Plomin et al's Heritable IQ Study

From Bryan Caplan EconLog "How Family Environment Works"

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. ... ... 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.

Commenter "eric":

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

Commenter "Tim Lundeen":

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.

I am trying to make sense of heritable IQ. 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?

Commenter "Chuck":

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.

Karl Smith via commenter "TGGP":

...I am sticking a flag in the sand and declaring my hypothesis that the genetic component of intelligence is preference. That is, smart people are people who like intellectually stimulating experiences. My guess is that they are more sensitive to the pleasure chemical released when successfully solving a hard problem. Thus they solve more hard problems. ... For those who have trouble seeing how this might work consider this: Obesity has a strong genetic component. Obesity like IQ has been rising over time. However, does anyone believe that obesity is not completely determined by your food consumption and exercise patterns? Genes can modify that function, in particular they can modify your equilibrium levels of food consumption but they cannot act outside of the environmental regime.