Bryan Caplan  

How Family Environment Works

PRINT
My Ideas on Health Care Delive... The Dollar and the Gas Pump...

I finally got around to reading Plomin et al's classic 1997 parent-offspring adoption study. Background: Way back in 1975, Plomin and co-authors launched the Colorado Adoption Project (CAP). They put together a sample of 245 adoptees, their biological mothers, their adoptive parents, plus a control sample of 245 non-adoptive children and their parents. Then they followed the children through life, administering various tests along the way.

If the only result from this study had been the "IQ is heritable," it would have been just another study. But its special methodology - studying adoptee's development from birth to adulthood - confirmed a shocking finding: As children grow up, the heritability of IQ rises, and the influence of family environment on IQ literally vanishes. Here are the custodial parent-child IQ correlations, by age, for adoptees and the biological control group:


Age
Adoptees
Controls
1-2
.13
.09
3-4
.20
.17
7
.07
.17
12
.00
.30
16
.00
.30
We naturally think about the effects of family as cumulative: The longer you're in a family, the deeper the impression. At least for IQ, though, this "natural" thought turns out to be wrong. Family affects the very young, then fades out.

In hindsight, should this pattern really have been so surprising? Yes and no. Consider the parallel case of church attendance.

For a young child, family has near-absolute control over church attendance (unless you're Damien in The Omen, of course!): If your parents go, so do you; if they don't, you don't. As you get older, though, you gain some independence - and with it, a chance to show your true colors. By the time you're an adult, you only go to church if you want to. So it's not surprising that family matters less over time.

Even so, though, you would expect attending church to be at least somewhat habit-forming. Adults only go to church if they want to, but what they want has something to do with what they've experienced. The surprising thing about family influence on IQ is that the effect actually goes to zero. As you grow up, you find your own cognitive level, and the cognitive level you're looking for has nothing to do with the cognitive level you grew up with.


Comments and Sharing





COMMENTS (16 to date)
eric writes:

That study was featured prominently in Judith Rich Harris's book, the Nurture Assumption.

dearieme writes:

I'm not surprised that the effect is pretty small: I am sceptical of all propositions that look like mere wishful thinking. I am surprised that it's nil. But should I believe correlation coefficients that are good to 2 sig figs? 0.00 plus or minus what?

dearieme writes:

Why does the coefficient rise with age for the control group?

aaron writes:

Evolution?

aaron writes:

Leaving the opressive public school environment?

Tim Lundeen writes:

Re: Why does the coefficient rise with age for the control group?

Because the controls are more similar genetically to their parents than the adoptees, and there is a genetic component to IQ. Cognitive ability develops in fits and starts (just as physical ability does); a child may be ahead or behind of the curve at younger ages, so has lower correlation at younger ages.

manuelg writes:

How do we model this?

is heritable IQ best modeled as a:

* _necessary_ to act at a certain level of effectiveness

* a _multiplier_ that increases or decreases effectiveness

* a factor that is best teased out by a _feed-back loop_, where successes are built upon towards larger successes, ad infinitum, and failures lead to larger failures, ad infinitum, based on higher or lower heritable IQ

* is it even clear that heritable IQ is _sufficient_ for a certain level of effectiveness?

(what model can explain some of the confounding problems of IQ, like that IQ scores have risen from generation to generation?)

aaron writes:

Brain development generally happens at a high, generally consistent rate early on. The timing and type, and whether it happens at all, of further development probably varies more later in life.

Chuck writes:

It would be intersting to know if, as IQ migrates to a heriditary value from the family value, is it predominantly increasing or decreasing.

To put it another way, does the family environment cause IQ to predominanty deviate lower or higher than the nominal hereditary value.

I would guess that family environment has a bias to suppress hereditary IQ. My hypothesis is that stress interferes with learning, and that most parents are 'mean' authoritarians. (As a point of distinguishment*, there is a differece between authoritarian and disciplinarian.)

So, the idea is that in the authoritarian home, our IQ is suppressed until we finally leave it, at which time it rises to it's inherited normal value.

I would be surprised to find that it was elevated in childhood and then dropped.

* I'm not positive I invented that word, the preznit might have beat me to it.

TGGP writes:

Karl Smith's explanation is that the genetic portion of IQ is preference. Once you're an adult they can't make you learn what you don't want to.

Steve Sailer writes:

One question is whether there is some advantage to having a higher IQ as a child than as an adult compared to having the same low IQ all along. I suspect there is. For example, the chance of remaining illiterate for life is lower in a good childhood environment.

dearieme writes:

In case our esteemed bloggers don't thank you for your comments, chaps, permit me to do so. Very interesting.

MattJ writes:

I'm pretty sure Damien's birth father dictated his churchgoing habits.

aaron writes:

Oh, I should recommend the book I'm currently reading The Origin of Mind. Highly recommended.

Floccina writes:

John Dewey those results just show that the students do better on tests.

From your link:

Yet thousands of families took advantage of them. One result, according to Rouse's report: The schools that were losing students quickly changed their ways and generally improved on test scores – even though they had lost many of their top students to other schools.

The gains from schooling are mostly relative and short lived. We do not teach enough practical knowledge in schools. Our schools after the 2nd of 3rd grade are just a long test. They do not even pretend to teach much useful practical knowledge, they pretend to teach subjects that raise aptitude (it does not work), in the theory that they prepare the student to be better able to learn what they need to after they leave school.

Studies like discussed here:

http://econlog.econlib.org/archives/2008/06/how_family_envi.html

If the only result from this study had been the "IQ is heritable," it would have been just another study. But its special methodology - studying adoptee's development from birth to adulthood - confirmed a shocking finding: As children grow up, the heritability of IQ rises, and the influence of family environment on IQ literally vanishes. Here are the custodial parent-child IQ correlations, by age, for adoptees and the biological control group:


We naturally think about the effects of family as cumulative: The longer you're in a family, the deeper the impression. At least for IQ, though, this "natural" thought turns out to be wrong. Family affects the very young, then fades out.


In hindsight, should this pattern really have been so surprising? Yes and no. Consider the parallel case of church attendance.


For a young child, family has near-absolute control over church attendance (unless you're Damien in The Omen, of course!): If your parents go, so do you; if they don't, you don't. As you get older, though, you gain some independence - and with it, a chance to show your true colors. By the time you're an adult, you only go to church if you want to. So it's not surprising that family matters less over time.


Even so, though, you would expect attending church to be at least somewhat habit-forming. Adults only go to church if they want to, but what they want has something to do with what they've experienced. The surprising thing about family influence on IQ is that the effect actually goes to zero. As you grow up, you find your own cognitive level, and the cognitive level you're looking for has nothing to do with the cognitive level you grew up with.

and others that study “head start” ect. show that the gains to schooling/early education are fleeting.

I suggest that if we cannot get children to learn more, perhaps we should try to get them to learn more practical and useful stuff. But even this will most likely fail.

Half Sigma writes:

Tests given to young children test a much smaller range of cognitive ability, and thus are much more susceptible to improvement through coaching.

They also have a lower correlation with actual intelligence, so as children grow older, the tests given to them correlate better with their natural genetic IQ and less with their family environment.

Comments for this entry have been closed
Return to top