Were You The Firstborn? A New Study Says It May Have Affected Your Health
Were you the oldest child, the youngest, or somewhere in the middle? It's a question that usually comes up in the context of personality (i.e. the responsible firstborn, the free-spirited youngest).
The effect sizes are modest and birth order is far from destiny, but here's what you need to know.
What the researchers looked at
Birth order and health isn't a brand-new area of research. Scientists have long suspected that where you fall in the family lineup could influence things like allergy risk and immune development.
Researchers pulled data from over 10 million individuals across 5.1 million two-child families, drawn from a large U.S. insurance claims database spanning two decades. They tested associations between birth order and 569 diseases using two different analytical approaches: one comparing firstborns and secondborns across different families, and one comparing siblings within the same family.
Of the 418 diseases with enough cases to analyze, 150 showed significant links to birth order.
What they found
Birth order showed up across nearly every clinical domain tested, not just the handful of conditions researchers expected.
- Firstborns showed higher rates of neurodevelopmental conditions, including autism, ADHD, OCD, and tics. They also had higher rates of immune and allergic diseases: food allergies, hay fever, asthma, and acne all showed firstborn excess. Anxiety and depression also appeared more often in firstborns, though with smaller effect sizes.
- Secondborns showed higher rates of certain digestive conditions, including gastritis and irritable bowel syndrome, as well as substance abuse disorders and migraine.
These associations were significant at a population level, but not the kind of difference that doubles or triples individual risk. Birth order is one factor among many.
Why these patterns exist
The allergy findings have the clearest explanation. Early exposure to a wide variety of microbes helps the immune system learn to distinguish real threats from harmless ones. Secondborns tend to get that exposure sooner, partly because an older sibling brings home more germs from school. That early microbial diversity is likely why secondborns have lower rates of allergies.
The neurodevelopmental findings are less straightforward. The researchers don't point to a single cause. It's likely a combination of biological effects from pregnancy order, how attentively parents monitor a firstborn's development, and differences in when diagnoses tend to happen.
There's also a statistical factor, as parents who have a child diagnosed with autism are somewhat less likely to have a second child (which can make autism appear more common in firstborns than it actually is).
For substance use, the research doesn't yet have a definitive explanation. Family dynamics, social environment, and siblings influence likely all contribute in some capacity.
The takeaway
Birth order does not predict your health The differences found in this research exist at a population level; they describe patterns across millions of people, not predictions for any individual. Your lifestyle, environment, diet, sleep, stress levels, and early-life habits have a far greater influence on your long-term health than whether you were born first or second.
For parents, rather than worrying about birth-order risk differences, focus on building healthy habits for every child. This includes diverse, nutrient-rich food, time outdoors, consistent sleep, and strong social connection. These factors shape health in ways that birth order simply can't compete with.