Bryan Caplan points to a study showing that the cognitive effects of upbringing evaporate over time.
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.
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.
This is pretty well replicated for both home environment and early childhood education. There are lots of things that early childhood environment does affect: the Perry Preschool Project, for example, produced significant reductions in criminality, while improving high school graduation rates and modestly increasing future income. But we're talking about moving from Popeye's to a steady job in a warehouse or at the Post Office, not mass movement into the professional class.
I confess I still don't understand it--if IQ is so heritable, why is it ever plastic? But I'd say that dealing with this problem is the biggest social policy problem America has, whether we're dealing with true heritability or some masking factor.
I certainly hope that dealing with something that we barely understand, and haven't the faintest idea of how to change, isn't our biggest social policy problem. And how could it be, since we are talking about something that, so far as we know, has existed forever and ever.
I confess I still don't understand it--if IQ is so heritable, why is it ever plastic?
Very insightful question.
You might not be able to raise IQ in the young years, but you can certainly lower it.
But we're talking about moving from Popeye's to a steady job in a warehouse or at the Post Office, not mass movement into the professional class.
Any chance you could be slightly more condescending next time you post on this or a related topic? I'm just curious to find out if you can take your obviously highly tuned patronizing smugness to "11."
For that matter, how did they determine that the adult IQ was the 'real" one?
It also occurs to me that, assuming the study is correct, a child whose IQ rises over time would therefore have been having his IQ reduced by his home environment. Right? Which, if he is raised by his genetic parents, is a bit counter-intuitive. (The scenario occurs to me from having my own measured IQ rise about 40 points over the course of my first 18 years.)
Perhaps it would be easier to just conclude that IQ is a worthless metric. And probably more correct, too.
A possible analogy to consider is physical fitness -- aerobic endurance, let's say, which is both highly heritable and highly trainable. We all know some people are fitter than others, and it's definitely the case that some are born innately fitter: if you compare the aerobic capacity of a large, completely sedentary population, you'll see some that have very poor endurance and some that are quite fit even without exercising.
Training (environment) improves aerobic fitness, but the interesting thing is that it turns out it doesn't improve it for everyone: if you take five hundred sedentary people and put them all through an exercise regimen, some will become a little bit fitter, some will become a lot fitter, and some won't ever get any fitter no matter how much they train. So, physical fitness is trainable, but the degree of trainability is itself inherited.
There's no similar study for IQ as yet that I know of, but it's possible a similar genetic mechanism may be at work. Genes may control not only how smart you are to begin with, but also how much IQ responds to family environment.
Training (environment) improves aerobic fitness, but the interesting thing is that it turns out it doesn't improve it for everyone: if you take five hundred sedentary people and put them all through an exercise regimen, some will become a little bit fitter, some will become a lot fitter, and some won't ever get any fitter no matter how much they train. So, physical fitness is trainable, but the degree of trainability is itself inherited.
There's no similar study for IQ as yet that I know of, but it's possible a similar genetic mechanism may be at work. Genes may control not only how smart you are to begin with, but also how much IQ responds to family environment.
If expressed intelligence is 75% inheritance and 25% environment, that would seem to me to be a pretty high level of heritability. But that 25% might put someone with a fairly average inherited dollop of intelligence anywhere from very very dim to brilliant.
Maybe it is useful to compare to the idea of set points for weight - your actual weight can be pushed one way or the other off your underlying set point by environment (e.g.: the meat lover's pizza, subsistence farming), but without unusual influences your weight will tend to go back to your set point.
The set point theory of weight includes the notion that prolonged weight gain or loss away from your set point has the effect of shifting the set point. As I recall, that was a fairly revolutionary idea for weight - but it is exactly the idea that many people intuitively have of intelligence: you may be born with a certain native potential, but a good or bad childhood environment can significantly and permanently change your expressed intelligence.
Essentially, the study suggests that (i) people have an inherited intelligence "set point" that doesn't permanently shift in response to environment, (ii) the only environment that may make expressed intelligence differ from inherent intelligence is the environment you are in now, and (iii) as we get older, we create an (intellectual) environment for ourselves based on inherent traits more than learned habits from childhood. And if intelligence has a strong inherent component, that last doesn't seem that surprising, since most of us prefer not to be either very strained or bored, once we have something to say about it.
We should be very careful about drawing conclusions from the Perry Preschool Project. It was a small scale research project (N = 48). No one has ever been able to replicate its results. The developers of the project were also the researchers. The developers designed a similar K-3 program that failed miserably when tested on a large scale by independent researchers. methodolocial abnormalities have been caught by independent reviewers for both the PPP research and other related research by the same researchers.
If you look at Fig 2 in the paper you see that the divergence between the control occurs between 4 and 12 years. Parents are still a primary determinant of environment during this period and what is not determined by parents is determined by the schools, so it's unlikely to be due to an increase in personal autonomy.
One alternative explaination is that the part of development influenced by family environment is a cumulative process with saturation - environmental factors influence the rate of development, but everybody eventually makes it to their end-state. The explaination that the environment becomes much more homogenous when children start attending school is tempting, but the increased correlation between biological parents and children, since if environmental factors were still important, we'd expect both biological and adoptive parent correlations to decrease.
If IQ becomes "less trainable" with age, does it necessarily follow that it becomes "more heritable"? Are these really the only options?
Let me refer you to Judith Rich Harris' "The Nurture Assumption" which goes into this and the rest of the nature/nurture controversy. The mentioned study reinforces her conclusion that parenting is massively overblamed/overpraised in just about every domain. Her then-stunning but well-supported conclusion was that parenting has very little effect on the life outcomes of the children.
The numbers I've seen on intelligence heritability are in the vicinity of 50% without high precision, but parenting has very little to do with the other 50%.
IQ is all very well and good, but there are other aspects that are desirable in a citizen, like hard working, rationality, politeness, etc.
I'm guessing that work habits, reasoning and manners are all effected by the family you are raised in. Are these worth encouraging/nurturing?
It doesn't seem so hard for me to understand - I knew a kid in high school whose dad was a (HS) basketball coach. This kid was short, not very athletic or talented, but knew everything about basketball strategy, had picture-perfect shooting technique, did everything by the book. So when he was in junior high, and younger, he was always among the best players on the team. But as he got older, other kids that were taller, stronger, faster caught up mentally with him and he eventually became a backup and then not really good enough to compete anymore.
I could see something similar happening with intelligence - having a parents that can "coach" you intellectually can put you ahead of your peers for a while, but eventually your peers catch up and their natural ability takes them further.
Scores on TESTS are plastic, underlying intelligence is not.
For example, the types of educational toys that yuppie parents give to their children mimic the types of questions on IQ tests given to young children, so the children are being coached for the tests, but this doesn't make them more intelligent.