As many parents can attest, siblings tend to be more different than alike. Some of this may be the result of our birth order, and how we’re subsequently raised. What’s more, birth order may influence our health and sexuality too. Here’s what you need to know about how your birth rank affects your life.

Birth order is an incredibly difficult area to study, and as such, is considered highly controversial.


There are so many factors to consider outside of a person’s familial rank by age, including the spacing in years between children, the total number of children in a family, socioeconomic status, the sex of siblings, and environmental circumstances during upbringing. It’s not easy to isolate traits that are dependent on birth order.

And indeed, a 1983 meta-study by Ernst and Angst, which looked at birth order studies done between 1946 and 1980, threw much of this area into question. Many psychologists, to this very day, minimize the role of birth order and its effects on our personalities. And the preponderance of conflicting literature on the matter hasn’t helped, either.


But over the course of the past four decades, psychologists have continued to look into the issue — and they’re discovering that there may in fact be something to it.

Birth Rank and Personality

Alfred Adler, a peer of Freud and Jung, was one of the first theorists to use birth-order position for assessing clients. But the most important modern psychologist to rigorously study the topic was MIT’s Frank Sulloway.


In his 1996 book, Born to Rebel, he considered five major personality traits, namely openness, conscientiousness, extraversion, agreeableness, and neuroticism. Birth rank, he argued, has an influence on all of them.

Sulloway showed that firstborns were more conformist, while laterborns were more creative and more likely to reject the status quo. He also made the case that people tend to have more in common with someone of their own birth rank than their own siblings.


This seminal book, while celebrated by many, also kindled a firestorm of controversy. Researchers like Jeremy Freese attacked Sulloway’s methodology, accusing him of manipulating of the data. Since the publication of Born to Rebel, there have been hundreds upon hundreds of studies exploring the topic, with conclusions spanning the gamut.

But back in 2010, in a much-needed update to the Ernst and Angst meta-study, Daniel Eckstein and colleagues decided to do another exhaustive analysis of the existing literature. They looked at the results of 200 birth-order studies to see if any consistency could be found in lifestyle characteristics. They discovered that some personality characteristics were indeed being consistently matched according to birth rank.

First-borns, they learned, tend to experience high success and achievement. Only-borns desire achievement. Middle-borns are highly sociable. And the youngest children have a pronounced desire for a social life.


The authors also put together this tidy chart showing the number of times that research studies were able to match specific personality traits to birth rank:


Looking at other studies, there appears to be a connection between birth order and career interests. A 2001 study showed that laterborn children go in the direction of arts and outdoor related careers, while onlyborns, and possibly firstborns, tend to prefer intellectual pursuits. The researchers aren’t saying that it’s an inborn, genetic effect, but rather something that’s enforced by parental guidance.

Relatedly, birth order position also appears to influence our competitiveness. Researchers have found that firstborns pursue “mastery goals,” where competence is determined by others.


Psychologist Bernd Carette writes:

These results are in line with the notion that, presumably due to a differential treatment by their parents during early childhood, firstborns prefer self-referenced standards to evaluate their competence. That is, they approach tasks with the desire to develop knowledge, skills, and task mastery. On the other hand, secondborns tend to evaluate their competence in terms of other-referenced standards. They are more strongly inclined to approach tasks with the desire to demonstrate competence relative to others.


Interestingly, birth rank also affects how dominant and extroverted we are. A recent study showed that firstborns show more of the dominance side of extraversion, while laterborns exhibit more of the sociability aspects of extroversion. Firstborns, it would seem, are actually less dominant or assertive than laterborns. The researchers suspect that strict and overprotective parenting of firstborns may be the reason, which causes them to grow up submissive.

Depends On Who You Ask — And When

A 1998 Canadian study, after looking at 1,022 families, found that firstborns are more conservative, more achieving, and more conscientious. Laterborns were assessed as being more rebellious, liberal, and agreeable. But these results came in from intra-family assessments — and that's a potential problem.


For example, a different study found that people outside the family, such as spouses, friends, and peers, provide different evaluations — which often show none of the expected birth-order differences!

Which is not a complete surprise. It’s possible, if not blazingly obvious, that personality assessments are inherently problematic by virtue of the fact that people behave differently around different people. What’s more, family dynamics will have an impact on both behavior and perception — including characteristics that don’t get seen or expressed outside the family context. So, while birth order has a measurable impact on personalities and the perception of them within a family, it’s an effect that may not often transcend the immediate family environment


What’s more, as Michael E. Lamb and Brian Sutton-Smith have argued, sibling relationships evolve over the course of a lifetime. They’re continually adjusting to changing dynamics and circumstances — adjusting to competing demands of society and inherent biology.


One particularly controversial area of birth order studies is the ongoing debate about intelligence. Firstborns consistently rank higher on intelligence tests. The going theory is that they get more attention and resources from parents.


Indeed, Robert Zajonc says that firstborn children are almost exclusively exposed to adult language, whereas laterborn children experience the less mature, childish speech of their older siblings. This may subsequently explain why firstborns tend to score higher on tests of verbal ability. As for non-firstborns, their older sibling(s) frequently assume the role of parents, answering their questions and offering perspectives, albeit less capably.

Other factors, of course, include socioeconomic status (the smaller the family, the more time and resources that can go around; also, large families tend to fall within lower socioeconomic groups). As well, the age of the mother at the time of birth also appears to be a factor (younger moms tend to be less educated, have lower incomes, and are more capable of producing larger families).


Not surprisingly, birth order can also influence our relationships. In 2009, Timothy Hartshorne and his team showed that, similar to Sulloway, people have a lot in common with people of their own birth ranks. Furthermore, the psychologists found that we’re more likely to form long-term platonic and romantic relationships with other people of the same birth order — an effect the researchers say cannot be explained by other factors, such as family size.


Studies have also shown that sexual orientation correlates with a man’s number of older brothers. And in fact, each additional older brother increases the odds of homosexuality by about 33%.

The going theory is that mothers become increasingly immune to certain antibodies with each subsequent pregnancy. Accordingly, the anti H-Y antibodies produced by the mother during a pregnancy pass through the placental barrier to the fetus, which in turn affects various aspects of sexual orientation in the fetal brain.


A follow-up study by the same research team noted:

The results indicate that the proportion of homosexual men whose sexual orientation is attributable to fraternal birth order constitutes a minority, but not a negligible minority, of all homosexual men. The fraternal birth order effect may reflect the progressive immunization of some mothers to Y-linked antigens by each succeeding male fetus, and the concomitantly increasing effects of antimale antibodies on the sexual differentiation of the brain in each succeeding male fetus.


Interestingly, only biological older brother’s predict men’s sexual orientation, strongly suggesting a prenatal origin to the fraternal birth-order effect.


Birth order can also have an impact in unexpected areas.

For example, researchers found that firstborn children have a greater difficulty absorbing sugars into the blood and have a higher daytime blood pressure than later born children. Firstborns, therefore, may be at a greater risk of metabolic and cardiovascular diseases in adult life. They surmise that this difference may be attributable to physical changes in the mother's uterus during her first pregnancy.


Japanese researchers have also discovered that first-borns may be more susceptible to food allergies.

First-borns may also be predisposed, for unknown reasons, to high-functioning autism (or what used to be called Asperger’s syndrome). It may have something to do with birth stoppage, obstetric complications, or immunological processes — but scientists aren’t really sure. Relatedly, closely spaced pregnancies have been linked to autism.

Researchers have also found a decreasing risk with increasing birth order for certain childhood cancers (but the opposite for acute myeloid leukemia). In terms of an explanation, the researchers write, "It is possible that firstborn children have higher estrogen exposures that may contribute to greater risk of cancer than later born children. Estrogen levels in maternal and umbilical cord blood samples are somewhat greater in first pregnancies compared with second or third pregnancies."


Also, children with older siblings are more likely to experience respiratory symptoms at four years of age. One possible explanation is that children with older siblings have more exposure to respiratory infections at an early age than oldest or only children.

Additional reporting by Joseph Bennington-Castro.

Images: LiliGraphie/Shutterstock.