Does Birth Order Affect Disease Risk? Study Finds First-Born and Second-Born Children May Face Different Health Risks

Birth order and health: What new research reveals

For generations, families have wondered whether being the oldest, youngest, or middle child influences personality, behaviour, and life outcomes. A new scientific study suggests that birth order may also be linked with differences in disease patterns, with first-born and second-born children showing different risks for certain health conditions.

The research indicates that first-born children may have a higher likelihood of developing neurological and mental health conditions later in life, while second-born children appear to have increased risks linked to substance abuse and some physical health problems.

However, scientists stress that birth order does not directly cause disease. Instead, it may interact with biological, social, environmental, and family-related factors that influence health outcomes.

Large-scale study examines millions of families

The study analysed health records from more than 10 million people belonging to approximately five million two-child families. Researchers examined two decades of commercial insurance claims data to investigate whether birth order was associated with the risk of developing a wide range of diseases.

To improve accuracy, scientists compared siblings within the same families and also compared children from different families. They adjusted for factors such as sex, year of birth, geographic location, and parental age.

The findings suggested that birth order was associated with differences in the risk of developing around 150 diseases.

The research was published in the journal Nature Health, where scientists explored how family position could influence long-term health patterns.

First-born children and neurological conditions

According to the study, first-born children showed higher rates of several neurological and mental health diagnoses compared with their younger siblings.

Conditions associated with higher rates among first-born children included:

• Autism spectrum disorder
• Attention deficit hyperactivity disorder (ADHD)
• Tourette syndrome
• Depression
• Anxiety disorders

Researchers suggested several possible explanations for this pattern.

One possibility is that parents may monitor their first child more closely, making them more likely to notice developmental differences and seek medical advice earlier. This increased attention could result in higher diagnosis rates compared with younger children who may not receive the same level of early observation.

Another explanation involves biological differences between pregnancies. The maternal immune system, hormonal environment, and pregnancy conditions can change between a first and later pregnancy, potentially affecting child development.

Scientists emphasise that these factors are complex and that more research is required before drawing firm conclusions.

Why second-born children may face different risks

The study found that second-born children appeared to have higher rates of certain conditions, particularly those associated with behaviour and lifestyle factors.

These included:

• Substance abuse disorders
• Digestive problems such as gastritis
• Migraines
• Shingles
• Some joint-related conditions

Researchers suggested that younger siblings may experience different social environments compared with first-born children.

For example, second-born children may be exposed to older siblings and their peer groups at an earlier age. This could influence social behaviour, risk-taking tendencies, and exposure to certain experiences.

The findings support previous theories suggesting that younger siblings may sometimes develop more adventurous or risk-taking behaviours because of their different family dynamics.

Birth order is not destiny

Although the research highlights statistical patterns, experts warn against interpreting birth order as a prediction of an individual’s future health.

Being a first-born child does not mean someone will develop neurological conditions. Similarly, being a second-born child does not mean a person is more likely to experience substance abuse problems.

Health outcomes are shaped by many factors, including:

• Genetics
• Lifestyle choices
• Environment
• Access to healthcare
• Education
• Family support
• Mental health resources

Birth order is only one possible factor among many.

Researchers described the relationship between birth order and disease risk as a combination of biological, social, and healthcare-related influences rather than a single cause.

How family environment may influence health

Family structure can affect children in different ways. First-born children often experience a period where they receive complete parental attention before younger siblings arrive. This can influence early development, expectations, and family responsibilities.

Younger siblings may grow up in a different environment, where parents have more experience but less individual time available. They may also learn from older siblings, interact with wider social circles earlier, and develop different coping strategies.

These differences do not necessarily create advantages or disadvantages. Instead, they highlight how childhood experiences can vary even within the same household.

The importance of future research

The researchers behind the study say additional studies are needed to understand whether these patterns appear in larger families with three or more children.

Future research could examine how factors such as family size, parenting style, cultural background, socioeconomic conditions, and healthcare access influence the relationship between birth order and health.

Scientists also hope to explore whether similar patterns appear in different countries and populations.

What parents should know

Parents should not worry that their child’s birth position determines their health future. The most important influences remain healthy development, emotional support, regular medical care, and creating a positive home environment.

Recognising early signs of developmental, behavioural, or physical health concerns is far more important than focusing on whether a child is the oldest or youngest.

Conclusion

The latest research suggests that birth order may be linked with differences in disease patterns, with first-born children showing higher rates of some neurological and mental health diagnoses and second-born children showing increased risks for certain behavioural and physical conditions.

However, birth order alone does not determine health outcomes. The relationship appears to involve a mixture of biology, family environment, parental attention, and social experiences.

Understanding these patterns may help researchers discover more about how early-life experiences influence lifelong health, but individuals should view their birth order as one small part of a much larger health picture.

Source

The Independent, 6 August 2026, and research published in Nature Health (2026) examining associations between birth order and disease risk using large-scale health insurance data.

Disclaimer

This article is for informational purposes only and does not provide medical advice. Birth order is not a confirmed cause of any disease, and individual health risks depend on many genetic, environmental, and lifestyle factors. If you have concerns about your health or your child’s development, consult a qualified healthcare professional.