Study Links Birth Order, Test Results A new study found that boys who are the eldest siblings in their families score higher on IQ tests than their younger brothers. Dr. Petter Kristensen, an epidemiologist at the University of Oslo in Norway, talks about his study.
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Study Links Birth Order, Test Results

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Study Links Birth Order, Test Results

Study Links Birth Order, Test Results

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  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
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This is DAY TO DAY. I'm Madeleine Brand.


And I'm Anthony Brooks. Coming up, you can get your "Charlie's Angels" fix in five minutes or less. The pros and cons of Sony's new product, shrunken TV shows from the past.

BRAND: But first, every oldest child already knew this. A new study proves it. The oldest kid in the family is the smartest.

BROOKS: Hang on. Hang on, Madeleine. You are an oldest child, I think?

BRAND: That's right. But you know, I'm talking hard science here. The researchers measured the IQs of more than 240,000 men and they found on average a three-point difference in IQs between older and younger brothers. Three points might not seem like a lot, Anthony, to you. But as the New York Times points out - and I've always considered the New York Times to be the oldest sibling among newspapers - the New York Times says that could mean the difference between a high B average and the low A average, and the difference therefore - get this - between going to an elite private liberal arts college and the less exclusive public one.

BROOKS: I'm skeptical.

BRAND: You would be because you're a younger sibling.

BROOKS: Well, no comment, but I am going to say this, point this out, that according to other studies, as an older sibling, Madeleine, you are less adventurous, more conventional. So of course you'd put a lot of stock in that big official study. And then of course you'd seek out an expert to interview about the study, whatever.

(Soundbite of laughter)

BRAND: Yeah, you slacker. Anthony, Anthony, of course you go to the horse's mouth. This case we go to the study's authors; one of the lead authors, Dr. Petter Kristensen, he's an epidemiologist at the University of Oslo in Norway, and I asked him if this difference in IQ is biological or if somehow parents favor older siblings?

Dr. PETTER KRISTENSEN (University of Oslo): We believe it's not biological because we were comparing young men who were in the social sense the first, and in a biological sense the second. And it's the social rank that counts. What is less than would be that it has to be some kind of social interaction within the family that has this effect on birth order and intelligence.

BRAND: When you say social first, biological second, you're talking about men whose older siblings died?

Dr. KRISTENSEN: Yes, that's right.

BRAND: So what is it that could account for this? What kind of social factors?

Dr. KRISTENSEN: It has to do with time and resources of stimulation of parents towards the child.

BRAND: So basically the first one gets kind of the undivided attention of the parents for a while until the second one comes along. And then it's all over.

Dr. KRISTENSEN: It's not all over. But it has to be shared. But you're right about that.

BRAND: IQ tests are notoriously culturally biased.


BRAND: Can you really deduce then just through IQ tests whether the oldest sibling is indeed objectively smarter than the youngest?

Dr. KRISTENSEN: I would not believe this to be a problem comparing siblings or comparing young men approximately at the same time in the same society.

BRAND: There are, of course, famous smart people who were younger siblings - Charles Darwin, Rene Descartes, Bill Gates...


BRAND: What does that say?

Dr. KRISTENSEN: Well, you see, this birth order effect is quite small. But it has probably some impact on the population level. If all other things were equal, a mean difference of two IQ units would have some impact, whereas on the individual level it would not mean almost anything. It would have almost no predictive power in an individual setting.

We looked at more than 60,000 brother pairs, one elder, one younger. And when they had different scores in the IQ test, it would be a 57 percent probability that the best one would be the eldest. But there is still 43 percent probability that the youngest is the better.

So you see, on the individual level it's very dangerous to say that it automatically has to be the eldest that is the smarter.

BRAND: Except in my case.

(Soundbite of laughter)

Dr. KRISTENSEN: Okay. I'm not the eldest myself, so that's why I stress this point.

BRAND: I see.

Dr. KRISTENSEN: My older brother is very pleased with this study, I can tell you. Yeah, yeah.

BRAND: And you only studied men, right? But this would hold true for women as well?

Dr. KRISTENSEN: Well, I have to rely then on the other studies and there seems to be very little difference concerning this birth order effect; it's therefore women as well.

BRAND: Okay, well, thank you very much.

Dr. KRISTENSEN: Okay, thank you.

BRAND: That's Dr. Petter Kristensen, the lead author of a new study that shows higher IQ points for children who were born first.

(Soundbite of music)

BRAND: More smart stuff coming up on DAY TO DAY from NPR News.

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