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Take a Younger Sibling to Play Day

I've written before about the effect of birth order on intelligence. It's not my favorite topic, because it pits older siblings against younger siblings and inevitably makes parents feel guilty. Here's a new study from Brigham Young University economics professor Joseph Price that offers a possible explanation for the IQ edge that firstborns supposedly have, on average, in addition to higher earnings and educational attainment. The central finding is that "first-born children get about 3,000 more hours of quality time with their parents between ages 4 and 13 than the next sibling gets when they pass through the same age range." More inequity. More guilt. Parents spend time evenhandedly on any given day. But, the study found, parents spend less time with children daily as families grow older. "First-born children get more quality time simply because they pass through childhood when there is more overall family time to be shared." What's more, the time that younger siblings do spend with their parents more often involves TV. Lucky them. I guess the good news is that more time with parents is good for kids' brains.

More on Sibling Rank and IQ

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).