IQ.[[1]](https://www.newscientist.com/article/2522884-from-autism-to-migraines-birth-order-may-have-wide-reaching-effects/)[[2]](https://www.newscientist.com/article/2522884-from-autism-to-migraines-birth-order-may-have-wide-reaching-effects)

Key points

Study compared 1.6 million matched sibling pairs (by sex, birth year, parental age, age gap) plus 5.1 million families, controlling for some family confounds.
79 conditions more common in firstborns: autism, Tourette syndrome, childhood psychosis, acne, allergies, hay fever, anxiety disorders (e.g., 3.6 per cent relative increased depression risk).
71 conditions more common in second-borns: substance abuse, shingles, biliary tract disease like gallstones, gastritis, migraine.
Past 2015 study by Julia Rohrer found birth order has little effect on personality and only a small IQ drop (1-2.5 points) from oldest to youngest.
Allergies in firstborns may stem from less early microbial exposure via the "friendly foe" hypothesis; risk drops with wider sibling age gaps.
Substance abuse risk in second-borns lessens with age gaps, possibly from risk-taking or career exposures, though evidence is shaky.
Autism link in firstborns could involve stronger maternal immune response in first pregnancies or "diagnostic substitution" due to slight IQ edge.

Details and context

The study appears on medRxiv (DOI: 10.64898/2026.03.26.26349438) and is called rigorous by Rohrer, but she stresses modest effects prevent firm predictions—everyone experiences only one birth position.[[1]](https://www.newscientist.com/article/2522884-from-autism-to-migraines-birth-order-may-have-wide-reaching-effects/)

Mechanisms remain speculative: firstborns get fewer germs from siblings, possibly weakening immune tolerance; second-borns might face more peer influences. For autism, families may stop after a diagnosis, biasing data, or IQ differences could shift diagnoses from intellectual disability to autism or ADHD.

Critics like Ray Blanchard note limits: study ignores sibling sex order (e.g., older brothers raise homosexuality odds in later boys via maternal antibodies) and only-children.

Key quotes

“Overall, this seems like a really rigorous study,” says Julia Rohrer at Leipzig University.[[1]](https://www.newscientist.com/article/2522884-from-autism-to-migraines-birth-order-may-have-wide-reaching-effects/)

“We will only observe every person in one birth-order position. We will never know how their life would have played out differently in another position,” says Rohrer.[[1]](https://www.newscientist.com/article/2522884-from-autism-to-migraines-birth-order-may-have-wide-reaching-effects/)

Why it matters

Birth order emerges as a subtle factor in health risks across neurodevelopment, immunity, and chronic conditions, challenging views that it mainly affects personality. People may weigh family size or spacing against modest risks like higher autism odds for firstborns or migraines for seconds, though causes are unclear. Watch for peer review of the preprint and studies adding sibling sex, larger families, or mechanisms like maternal immunity.

LANG: en