L.C. sent us this suggestion, mentioning that she has been lording it over her siblings since she read about this study.
A Norwegian study looking at 240,000 teenagers (from military conscript records for males in the 18-19 year old range) found that firstborns are 2.3 I.Q. points smarter, at 103.2, than their next-eldest siblings, who average 101.2, which is higher than third-borns, who come in at 100. Studies support this relative dummying-down in female children as well.
Since I thought the average I.Q. was 100, I'd like to know where they find the children who score below 100 to pull the average down ("Lake Wobegon, where all the women are strong, the men are good looking, and the children are above average.").
There has been some speculation that this I.Q. bump comes about because parents shower their first children with so much attention. This does not seem to be the case, however. Only children, those whom you would think get the most attention, don't score as highly as a firstborn with siblings. I'd expect only children to get even more attention from parents than a first child with subsequent siblings. And a second child who becomes eldest on the death of an older sibling scores higher than the average second child, but not as high as those born into that lofty position.
I wonder if being the first or only child confers the advantages of parental attention, but that subsequent siblings help hone the intelligence and keep the first child challenged in ways an only child can only dream of. I've read that those with the most profound impact on us are our siblings.
One thing not taken into consideration is the possibility of a physiologic cause. I've been reading a lot of Natalie Angier lately, so I have to consider that possibility. I wonder if the age of a mother's eggs could have an impact on intelligence. After all, it's a rather subtle difference. Mothers of families with multiple children obviously have their youngest eggs with the first child, and the ova get subsequently older with each child. Maybe parents with a single child tend to decide to have that child at a slightly advanced age over those having several. I wonder if there is a difference in the I.Q. of a firstborn who is born when his mother is 20, and that of a firstborn whose mother is 35 at the time of birth.
Or maybe the father's age has an effect.
Who knows? But it sure is interesting speculating.
No matter what, I get to remain the eldest sibling.
[Image from TIME magazine. That story is not related to this one.]