While birth order is known to influence people’s personality traits and behavior, whether you’re the cautious firstborn or the risk-taking youngest child, that’s not the only way it affects your life. Research suggests it may also affect some aspects of your health, including your immune system, obesity risk, anxiety levels and more. “Although health outcomes are influenced by myriad factors, an individual’s personality traits, coping mechanisms and stress management, which greatly impact overall well-being, are layered by the intricate role of birth order,” Dr. Michael Valdez, medical director of Detox California, tells Yahoo Life.
Whether you’re the oldest child, the youngest or are right in the middle (and even if you’re an only child), here are some of the surprising ways birth order can have an impact on your physical and mental health.
Firstborn children tend to be more on top of their health care — but may also be more likely to have anxiety. (Getty Images)
The firstborn child
Firstborn children tend to be responsible, or rule followers. They often take on the caretaking role among their siblings. They can be prone to worry and are often seen as mature for their age. They are known to be comfortable taking charge. One study found that CEOs are more likely to be the firstborn child. Another showed they are more likely to get higher education and make more money than their younger siblings.
But being firstborn doesn’t necessarily mean an easy life start. One study found that firstborns were more likely to be born preterm and with lower birth weight compared to their later-born siblings. However, this gets balanced out because parents tend to be more attentive with their firstborn when it comes to doctor’s visits and preventive care than their subsequent children — specifically, the study authors found that “earlier-born children are more likely to participate in preventive medical screenings, their vaccine uptake rates are higher and they see their general practitioner (GP) or pediatrician more often.” Researchers say that, as parents have more children, this may change likely due to waning resources, including time and money.
Beyond the physical health aspects, Alyson Curtis, a New York-based therapist and founder of Attuned Therapy, says that because they’re born first, the oldest sibling is often “parentified,” or pushed into a parent-like role. For girls, there’s even a specific term for this — eldest daughter syndrome, which describes the disproportionate responsibility often given to the oldest girl in the family.
“Essentially, the eldest child grows up too fast as a form of survival,” Curtis tells Yahoo Life. “They learned at a young age that taking care of others is where their self-worth lies since that is what their caregiver valued in them.”
Because of this, older siblings may end up being more “conscientious or careful, possibly because of the higher expectations placed on them, and may be more proactive in managing their health,” Claudia Giolitti-Wright, founder of Psychotherapy for Young Women, tells Yahoo Life.
But this kind of upbringing can have “dangerous implications on their mental health,” says Curtis.
A report from Epic Research published in 2024 found that the oldest children were 48% more likely to have anxiety and 35% more likely to have depression than those who came later in the birth order.
Middle children tend to have stronger immune systems. (Getty Images)
The middle child
Ah, the middle child. If you are one, then you already know the struggle of being overlooked and forgotten. There’s even a term for it: middle child syndrome.
Sandwiched between the responsible eldest and the free-spirited youngest, middle children tend to be the peacemaking mediators of the family. While they can be people pleasers, they are also known to be rebels who try to carve out their own path. Research shows middle children engage in riskier behavior than their older siblings.
It might be a tough place in the birth order, but being the in-between also has its benefits. A large study published in February showed that middle children ranked highest in honesty, kindness and cooperation.
Still, Giolitti-Wright says she has seen patterns of self-silencing and feeling overlooked in middle children. They “tend to ignore their needs because it wasn’t ever about them.”
This can affect mental well-being, sometimes manifesting as anxiety or depression, especially if the middle child’s role in the family wasn’t clearly defined or validated, adds Giolitti-Wright.
When it comes to physical health, middle children tend to have better immune systems than the oldest because of the early exposure to germs.
Being the baby of the family may make you less likely to have a food allergy, but you’re also more prone to accidents that send you to the hospital. (Getty Images)
The youngest child
Youngest children are often seen as charming free spirits. As the babies of the family, no matter their age, they are generally given more leniency than their older siblings. They are known to be fun, rebellious risk-takers.
The good news for the youngest ones is that they, just like middle children, tend to have better immunity than their oldest sibling thanks to the germs they are exposed to from the get-go, early and often. This is known as the Hygiene Hypothesis. One paper found that younger siblings are less likely to have asthma and allergies than their oldest sibling, while other research suggests that having older siblings is associated with a lower risk of food allergies.
Still, while the oldest sibling tends to have worse health outcomes at birth, that role switches as kids get older, with research showing that younger siblings are more likely to have hospitalizations because of risky behavior, including accidental injuries and drug use.
While youngest siblings are often indulged for being “the baby” or the “protected” one, they “might receive less one-on-one medical attention simply because parents are stretched thinner or less anxious than they were with their first child,” says Giolitti-Wright.
When it comes to mental health, Curtis says younger siblings can be prone to struggles with self-esteem and self-efficacy. “Essentially, the pervasive dynamic of ‘coming last’ to older siblings leads to a reduced sense of one’s own abilities,” she says. “When you consistently can’t influence outcomes due to the developmental disadvantage of being last in the birth order, you adapt by ‘going with the flow.’”
A younger sibling may go on to be an adult who speaks up less, continues Curtis, believing that their opinion doesn’t matter as much because it didn’t when they were learning how the world worked. “A self-fulfilling prophecy is then enacted over and over again, creating a vicious cycle, wherein the individual doesn’t feel very powerful in their own life. … These types of thoughts have the capacity to alter one’s entire life, from what profession they choose to what kind of partner they marry,” says Curtis.
Only children can be more mature and verbal than their peers, but may also experience anxiety or depression. (Getty Images)
Only children
Are only children really “lonely onlys”? The idea of being an only child has strangely gotten a bad reputation in the past, even gaining the term “only child syndrome.” This refers to the idea that children who do not have siblings are bossy, spoiled, selfish and lack coping skills. But there isn’t much evidence to support this claim.
“One of the benefits I often see in both research and my own clinical work is that only children tend to be highly verbal, mature for their age and comfortable in adult spaces,” says Giolitti-Wright. “They’re often great at entertaining themselves, thinking creatively and developing a strong sense of identity because they’ve had a lot of solo time and adult interaction growing up.”
Still, if an only child comes from a dysfunctional household, Curtis says this is where things can get tricky. “I … see in my practice how only children take a greater hit coming from households with complex trauma than their counterparts who had siblings. … Siblings can be a life vest for any intense pressure or dysfunction the parents are knowingly or unknowingly putting on the children.”
Like firstborn children, research shows only children are more prone to anxiety and depression. “Only children may have better access to resources but might also carry higher pressure to excel,” says Giolitti-Wright.
When it comes to physical health, research suggests there’s a link between only children (and last-born children) and a higher risk of obesity in childhood and adulthood. In another study, researchers found that only and last-born children are more likely to be overweight or obese if their mothers tended to discourage them from eating and were less likely to praise them during meals.
What should parents take away from this?
While birth order can influence a person’s life, there are no absolutes, Amy McCready, founder of Positive Parenting Solutions, tells Yahoo Life. “As parents, we have tremendous power to help our kids avoid falling into those common stereotypes through the way we interact with them.”
When it comes to their kids, McCready says it’s important for parents to try and avoid labels and comparisons, which can pigeonhole kids into roles in which they feel stuck, even into adulthood. Instead, celebrate individual strengths.
Next, check in with them. “It’s not just what we do, it’s how our kids interpret it,” McCready says. She suggests asking: “How do you feel about the jobs we each have in the family?” and “Do you ever feel like I expect more from you than your siblings?”
For families with multiple children, it’s important to “prioritize one-on-one time,” says McCready. “Even just 10 or 15 minutes fosters deep emotional connection and helps each child feel truly seen, heard and valued. Because it’s one-on-one, your child doesn’t have to compete with siblings for your attention — they get you all to themselves, reinforcing their sense of belonging and significance, no matter their birth order.”
