The weaknesses of Kristensen and Bjerkedal's new birth-order study.

Snapshots of life at home.
June 26 2007 5:30 PM

Out of Order

Are firstborns really smarter than their siblings?

Emily Bazelon was online June 28 to discuss this story. Read the transcript.

Illustration by Robert Neubecker. Click image to expand.

Last year, I dismissed the relationship between birth order and intelligence, relying on such experts as University of Oklahoma psychology professor Joseph Lee Rodgers, who called the finding a "methodological illusion." While that view was not the consensus, it was far better supported than the bedeviling claim that older siblings have higher IQs. Now there's a new study from Norway, reported in two parts in Science and Intelligence, that makes the illusion seem real. According to the New York Times and the Boston Globe, and just about all the other press coverage, the Norwegian research does more than that. It settles the question: Firstborns are smarter.

I hate this idea. It's a vehicle only for resentment and envy, seemingly designed to pit siblings against one another and, inevitably, increase the drip drip of parental guilt. You can't control which of your kids is born when, and now you have to feel bad about it. Nor do I take comfort in the suggestion that you can boost your younger kids' IQs by giving them opportunities outside the family to "tutor" other children, as urged by Stanford University psychology professor Robert Zajonc. The kind of tutoring that older siblings do—and that Zajonc thinks lifts their own IQ more than that of their tutees—isn't about working through math worksheets and swimming strokes. What big brothers and sisters have to teach, if that's the right word, is how to win the camp burping contest or wheedle more dessert. I'm not sure you can stage that sort of, um, instruction.


So, is the study legit? Its authors, Petter Kristensen and Tor Bjerkedal, had a large sample size—more than 60,000 pairs of Norwegian brothers. Their work seems careful, and they don't pump up their findings with speculative conclusions. Still, this is one study among many, and the results were colored by the decision of Science and the newspapers to turn to Frank Sulloway for expert commentary. As the author of Born To Rebel, he's a longtime believer in the importance of who's born when: He told the press that the Norwegian study "should put an end to an unnecessarily heated debate over intelligence and birth order." But why should one study have that kind of power, when other well-done research has come up with the opposite result? The Norwegian research, for all its strengths, has some weaknesses, too.

The report in Science relies on a clever comparison to prove its key point: that the average 3-point IQ difference between firstborn and second-born brothers comes from the boys' varying "social rank" in the family, not differing biology. Kristensen and Bjerkedal looked at second- and third-born brothers who had an older sibling (male or female) who died in infancy. They found that second-borns who grew up as the oldest child in the family, because of a sibling death, had average IQ scores equivalent to firstborns. And third-borns who moved into second place in the family had average IQ scores like second-borns (one point higher). This is supposed to show definitively that family environment and expectations account for the intelligence boost.

The Science article doesn't tell us how many brothers it counted whose siblings died, so we don't know the size of this subset. (Psychology writer  Judith Rich Harris mentioned this to me; I e-mailed Kristensen to ask about it, but haven't heard back from him.) Still, it's an inventive comparison. Psychology professor Joe Rodgers gives the study points for cleverness and novelty. He notes, however, that this comparison is necessarily cross-sectional: It looks at brothers from family to family, as opposed to pairs of brothers within the same family. Rodgers has argued for years that between-family studies of birth order, as opposed to within-family studies, just aren't valid. There are too many differences between families to account for, he thinks. Not just the obvious ones, such as class, education level, and maternal age, which the Norwegian study controlled for, but subtler ones that can't adequately be captured.



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