The truth about birth order and its impact on your personality and health
We’ve all heard the stereotypes about firstborns, middle children and youngest siblings – but growing research suggests birth order may influence more than personality alone. Experts reveal what it could mean for behaviour, wellbeing and future health.
- Hollie Bond
Published: June 29, 2026 at 4:38 PM
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There’s long been a fascination with how birth order shapes siblings. We joke that eldest children are bossy future CEOs, middle children are natural peacemakers, youngest siblings charm their way out of trouble and only children are perfectionists who like things just so.
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And in plenty of families, those labels seem to fit uncannily well. The oldest child takes charge of every game, the middle avoids conflict at all costs, while the youngest somehow gets away with behaviour their older siblings insist they’d never have been allowed.
But is birth order really powerful enough to shape who our children become or are these just stories families tell themselves?
"Birth order is less about fixed personality traits that people are inherently born with, and more about how a child experiences their role within a particular family system," says Helen Wells, Psychotherapist and Clinical Director at The Dawn Rehab Thailand.
"Rather than birth order determining who we are, it often shapes the narrative we carry about who we're supposed to be – and that narrative can have a powerful influence on behaviour, coping styles and self-worth over time."
The science behind birth order, health and personality
Researchers have spent decades trying to find out how birth order affects us, and a growing body of evidence suggests where a child falls in the family line-up may be linked not only to personality and behaviour, but also to aspects of physical and mental health.
In fact, a brand new 2026 study (still in pre-print stages and not yet peer-viewed) has explored connections between birth order and everything from allergies and eczema to neurodevelopmental differences, including autism.
“Birth order is one of those concepts people tend to laugh off, but there is actually some fascinating science behind it,” says Dr Nadia Ahmad, founder of The Weight Care Clinic. “While it is not destiny, there are consistent patterns we see.”
That doesn’t mean being firstborn, middle or youngest determines a child’s future. Experts stress that genetics, parenting, life experiences and plain old temperament matter far more. But birth order may be one small piece of the puzzle.
“There is some evidence suggesting that birth order can be associated with small differences in both physical and mental health outcomes,” says Dr Suzanne Wylie, GP and medical adviser for IQdoctor, “although these associations are often modest and heavily influenced by environmental, social and family factors rather than birth order itself acting as a direct cause.”
So how much truth is there in the stereotypes, will this new study change how we view birth order and our health, and what have real families noticed when it comes to birth order in their own homes?
We asked psychologists, doctors, health experts and parents to find out whether your number in the family has made you who you are.
What the new study found
The new study posted on MedRxiv at the end of March 2026 – Birth order and disease risk across the human phenome: evidence from 10 million siblings – looked for 569 conditions within over 5 million families with two children.
The topline findings suggest that firstborns are more likely to be diagnosed with autism and ADHD and suffer from allergies, acne and childhood psychoses.
The study showed that there was a higher chance of second born children being diagnosed with substance disorders, shingles and gastrointestinal issues than their older sibling too.
Wells urges caution when interpreting the findings. "Birth order is a proxy variable that often reflects a complex interplay of factors including parental attention, family stress, genetic vulnerability and sibling influence," she says. "While birth order may contribute to shaping developmental context, it is not viewed as a primary determinant of mental health."
The researchers also found that age gaps played a role, with smaller age gaps of less than four years appearing to provide some protection against allergies and asthma. There is a clear reason for why this may be the case though – the hygiene hypothesis.
Earlier microbial exposure may help train and regulate the developing immune system.
Dr Suzanne Wylie
“The so-called hygiene hypothesis proposes that younger siblings are exposed to a wider range of microbes and minor infections earlier in life because older brothers and sisters bring viruses and bacteria home from nursery or school,” explains Dr Wylie. “This earlier microbial exposure may help train and regulate the developing immune system, potentially making it less likely to overreact in the form of allergic disease.
“We do sometimes see this reflected epidemiologically, with children from larger families appearing to have lower rates of some allergic conditions, although again this is only one part of a much bigger picture because genetics, breastfeeding, antibiotic exposure, household smoking, pollution and even pet exposure can all influence allergy risk as well.”
What other recent studies say about birth order and health
Another recent study from 2024 looked at mental health and birth order and found that firstborn and only children are at a higher risk of facing anxiety and depression.
The study, conducted by Epic, analysed data from over 182,000 children aged 8 across the United States, and found that firstborn children were 48% more likely to be diagnosed with anxiety and 35% more likely to be diagnosed with depression compared to children born second or later.
Only children, those without any siblings, were 42% more likely to have anxiety and 38% more likely to have depression compared to children with siblings.
The study's results suggest that birth order may play a significant role in the development of anxiety and depression in children.
While the exact reasons for this association are not fully understood, researchers speculate that factors such as limited interactions with parents and less parental attention may contribute to these mental health issues.
These circumstances can have adverse effects on mental health throughout sensitive development periods in childhood, potentially lasting across the life course.
Why might health implications differ depending on birth order
There’s still a huge amount of research to be done in this area, but there are several reasons that health and birth order might be linked. These include:
Different parenting attention – firstborns and only children might benefit from closer attention and are more likely to be taken for neurodiversity assessments and health-related appointments.
Family resources spread differently – as families grow, parents usually have to divide their time, money and energy between more children. That can mean different opportunities, responsibilities or experiences depending on when a child is born into the family.
“Parenting style often shifts with each child, expectations change, resources are divided differently, and cultural factors play a huge role,” says Dr Ahmad. “In many South Asian households, for example, eldest children may carry a greater sense of duty towards family, which can influence career choices, stress levels and even how they prioritise their own health.”
Exposure to germs from siblings – Younger children are often exposed to common bugs earlier because older siblings bring viruses home from nursery or school. Some researchers believe this early exposure may help train the immune system, which could explain lower allergy risks.
Maternal age and pregnancy factors – Parents are naturally older with later pregnancies. Factors such as maternal age, spacing between children, stress levels and pregnancy health may all influence outcomes more than birth order itself.
“Maternal age and spacing between pregnancies are likely to be much more clinically significant than birth order alone,” says Dr Wylie. “Advanced maternal age is associated with increased risks of chromosomal abnormalities, gestational diabetes, hypertension and some obstetric complications, while very short gaps between pregnancies can affect maternal nutritional recovery and may increase risks such as preterm birth or low birth weight.
“Conversely, very long intervals between pregnancies may also carry certain risks. In practice, these reproductive and maternal health factors tend to have a more direct and measurable effect on child outcomes than ordinal birth position itself.”
Children reacting to sibling roles – in terms of mental health a lot of it may be down to family dynamics and the stories we create for ourselves. Siblings naturally compare themselves with one another, and children (and their parents) may carve out different identities within the family, which can then be reinforced over time.
"At The Dawn, we recognise that adult mental health challenges are often rooted in unresolved childhood experiences," says Wells. "The ways children adapt in order to feel safe, valued or noticed within their family can become deeply ingrained. When these adaptations aren't consciously examined later in life, they can quietly shape how someone relates to themselves and others well into adulthood."
I often see firstborns who are high achieving but struggle to switch off, which can impact sleep, cortisol levels and long-term metabolic health.
Dr Ahmad
It’s important to remember that nothing sits in isolation and the psychological impact of where you sit within your family can have a direct link to your health. As Dr Ahmad explains:
“Firstborns often take on responsibility early, sometimes carrying higher levels of stress or perfectionism into adulthood. Middle children can become natural negotiators, while younger siblings may develop more risk taking or socially confident traits. Only children often present more like firstborns, but without sibling competition,” she says.
“From a medical perspective, what is interesting is how these patterns can translate into health behaviours. For example, I often see firstborns who are high achieving but struggle to switch off, which can impact sleep, cortisol levels and long-term metabolic health.
“Younger siblings may be more relaxed but sometimes less consistent with routines like diet or exercise. These are not rules, but they are trends worth understanding.”
Birth order and personality: are the stereotypes true?
While the recent studies focus on birth order and health, psychologists have long been interested in whether your place in the family can also shape your personality.
It’s a difficult theory to prove. It can be hard to untangle how the temperament and traits you’re born with and the personality you grow into after years of hearing family labels and expectations merge to make you who you are.
“Labels given by family members play a huge part in developing an individual’s sense of self,” says Janee Young, mental health care specialist and Clinical Director of Wellness Detox of LA.
“If a person is labelled “the smart one”, then this label can provide academic confidence. However, this same label can create fear of failure because this person believes their self-worth is directly linked to their academic success.
“If a person is labelled “the difficult one,” than they will internalize this and act accordingly in all aspects of life, even though the true need behind the behaviour is for attention/validation of emotions.”
Over time, these labels can solidify into roles that children carry within the family – and beyond.
If a child hears the same description often enough, it stops sounding like an opinion and more like the truth.
Helen Wells, Psychotherapist and Clinical Director at The Dawn Rehab Thailand
Wells says repeated labels can become surprisingly powerful. "A child known as 'the clever one' may feel intense pressure to perform or fear failure. 'The baby' may struggle to feel taken seriously or capable, even as an adult, while 'the difficult one' may internalise shame and expect rejection before it happens. If a child hears the same description often enough, it stops sounding like an opinion and more like the truth."
Matthew Bernarda, family therapist at 12 South Recovery, has seen these repeated labels play out amongst his clients: “Birth order is not a personality diagnosis, but it can quietly shape the role we learn to play in the family. I notice these roles in therapy all the time. One child is usually the peacekeeper, another is the one who needs attention all the time, and another learns how to get away with murder with charm and humour.
“These roles can follow us into adulthood. Sometimes they serve us, and sometimes they limit us. We are often completely unaware of them, but when we identify them, we can see where they are good for us and where we need to break out of them.”
So, while it’s true that birth order can affect our personalities it’s just one factor in a more complex system of family dynamics, genetics and the environment we grow up in.
“Discussions around birth order can easily drift into stereotypes, particularly around personality traits,” says Dr Wylie. “While these ideas are culturally popular and may resonate anecdotally with families, robust scientific evidence for strong personality differences purely based on birth order is much weaker than many people assume.”
Real mums reveal how birth order affects their kids
When it comes to birth order in practice, many parents say the stereotypes feel surprisingly familiar:
For some families, the classic responsible eldest, carefree youngest dynamic plays out almost exactly as expected. “My eldest is sensible, doesn’t like breaking rules and does as she is asked,” says Sarah Singleton, mum of two. “My youngest is wild, spirited, cheeky and likes breaking rules!”
That contrast is something many parents recognise. Amy Brooks describes her eldest as “shy, follows the rules and wants to please”, while her youngest is “loud and confident, assertive and a leader not a follower”.
Others notice how younger siblings seem to develop confidence more quickly. Emma Gatenby says her youngest “definitely knows what he wants and is much more confident and boisterous than his brother and has definitely learned a lot quicker by having an older sibling”.
My two are the complete opposite of the stereotypes.
Mum Claire Thompson
Middle children, meanwhile, are often described as carving out their own distinct space. Hannah Hutchings calls her middle child “wild, constantly on the go, pushing boundaries, and fearless, but extremely funny”, while Olivia Parker says her middle son is “definitely more independent… he’s a leader for sure”.
But not every family fits the mould. “My two are the complete opposite of the stereotypes,” says Claire Thompson. “My eldest doesn’t like rules and isn’t responsible at all. My youngest is so much more sensible and does what she’s told.”
Only children also challenge expectations. “He does not fit the only child stereotypes at all,” says Louise Hodges of her 7-year-old son. “He is incredibly kind, caring and very empathetic.”
Alongside personality, some parents have noticed differences in health between siblings — though patterns vary widely. Leah Dean says her middle child “struggles with asthma and has spent a lot of time in hospital”, while her youngest “has the best health so far”. Olivia Parker shares that her “middle child had bad eczema as a baby”, while her eldest has “very mild asthma”.
In other families, health similarities stand out more than differences. “All have allergies!” says Jennifer Ann of her three children.
For many, the reality is a mix of patterns, surprises and individuality. As Connie NaKusa puts it: “Stereotypes are holding true here at the moment but my eldest used to be the complete opposite, which makes me think a lot of it down to age.”
Taken together, these experiences highlight what experts say too: while birth order can shape family dynamics, every child’s personality and health is ultimately their own.
What matters far more than birth order
Before you start looking at your children and worrying that they’ll be adversely affected by the completely uncontrollable fact of when they were born, remember birth order is a miniscule piece of a much larger puzzle.
“When thinking about child health overall, factors such as stable caregiving relationships, emotional security, good nutrition, safe housing, parental mental health, vaccination uptake, opportunities for physical activity and early developmental support are overwhelmingly more important than birth order,” says Dr Wylie.
“A child growing up in a secure, nurturing and economically stable environment is likely to have better long-term outcomes regardless of whether they are the eldest or youngest sibling.”
Wells agrees that birth order is only one influence. "It's important to remember, though, that these roles aren't the be all and end all, she says. "You can outgrow or reshape them through conscious effort, for example through therapy."
And if you’re grappling with aspects of your own personality or health and think birth order may have a significant part to play, the good news is that you can use this as a tool for change.
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“What I always say to patients is this. Birth order may shape tendencies, but it does not define outcomes,” says Dr Ahmad. “Awareness is powerful. Once you understand your patterns, you can work with them rather than against them.”
Authors
Hollie BondSenior Digital Journalist
Hollie is Senior Digital Journalist at MadeForMums. She writes articles about pregnancy, parenting, child health and getting pregnant. She has written for a number of national lifestyle magazines and websites over the past 12 years including Family History Monthly, You and Your Wedding and Muddy Stilettos. She has two children aged 5 and 8 and hasn't slept since 2017!
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