For decades, researchers have observed that, on average, firstborns score higher on intelligence tests than their later-born siblings. The further down a child is in birth order rank, the lower his or her IQ compared to older siblings.This is nothing I’d brag about. I think it inspires resentment and eye-rolling among later-borns. (Not to mention that intelligence tests and what they really measure are a controversial bugaboo.  But let’s put that aside for now.)  There’s no obvious reason for the difference in test scores because siblings often have the same parents and grow up in the same family environment.

The largest study on birth order and intelligence comes from Norway, where psychologist Petter Kristensen and Tor Bjerkedel studied data from a vast sample of more than 240,000 brothers conscripted by the military. Eldest children, it turned out, had an IQ nearly three points higher on average than the second-born siblings and about four points higher than third-borns, after controlling for parental education, marital status, income, mother’s age, and birth weight. (The effect of birth order on IQ does not differ between boys and girls.) A three-point difference in IQ doesn’t sound very significant from a personal perspective, but it is in the big picture. All else being equal, three points can translate into a thirty-point difference in SAT scores.  That may make all the difference between admittance into an elite college or a second-tier one, for instance.

The IQ boost was strong when Kristensen compared scores between families (your firstborn child versus everyone else’s first-born, and your second-born versus all other second-borns and so on) and remained strong in a later study when he compared the scores of kids within families (my first-born versus my second-born versus my third-born and so on).  This doesn’t mean that every firstborn in every family was brighter than her siblings.  Many later-borns scored higher on IQ tests than their older sib.  We’re talking about huge populations here, not individuals.

 Even so, you’d think there is something special about the wombs of first-time mothers.  It’s like a biological birthright, the first-born as know-it-all.  They’re preachy and self-righteous.  But they can be useful, too. They show the younger kids how to wheedle ice cream from an addled parent and properly glitter and glue.  They know how to make killer snowballs, open a lemonade stand, and coax the dog to stand on its hind legs.  They know what people do when they have sex.

All this teaching and preaching is key to Kristensen’s explanation of why first-borns score better on tests. Whether sanctimonious and bossy or generous and caring, the interaction between older and younger sibs helps the elder. They learn by telling.  In tutoring others, we all make sense of the world, and this in turn affects how well we do on intelligence tests.  “Smarter” comes from being the explainer. In Kristensen’s view, this explanation beats the other big three theories that involve family dynamics -- first-borns get more parental attention and are exposed to a more intellectual environment longer; parents have higher expectations of them; and they’re more achievement-oriented.  It beats them for one major reason that Kristensen discovered when he dug deeper.

On average, the IQs of second-borns are higher in families in which the first-born has died.

Not only did the second-born rise in family rank when a first-born died, but also in IQ, topping the scores of any younger sibs. Third-borns moved into second-place IQ rank in their families (one point higher) in the few cases in which second-borns in a family died.

So what can we really learn from this? We can imagine that birth order affects a baby throughout life, and performance on IQ tests is only a sliver of it. For those who buy into the claims of birth order psychology, even choice of career can be explained by one’s ranking in the family (which some studies confirm and others do not).

Every rank has its niche.

First-borns are said to be more conscientious, conservative, performance-and-power-oriented, disciplined, fearful of losing face, and generally more anxious. This helps explain why more first-borns are presidents, Nobel Laureates, and CEOs.

Middle children are born to rebel. They are less conscientious, less religious, and don’t do as well in school. They’re more sociable, impulsive, and open to fantasy. They’re good negotiators.

The baby of the family shows more interest in others and is more empathetic. Lastborns are more creative, flexible, risk-taking, impulsive and more extroverted, perhaps because they arrive into a larger, more stimulating household and must compete for attention.

Children without siblings are similar to first-borns in that they tend to be ambitious and performance-oriented, but they are often lonely, independent individualists, or, depending on their situation, rely more on family.

But is there anything about being a firstborn or later-born that affects success and happiness in life?  This is ultimately what matters most for our kids -- and the answer is that birth order has no impact here. The planet needs explainers and extroverts, contemplatives and can-do types, rebels and realists, visionaries and ancillaries.

What one achieves in the world, beyond birth order and standardized exams, is the real test.

 *If you like this blog, click here for previous posts (including this one) and here to read a description of my most recent book, Do Gentlemen Really Prefer Blondes?, on the science behind love, sex, and attraction. If you wish, check out my forthcoming book, Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy. 

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