Birth order matters, right?

Here’s the science on how it impacts personality, sexuality and intelligence.

My brothers and I.

Firstborns are responsible and natural leaders.

Middle born are neglected and peacekeepers.

The youngest is spoilt and uses charm to get away with murder.

Only child? Well, don’t even get me started!

Recognise these stereotypes? Or perhaps you have your own. Either way, birth order stereotypes are highly prevalent. A recent campfire chat I had turned to the topic of older vs younger siblings, with questionable opinions on the matter being thrown around (“I’m telling you, firstborns are more likely to become teachers!”). So I figured an article was needed to lay out the latest research.

Full disclosure: I don’t have kids and am the oldest of three brothers.

The research doesn’t stack up

Let’s start with whether birth order impacts personality traits. After all, these traits would influence behaviours like the firstborn being ambitious or the youngest being a risk-taker. The short answer is — it does not. Decades of research has failed to show a consistent relationship between birth order and personality traits such as openness to experience, consciousness, extraversion, agreeableness and neuroticism.¹

So why is the idea so ingrained in people’s psyche?

The technical answer is Illusory Correlation — the conviction that two things are associated when they are not. We are so convinced of these associations because they are somewhat intuitive. For instance, we accept that firstborns are likely to be leaders because they are the oldest of the siblings, and we correlate age with taking charge. Or that the youngest is the most carefree of the siblings. Of course, they have no responsibilities! But these broad generalisations about humanity simply aren’t accurate.

Confirmation bias is another reason these stereotypes are so prevalent. There is no shortage of information available to reaffirm your long-held beliefs. For example, have you heard that 21 of the first 23 space astronauts were firstborns?² Guess that means firstborns tend to be natural leaders, given that’s a quality associated with astronauts. But, hang on, what if astronauts are more likely to have highly affluent and educated parents, which tend to have small families and therefore increase the odds of astronauts being firstborn. I’ve not seen data that explores these variables, but the point is that factors other than personality traits may have influenced the high rate of astronauts being firstborns.

Plus we probably know a spoilt youngest child or leader-like firstborn among our family and friends to really drive home our birth-order convictions.

Further fuelling the flame of common misconceptions about birth order and personality is historical research that seems plausible but hasn’t been replicated in modern scientific research. Alfred Adler — a protege of Sigmund Freud — proposed that firstborns are neurotic because they didn’t have to share their parents for years, then BANG, they get dethroned when a sibling comes along.³ I can relate to this one! Frank J. Sulloway — American psychologist — combed the history books analysing 6,500 historical figures.⁴ He made the case that later borns were more likely to be rebellious and lateral thinkers — such as Charles Darwin and Mahatma Gandhi. Conversely, firstborns were more likely to tow the party line with a conservative mindset. While these findings are logical and convincing, they don’t use modern scientific processes and controls.

The latest research shows:

Birth order impacts sexuality.

The higher the number of older brothers a person has, the higher their probability of being gay.⁵ According to the research, this also applies to both men and women. Analysis of nine million people in the Netherlands found that both birth order and the number of siblings affect sexual orientation. For example, the data showed that the likelihood of experiencing same-sex sexual attraction is 19.7% greater for men and 18.6% for women when they had an older brother compared with a younger brother. The mechanism is thought to be biological. One hypothesis is that the mother’s immune reaction to proteins produced by a male foetus triggers the production of antibodies that influence the sexual development of later children. The more pregnancies with male foetuses, the more antibodies and therefore an increased likelihood that later-born children experience same-sex sexual attraction.⁶ This is fascinating research and firmly squares up to any outdated notions that sexual orientation is a choice.

Number of men with varying birth orders who enter same-sex unions with up to three siblings

Birth order impacts intelligence.

Six times out of ten, the youngest in a pair of siblings will score lower on IQ tests. Being firstborn, I’ll keep this section short in fear of sounding like an ass. However, research shows that later-born children score lower on cognitive tests than their siblings, which increases until they enter school, and the difference remains consistent thereafter.⁷ The gap in IQ scores is statistically significant but small, and the research suggests that sibling bonds probably play a greater role in influencing our development.

Parental behaviour is often cited as the cause for this birth order difference in IQ score.⁸ Parents take more risks with later-born children and are less likely to breastfeed and provide cognitive stimulation. It’s speculated this change in parental behaviour is due to lessons learnt from earlier children, further constraints on time and resources, and a general relaxing on non-essential child-raising tasks. But fear not, my younger brothers, the research shows that parents support emotional development equally.

Birth order is also correlated with physical health, with a Swedish study showing firstborns have greater cardiovascular fitness than later borns.⁹ Another study showed that firstborns were less likely to die an accidental death or by suicide.¹⁰ I really didn’t mean to focus so much on firstborns, but that’s where the research seems to focus!

It’s clear that birth order matters. However, it’s not impacting personality like the stereotypes would have us believe. I’m afraid it doesn’t give us the neat explanation we crave to understand the often striking differences between siblings. Instead, birth order research sheds light on what makes people who they are, both from a biological standpoint and from the impact of environmental factors like our upbringing.

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REFERENCES

1. Rohrer, J. M., Egloff, B., & Schmukle, S. C. (2015). Examining the effects of birth order on personality. Proceedings of the National Academy of Sciences, 112(46), 14224–14229.
2. Leman, K. (2009). The birth order book: Why you are the way you are. Revell.
3. Eckstein, D., & Kaufman, J. A. (2012). The Role of Birth Order in Personality: An Enduring Intellectual Legacy of Alfred Adler. Journal of individual Psychology, 68(1).
4. Sulloway, F. J. (1996). Born to rebel: Birth order, family dynamics, and creative lives. Pantheon Books.
5. Ablaza, C., Kabátek, J., & Perales, F. (2021). Are sibship characteristics predictive of same sex marriage? An examination of fraternal birth order and female fecundity effects in population-level administrative data from the Netherlands. The Journal of Sex Research, 1–13.
6. Blanchard, R., & Bogaert, A. F. (1996). Homosexuality in men and number of older brothers. American Journal of Psychiatry, 153(1), 27–31.
7. Damian, R. I., & Roberts, B. W. (2015). The associations of birth order with personality and intelligence in a representative sample of US high school students. Journal of Research in Personality, 58, 96–105.
8. Lehmann, J. Y. K., Nuevo-Chiquero, A., & Vidal-Fernandez, M. (2018). The early origins of birth order differences in children’s outcomes and parental behavior. Journal of Human Resources, 53(1), 123–156.
9. Barclay, K., & Myrskylä, M. (2014). Birth order and physical fitness in early adulthood: Evidence from Swedish military conscription data. Social Science & Medicine, 123, 141–148.
10. Barclay, K., & Kolk, M. (2015). Birth order and mortality: a population-based cohort study. Demography, 52(2), 613–639.