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Sibling rank and IQ

I wrote last summer about a Norwegian study on birth order that was being treated as definitive proof that first-born kids have higher IQs than their siblings--and that the IQ edge is due to social rank, not biology. (Actually, the results are about brothers only, since the study was confined to boys, but much of the coverage generalized to girls, too.) At the time, I asked the authors about some numbers that were missing in the paper. I recently heard back from one of them, Petter Kristensen. He charmingly said that the omission "is embarrassing, but I have no one to blame but myself" and sent along a file with the figures.

I sent the numbers to psychology writer Judith Rich Harris, author of No Two Alike. She points out that the numbers Kristensen sent weaken the claim that social rank explains the IQ difference, if it exists. Here's the context from my piece last summer:

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.

It turns out that the number of third-borns in the study is only 81, and that the data point is shaky because there's a wide confidence interval, which means that the conclusion drawn from the data is relatively unreliable. Harris also points out that Kristensen and his co-author controlled for birthweight, which is a mistake if whatever causes younger brothers to have a slightly lower IQ (if they do) also causes them to be smaller at birth. That's what a biological explanation might show (scroll down).