Birth Order May Be Linked to Different Health Risks, Study of 10.3 Million Siblings Finds

Birth Order May Be Linked to Different Health Risks, Study of 10.3 Million Siblings Finds

A large study involving 10.3 million siblings found that birth order was associated with different disease risks, with first-borns showing higher rates of some neurodevelopmental and allergic conditions.

Family Rank May Matter More Than Previously Thought

Does being the first child in a family influence health later in life?

A major new analysis involving 10.3 million individuals from 5.1 million families has found that birth order is associated with differences in the likelihood of developing a wide range of diseases.

The study, titled “Birth order and disease risk across the human phenome: evidence from 10 million siblings,” examined health records covering 569 diseases. Researchers found statistically significant birth-order associations for 150 diseases among those with sufficient case numbers.

However, the findings do not mean that being born first directly causes a particular disease. The research is a large observational analysis, and the authors say the results could reflect a combination of biological, environmental, social and diagnostic factors. The manuscript is also currently a preprint and has not undergone peer review.

First-Born Children Showed Higher Rates of Several Conditions

One of the most striking patterns involved neurodevelopmental and immune-related conditions.

Compared with second-born children, first-borns showed higher odds of several conditions, including autism, ADHD and food allergy.

In the study's statistical analysis, an odds ratio below 1 for second-born children indicated that the condition was more common among first-borns. For example, the between-family analysis reported an odds ratio of approximately 0.74 for autism, 0.93 for ADHD, and 0.80 for food allergy when comparing second-borns with first-borns.

The researchers also observed first-born excesses for conditions including allergic rhinitis, anxiety or phobic disorders, tics and Tourette syndrome.

Importantly, these figures describe relative statistical differences, not a guarantee that a first-born child will develop any of these conditions.

Second-Born Children Had a Different Pattern

The study found that second-born children did not simply have lower disease risks across the board.

Instead, they showed higher rates of several other conditions.

The strongest associations included substance abuse disorders, migraine, gastritis and duodenitis, and biliary tract disease. The reported odds ratio for substance abuse was approximately 1.19, while migraine was approximately 1.13, meaning these conditions were statistically more common among second-borns than first-borns in the analysis.

This is one of the most interesting aspects of the research: birth order appeared to be associated with different patterns of disease, rather than simply making one sibling position universally healthier than another.

Why Could Birth Order Be Associated With Health?

Researchers do not believe there is one simple explanation.

Several possibilities could contribute to the observed differences.

1. Differences During Pregnancy

A first pregnancy is biologically different from subsequent pregnancies. Changes involving the mother's immune system, physiology and pregnancy history could potentially contribute to differences between children born at different points in a family's reproductive history.

However, the study does not establish that these biological differences directly cause the diseases identified.

2. Greater Parental Attention to First Children

Parents may monitor their first child particularly closely because they have no previous experience raising a child.

That could influence how quickly developmental or behavioral concerns are noticed and evaluated.

For example, researchers say that some of the differences involving neurodevelopmental conditions could partly reflect parental surveillance and diagnostic timing, rather than a straightforward biological effect of birth order.

3. Exposure to Older Siblings

Younger children grow up around older siblings who bring different microorganisms, behaviors and social experiences into the household.

This could potentially influence immune development and other aspects of childhood health.

The study notes that the pattern involving food allergies is compatible with previous ideas surrounding the so-called hygiene hypothesis, although this does not prove that sibling exposure is responsible for the association.

4. Social and Behavioral Influences

Birth order can also affect a child's social environment.

Second-born children, for example, may encounter older children's peer groups and behaviors at younger ages. Researchers suggest that this could potentially contribute to the observed association with substance-use disorders, although more research is required to establish the mechanisms involved.

Researchers Compared Siblings Within the Same Families

A major strength of the research was its use of a within-family sibling comparison.

Instead of simply comparing unrelated first-born and second-born children, researchers analyzed siblings from the same families. The study included 5.1 million families and 10.3 million individuals in this analysis.

This approach helps reduce the influence of certain family-level differences because siblings share many aspects of their family environment.

The researchers also conducted a separate between-family matched analysis involving approximately 1.6 million sibling pairs.

The two approaches generally showed similar directions for many of the significant associations, strengthening the researchers' confidence that the findings were not simply the result of one statistical method.

What the Study Does Not Prove

The headline “birth order determines disease risk” would be misleading.

The research found associations, not proof of causation.

That distinction is especially important when discussing conditions such as autism, ADHD, OCD, allergies or substance-use disorders.

A child's birth order is only one characteristic among countless biological, environmental, social and developmental factors.

The study's authors themselves emphasize that the neurodevelopmental findings could reflect a mixture of pregnancy-order biology, parental monitoring, diagnostic timing and other factors that were not completely measured in the data.

Also, although the analysis covered a huge number of people, the underlying data came from U.S. commercial health-insurance claims. Therefore, the findings should not automatically be assumed to apply identically to every population or country.

What About OCD?

The viral claim circulating about the study specifically mentions OCD alongside ADHD, autism and food allergies.

The broader research did identify numerous birth-order associations across psychiatric, neurological, allergic, gastrointestinal and other disease categories. However, the study's headline results emphasize autism, ADHD, food allergy and several other conditions rather than suggesting that birth order is a simple cause of OCD.

That distinction matters because social-media summaries can sometimes compress complicated scientific findings into a much stronger claim than the original research supports.

A New Perspective on Family Health

The study is important because researchers previously examined birth order in relation to individual diseases, but this analysis attempted to look across a much broader portion of the human disease landscape.

Researchers examined 569 diseases, with 418 having enough cases for the primary analysis. Of those, 150 showed statistically significant birth-order associations after stringent multiple-testing correction.

The findings suggest that birth order may be connected to health in more ways than previously recognized.

But they also demonstrate how complicated the relationship between family structure and health can be.

Being the oldest child does not automatically make someone unhealthy, just as being the second-born does not provide protection against disease.

Instead, the study points toward subtle population-level differences that may deserve further investigation.

What Parents Should Take From the Findings

Parents should not worry simply because their child is first-born or second-born.

Birth order is not a diagnosis and cannot be used to predict an individual's medical future.

If a child shows signs of developmental difficulties, allergies, persistent digestive symptoms, migraines, behavioral changes or other health concerns, the appropriate response is to seek professional medical advice rather than attributing those symptoms to birth order.

The study is best viewed as a starting point for further research rather than a guide for predicting the health of individual children.

FAQ....

Does birth order cause autism?

No. The study found a statistical association between being first-born and autism diagnoses, but it did not prove that birth order causes autism.

Are first-born children more likely to have ADHD?

The study found a statistical excess of ADHD among first-born children compared with second-born children. However, the difference should not be interpreted as proof that being first-born causes ADHD.

Are second-born children healthier?

Not necessarily. The study found different disease patterns between first- and second-born children. Second-borns had higher associations with several conditions, including substance abuse, migraine and gastrointestinal disorders.

How many people were included in the study?

The within-family analysis included approximately 10.3 million individuals from 5.1 million families.

Was the study peer-reviewed?

No. The research is currently available as a medRxiv preprint and has not undergone peer review.

Should parents be concerned about their child's birth order?

No. Birth order alone cannot determine an individual's health. Parents should focus on their child's actual symptoms, development and medical needs.

The enormous sibling study offers a fascinating look at the possible relationship between family rank and health.

First-born children showed higher statistical associations with several neurodevelopmental and allergic conditions, while second-born children showed higher associations with substance-use disorders, migraine and some gastrointestinal conditions.

But the findings should not be turned into a prediction tool.

The key message is not that “first-borns are unhealthy” or “second-borns are healthier.” Rather, birth order may interact with biological, environmental, social and healthcare factors in ways researchers are only beginning to understand.

As the evidence develops, larger and more diverse studies will be needed to determine which of these associations are genuine biological effects, which are related to family and social circumstances, and which may arise from differences in diagnosis and healthcare use.

For now, the research provides an intriguing new reason to look beyond genetics and individual behavior when scientists study why diseases occur differently among siblings.

Research source: Benjamin Kramer, Steven A. Kushner and Andrey Rzhetsky, Birth order and disease risk across the human phenome: evidence from 10 million siblings, medRxiv, 2026.

 birth order health risks, first-born children health, second-born children health, birth order study, 10 million siblings study, autism and birth order, ADHD and birth order, food allergy birth order, migraine and birth order, family birth order research.