## Friday, January 23, 2009

Family size, birth order, and IQ: Following Blode's line of thought from the last post, I calculated the vocab means for each category of birth order and family size:

Mean vocabulary test score

Only child 7.11

First of two kids 6.71
First of three kids 6.63
First of four kids 6.09
First of five kids 5.28
All firsts 6.29

Second of two kids 6.82
Second of three kids 6.94
Second of four kids 5.68
Second of five kids 5.87
All seconds 6.47

Third of three kids 6.36
Third of four kids 6.41
Third of five kids 5.14
All thirds 5.91

Fourth of four kids 6.53
Fourth of five kids 5.80
All fourths 6.10

Let me know if you see something else, but the main story here seems to be that smaller family size is correlated with higher IQ. I imagine that much of that is due to smarter parents, but it might also have something to do with resources and time being split among fewer children. There seems to be no intellectual benefit from growing up in a household with lots of people to interact with.

1. Anonymous2:41 PM

"There seems to be no intellectual benefit from growing up in a household with lots of people to interact with."

I imagine the social benefit far outweighs the lack of intellectual benefit.

2. I don't think you can get much on birth-order effects from the GSS. The effect of family size shows up pretty strongly in the GSS - whether using sibs or childs (see here.)

I think you really have to have data within families and compare the IQ's of actual siblings to get any good read on birth-order effects. Maybe if the sample sizes were bigger for the birth orders - this was only asked in 1990.

3. The Undiscovered Jew4:48 PM

"But what about the difference between an only child and a first one?"

It could be due to a number of factors such as higher IQ parents having fewer children. A good study would have to take into account IQ of parents as well as the children's IQ and birth order.

However, we do know that hormones and birth order can have an effect on the brain.

We know for instance that the more older brothers a boy has the more likely he is to be gay and homosexuality is estimated to be 60-70 hormonal in origin. The brains of homosexuals appear to be "feminized" because of hormonal effects in the womb.

4. Thanks for following up on that, R.G. I have found the GSS pretty easy to use, but you're much more aware of it's possibilities than I, since it has SO many survey questions I have yet to discover.

5. Anonymous7:43 PM

My mother was a high school teacher from the early 1960's to the mid-1990's, which means that for more than a decade she often had students from large Baby Boom families. It wasn't uncommon for her to have several siblings one after another. She's often said that it was as if parents had only so many smart genes to go around, which were used up on each child starting with the oldest, so that the siblings would get progressively stupider and stupider :)

Peter

6. "I imagine that much of that is due to smarter parents, but it might also have something to do with resources and time being split among fewer children"

A recent study showed that second-borns have IQs like first-borns, if first-borns die.

7. To the first Anonymous:

I doubt it, since only-children are just as socially adjusted as those with siblings.

8. Intelligence predicts the fate of nations, I've been told, reading this site and others I found via the links on Steve Sailer's blog.

So, what would happen if a large nation embarked on a program to raise a whole generation of only children?

I guess we'll find out in the next 20 years or so.

9. Pyloric stenosis is heavily concentrated among first-born males. So it's pretty clear that there *could* be non-sociological effects of birth order on intelligence.

10. Cleanthes:

China had a one child per family policy in place for a while. I wonder if there is any public data that can be found on it regarding IQ.

11. "So, what would happen if a large nation embarked on a program to raise a whole generation of only children?"

Yes, reductions in family size have made a direct contribution toward the Flynn Effect.

Smaller family sizes = higher national IQ.

12. BlondeMum3:19 PM

Hi smart guys,

googled pyloric stenosis and indeed, it is much more common in firstborn males. How would this be possible? The immediate connection that comes to mind is the link between nutrition and IQ -- the fetus starts feeding off its mum straight away, and perhaps the woman's body is more depleted of nutrients after each birth? After giving birth to one I certainly feel that way...
Keep up the good work guys! Only found out about this blog today and have spent the day reading it. I used to be a liberal feminist studying humanities. Then I figured out a lot of things independently and intuitively, and have only recently found out that the blimming data supports it! Like, high-IQ westerners are getting extinct. (While I'm here, what do you suggest we do? As a high-IQ woman who loves reading and writing and have made a career of it, I would like you to give some practical indications of what to do. Only have one kid so far but am contemplating having a brood of 6, God willing, but hey, if the IQ keeps decreasing by each kid, maybe my contribution would not even help at all?

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