Does birth order affect size?

The question of whether birth order affects size has fascinated researchers and parents alike. As parents welcome new additions to the family, they often wonder if the order in which their children are born will have any effect on their growth and development. For decades, studies have investigated the links between birth order and various outcomes, including personality, intelligence, and achievement. More recently, some research has started to explore whether a child’s birth order influences their size and growth patterns.

Does birth order affect height?

One of the most common questions about birth order involves its potential effects on height. Numerous studies over the years have aimed to determine if firstborn children ultimately grow taller than their younger siblings. The results have been mixed.

Some research has found that older siblings do tend to be taller, on average, than their younger brothers and sisters. For example, a Norwegian study published in the journal BMJ Open in 2015 reviewed height data for over 2 million siblings. They found that older brothers were on average 2.3 cm taller than their younger brothers. The height difference between older and younger sisters was less pronounced but still present, with older sisters being 1.7 cm taller on average.

However, other studies have failed to find meaningful differences in height based on birth order. A large Australian study followed children from birth to age 14 and found no evidence that firstborns were taller during childhood or adolescence. Similarly, an analysis of over 20,000 sibling pairs in the United States found no significant connection between birth order and final adult height.

Overall, the research to date suggests that birth order may have a small influence on height early in development, with older siblings being slightly taller on average. However, this effect likely diminishes over time and does not appear to impact final adult stature. Social and environmental factors during childhood probably play a much larger role.

Does birth order affect weight?

Along with height, researchers have also looked for links between birth order and weight. Only children have been found to have higher rates of obesity in some studies. But do firstborns tend to weigh more or less than their younger siblings? Again, the research findings are mixed.

Some studies have found that earlier-born siblings tend to have lower BMIs on average. For example, a study of over 12,000 participants from the National Longitudinal Survey of Youth found that firstborns had lower BMIs as adolescents and lower obesity rates as adults. This was true even when controlling for age, race, gender and socioeconomic status.

However, other large studies have found the opposite effect. An analysis of data from nearly 3,000 families in the United Kingdom found that firstborn children were more likely to be overweight or obese than second- or third-born children at age 7. This difference was more pronounced among boys in the study.

The reasons for these conflicting findings are unclear but may involve differences in family dynamics and parenting practices depending on birth order. Overall, birth order alone does not seem to be a major predictor of weight. Lifestyle habits around diet and exercise likely play a much larger role in determining size and weight for any child regardless of their birth order.

Do firstborns have bigger heads?

Parents may notice or wonder if firstborn children tend to have bigger heads at birth. Some research has suggested this may be true, for a couple of reasons:

First, some studies have found that firstborn babies have slightly longer gestations on average compared to subsequent births. This extra time in utero allows more brain growth before birth.

Second, research using imaging scans suggests that firstborn brains may contain more gray matter and score higher on cognitive tests than later borns. The increased gray matter could contribute to a larger head circumference.

However, the differences are quite small. One study found that the average head circumference of firstborns was less than 0.5 cm greater than later-born siblings. Other studies have found no significant difference in head size according to birth order.

In most cases, genetics and other factors play a far greater role than birth order in determining head size at birth as well as cognitive development over time. So while firstborns may have a slight initial advantage, it likely does not make a major difference in the long run.

Do firstborns have bigger feet?

Some parents notice that their firstborn child seems to have bigger feet than their younger siblings did at the same age. Is there any truth to this observation? A few studies have looked for evidence of a connection between birth order and foot size.

One study from the United Kingdom measured the foot sizes of 297 children aged 6 to 15. The researchers found that older siblings did tend to have bigger feet on average, even when accounting for age and gender. The length of firstborn feet was on average 5 millimeters longer than later-borns.

However, other studies have failed to find major differences in shoe size between firstborns and younger children. As with height and head size, it seems that any effect of birth order is minor compared to other factors like genetics, nutrition, and gender. Boys tend to have bigger feet than girls regardless of birth order.

Interestingly, some research suggests that older siblings receive more resources and investment early on, which could spur greater physical development including slightly bigger feet. But once again, definitive evidence linking birth order and foot size remains elusive.

Do firstborns have bigger muscles?

Physical size involves not just length measurements like height and foot size but also muscle mass and strength. Could birth order impact the amount or quality of muscle tissue?

A few studies provide some hints. Research from Germany found that firstborn boys ages 8 to 17 displayed greater hand grip strength on average than their younger brothers. This could suggest more muscular development, perhaps linked to receiving more attention and resources as the oldest sibling.

However, other research looking at jumping ability and muscle force generation in children failed to find differences based on birth order. As kids grow and become more active in sports and other physical pursuits, birth order advantages may fade.

In one interesting study, scientists measured muscle fibers in biopsy samples from adult men and found no differences in size or composition between firstborns and later-borns. This suggests that by adulthood, birth order does not impact actual muscle morphology and strength.

Overall, firstborns may exhibit slightly greater muscularity and strength in early childhood thanks to increased attention and investment. But by adulthood, too many other factors influence muscle and strength development to point to birth order as a key player.

Do only children grow bigger?

While birth order refers to a child’s rank among siblings, some research has looked at how only children with no siblings compare in terms of size and growth. Only children do not have to compete with siblings for parental resources and attention, so some scientists hypothesized they may grow bigger as a result.

Several studies provide support for this idea. Research from Brazil found that only children tended to be taller on average than children with one or more siblings. An American study found that only children scored higher on measures of both height and weight compared to children with siblings.

However, other studies show less clear results. A Swedish study tracking growth in children born in the 1970s found no differences in height or weight between only children and those with siblings. And a more recent look at thousands of children in China similarly found no significant differences in height, weight or BMI between only children and those with siblings.

As with birth order, family dynamics and other social factors likely play a larger role than simply having siblings or not when it comes to size. Only children may receive more resources and nurturing early on but have no siblings to compare themselves against or compete with, for better or worse.

How could birth order affect size?

If birth order does influence size in subtle ways, especially early in development, what might explain this connection? Scientists have proposed a few key factors:

Physical resources: Firstborn and only children may grow bigger because they have exclusive access to food and other resources for some period of time. Having to share with siblings could restrain growth.

Maternal health: First pregnancies occur when mothers are healthiest and least depleted physically. This could boost fetal development.

Hormonal exposure: hormones like testosterone could decline with each successive pregnancy, slightly reducing growth.

Psychosocial dynamics: firstborns receive undivided parental attention and expectations initially, which could spur faster development.

Comparison effects: older siblings provide a standard to strive toward, while younger ones have a role model’s example to follow.

Of course, birth order is just one factor among many affecting size and is clearly outweighed by genetics, nutrition, health care access, and other variables. Still, it does appear capable of subtly shifting growth patterns under some conditions.

Does birth order affect other body features?

Beyond the most common measures of height, weight, and head size, some research has also looked at birth order and other physical attributes including:

Teeth: Some evidence suggests firstborn children may get teeth earlier and have better overall dental health. Access to resources and vitamin D while oldest may promote tooth development.

Eyesight: Firstborns tend to have better visual acuity and lower rates of needing vision correction, perhaps linked to receiving more early attention and experiencing more stimulation.

Handedness: Firstborns are slightly more likely to be right-handed, while later borns show greater left-handedness and ambidexterity. Underlying brain laterality differences could be at play.

Allergies: Order of birth may impact early exposure to microbes, with firstborns showing higher rates of allergies and asthma. However, growing up with siblings seems to confer protection.

As with other physical features, strong evidence linking birth order to these outcomes is limited. More study is needed to determine if and how order of birth tangibly influences eyesight, teeth, handedness, allergies, or other attributes apart from the most basic measures of size.

Does birth order affect siblings of the opposite sex differently?

Most studies on the effects of birth order combine same-sex and opposite-sex siblings when looking at height, weight, and other physical differences. However, some research suggests that birth order could impact brothers and sisters differently.

One study found that the height difference between firstborn and secondborn siblings was more pronounced when the siblings were of opposite genders. The firstborn brother was taller on average than his younger sister, while firstborn sisters were not taller than their younger brothers.

For weight, some evidence points to firstborn brothers being heavier than their younger brothers, while firstborn girls tended to weigh less than their younger sisters. These gendered effects could relate to physical maturity and growth patterns differing between boys and girls.

Overall though, definitive evidence is lacking on whether birth order impacts same-sex versus opposite-sex siblings differently in terms of size and other physical attributes. Much may depend on the cultural and social context, including norms and expectations around gender. More research across diverse populations is needed on this question.

What does this mean for parents?

While the research on birth order’s effects on physical size remains inconclusive and contradictory in many ways, here are a few takeaways for parents:

– Don’t worry too much about birth order when predicting your child’s ultimate height and growth. Many other factors play a much larger role. Genetics and nutrition are most important.

– Firstborns may tend to be slightly bigger in early childhood on average, but any differences likely diminish over time and should not cause concern.

– Try not to compare siblings’ size and growth patterns too closely, especially same-age vs. opposite-age. Each child follows their own developmental timeline.

– Ensure each child gets sufficient nutrition, health care, activity and attention regardless of their birth order to help them meet their potential physically.

– Watch for potential effects of sibling competition like eating disorders or over-exercising and counteract by emphasizing self-esteem and body positivity for all children.

While interesting to explore, a child’s order of birth is clearly not destiny when it comes to their physical size and growth. With wise parenting, each child can thrive and flourish.


The question of whether birth order impacts size has fascinated parents and researchers and spurred lots of studies over the years. Some evidence suggests that oldest and only children may be slightly taller, heavier, and bigger overall early in development compared to their younger siblings. However, any differences are quite small and seem to disappear by the time children reach adulthood.

Many studies show no effects of birth order on height, weight, head circumference or other physical measures. Overall, genetics, nutrition, healthcare access, and social dynamics within families play much larger roles than birth order alone in determining a child’s size. While the findings remain mixed, birth order does appear capable of subtly shifting growth patterns under some conditions. But parents should not worry too much about how their child’s birth order will impact their size. With attentive parenting and care, each child can grow and thrive physically regardless of their order of birth.

Leave a Comment