It’s widely believed that birth order has a powerful effect on personality. Firstborns are leaders, assertive and commanding, like George W. Bush, Winston Churchill, and both Bill and Hillary Clinton. Middle children are peacemakers, caring and social, like Barbara Walters, John F. Kennedy, and Dr. Martin Luther King Jr. Last children are rebels and nonconformists, born to think different, like Joan of Arc, Mahatma Gandhi, Mark Twain, and Stephen Colbert.

The theory is appealing on an intuitive level. Firstborns bask in the glow of their parents� undivided attention for the first years of their life. It seems reasonable that this might make them more assertive. Later siblings do have to compete for resources and attention, but have a benefit the firstborn did not: having a peer, companion, and playmate present from the day they are born.

Does it not make sense that they would be more social?

In a 1996 book called Born to Rebel, a psychologist named Frank Sulloway argued that firstborns, imitating parents, are more conservative and conscientious, whereas laterborns, forced to defend themselves from the depredations of older siblings, are more conciliatory and open[1].

But psychologists now believe that most research purporting to find links between birth order and personality is in fact flawed. Almost all such studies are subject to a statistical confound: the fact that birth order is linked to family size.

In a two-child family, a child has a 50-50 chance of being the firstborn. But in a five-child family, a child has a 20 percent chance of being the firstborn. Supposed birth order effects may occur simply because when parents have fewer children, they expend more resources on each one. It is also the case that wealthier, better-educated parents tend to have fewer children. The widely-reported fact that a majority of U.S. astronauts have been firstborns, then, may indicate not something about firstborns, as is often claimed, but rather something about small families, or wealthy, educated families[2].

Two large, more recent studies that did control for family size found null effects for all personality traits, and specifically found no birth-order effects for extraversion, emotional stability, agreeableness, conscientiousness, or imagination. The only correlation that was found was for intelligence: Firstborns score higher on objectively measured intelligence as well as on self-reported intellect[3,4]. Another study found that firstborns have IQs about 3 points higher than second-born children[5].

There is one other real birth order effect: fraternal birth order. The more older brothers a man has, the more likely he is to be gay. In fact, for each older brother he has, his odds of being gay go up by about 33 percent[6].

So then what explains why we feel so strongly that birth order must influence personality? Tellingly, a study found that people perceive birth order effects when they are made aware of someone�s birth order[7].

Additionally, birth order effects on personality may well exist within the context of the family of origin, argues psychologist Judith Rich Harris. When adult children go home and visit their parents for the holidays, she explains, firstborn children may indeed behave differently than later-born children. But these effects probably don�t reflect an enduring aspect of their personality outside the home[8].

?References

1. Sulloway, F. (1996). Born to rebel: birth order family dynamics, and creative lives. �New York: Pantheon.
2. Hartshorne, J.K. (2010, January 1). How birth order affects your personality. Scientific American Mind, 21, 18-19.
3. 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, 14224�14229.
4. Kristensen, P., & Bjerkedal, T. (2007). Explaining the relation between birth order and intelligence. Science, 316, 1717�1717.
5. Black, S.E., Devereux, P.J., & Salvanes, K.G. (2011). Older and wiser? Birth order and IQ of young men. CESifo Economic Studies, 57, 103�120.
6. Puts, D.A., Jordan, C.L., & Breedlove, S.M. (2006). O brother, where art thou? The fraternal birth-order effect on male sexual orientation. Proceedings of the National Academy of Sciences, 103, 10531�10532.
7. Jefferson T.; Herbst J.H.; McCrae R. R. (1998). Associations between birth order and personality traits: Evidence from self-reports and observer ratings. Journal of Research in Personality, 32, 498�509.
8. Harris, J.R. (2011). The nurture assumption: Why children turn out the way they do. New York: Simon and Schuster.