Sunday, July 31, 2011

Heritability 2.0

Current best estimates of the heritability of g come from twin and adoption studies. The table below, from the recent paper: Molecular Psychiatry (2010) 15, 1112, gives you an idea of the general consistency of results from a number of twins studies with large statistics.

But now that we have inexpensive genotyping, we can study heritability of a quantitative trait by looking at unrelated (or only distantly related) individuals, and asking to what extent similarity in genotype is correlated with similarity in phenotype. A simple way to think about this is to imagine that we have a sample of N people for whom both phenotype (measured g score) and genotype (e.g., SNP profile) are known. Form all possible pairs and plot magnitude of difference in g score against genetic distance between the individuals in the pair. The g score difference should (on average) decrease as the genetic distance goes to zero (at which point the pair are MZ twins; but we avoid the confound of shared prenatal environment). Even if we have no identical twins in the sample, and even if none of the people in the sample are closely related to each other, we can extrapolate to zero genetic distance to obtain an estimate of heritability. An analysis along these lines (more technically, a global fit across all SNPs of total heritability) for height yields a result which is consistent with the narrow sense heritability estimate from twin and adoption studies. The results for g have not yet been published, but rumor has it that they also support earlier estimates such as those given above.

With this new technique one can average over a much larger range of environments. (As we know, heritability is only defined with respect to a specific range of environments.) One can form the sample from individuals who grew up in very deprived or very privileged families. One can even compare cohorts born in very different eras -- as long as DNA samples are available. For example, we can compare people born 80 years ago (i.e., who are still living and whose early adult IQs are known from military or school records) to each other, or even to people born as little as 15 or 20 years ago (assuming raw scores or cross norms are available). This kind of analysis may reveal something interesting about the Flynn Effect.

Update: The paper I referred to is out. It yields a lower bound on narrow sense heritability which is consistent with the earlier twin and adoption studies.

Genome-wide association studies establish that human intelligence is highly heritable and polygenic.

General intelligence is an important human quantitative trait that accounts for much of the variation in diverse cognitive abilities. Individual differences in intelligence are strongly associated with many important life outcomes, including educational and occupational attainments, income, health and lifespan. Data from twin and family studies are consistent with a high heritability of intelligence, but this inference has been controversial. We conducted a genome-wide analysis of 3511 unrelated adults with data on 549,692 single nucleotide polymorphisms (SNPs) and detailed phenotypes on cognitive traits. We estimate that 40% of the variation in crystallized-type intelligence and 51% of the variation in fluid-type intelligence between individuals is accounted for by linkage disequilibrium between genotyped common SNP markers and unknown causal variants. These estimates provide lower bounds for the narrow-sense heritability of the traits. We partitioned genetic variation on individual chromosomes and found that, on average, longer chromosomes explain more variation. Finally, using just SNP data we predicted ∼1% of the variance of crystallized and fluid cognitive phenotypes in an independent sample (P=0.009 and 0.028, respectively). Our results unequivocally confirm that a substantial proportion of individual differences in human intelligence is due to genetic variation, and are consistent with many genes of small effects underlying the additive genetic influences on intelligence.


David Coughlin said...

The Rutgers Gifted Child Clinic called us and said that they opened up some evaluation slots for Sept-Oct.  Stella turned 5 in July so we are getting near the end of the up-to-6 tests.  You have probably mentioned it, but I need it rehashed.  How valid are IQ tests for 5 year olds?

I'm at the point where knowing the number isn't really going to change my high-level plan, but we could use more outlets [events, camps, classes] for her.

steve hsu said...

Not very reliable. I don't have a quick link for you but score at age 5 is not a great predictor of adult score. Since you are interested in this topic you might look for a longitudinal study (book) of Hunter College's gifted K-12 in NYC which admits using IQ scores at the kindergarten level, and again at the HS level. There is tons of regression among those admitted early. The kids admitted later tend to be stronger because the filter is more reliable.

MtMoru said...

"I'm at the point where knowing the number isn't really going to change my high-level plan, but we could use more outlets [events, camps, classes] for her."

"On a scale of 0 to 10, with 0 representing zero possibility and 10 representing complete metaphysical certitude," as John McLaughlin would say, are there enough parents of the David Coughlin type in twin studies to say whether such a type of parent can influence his child's IQ?Or does the David Coughlin type of parent lie on a bell curve of parental involvement?

The answer is: NO 10, NO 10.

MtMoru said...

I wonder if Steve has an embarrassing age 5-13 IQ score?

David Coughlin said...

You are looking in the wrong direction.  It could be 110.  It could be 170.  It doesn't change what we are doing for her. 

She got to spend a week at a drama camp for kids.  She spent two weeks in Spanish immersion for 1st-3rd graders [she would have gone to Chinese immersion if we had that kind of thing around here].  She's a good kid and her teachers like her and think well of her.  We pay the $8/month because she likes to play endgames on  Summer camps are over, bring on the summer camps.

MtMoru said...

"You are looking in the wrong direction."

I don't know what that means. Parents like you have children wit high IQs for two reasons. One, you've passed on high IQ genes and two, you are very involved in your child's intellectual development. Steve will look at twin studies and may say such involvement makes no difference or there is no evidence that such involvement makes a difference, but parents like you are not in twin studies and are so uncommon that no extrapolation can be made from the little difference that ordinary parents make to the difference that parents like you may make. 

"It could be..."

IQ is not an "it".

steve hsu said...

Nope, I was very precocious. I knew all about g and Terman when I was very young and have been waiting a long time for the genetic technology to catch up with the obvious questions. I took my first class at the university at age 12. But the facts are the facts re: regression and predictive power of very early testing.

JLOV said...

Quoting from this paper:

Similarly, Fagan, Holland, and Wheeler (2007) found an 18-year span stability coefficient, corrected for unreliability, of .87 for IQ scores estimated at age 3 and again at age 21.


Using the criterion of academic achievementmeasured at age 21, Fagan et al. (2007) found that IQ measured at age 3 was only slightly less predictive (corrected r=.68) than the concurrent validity coefficient based on IQ measured at age 21 (corrected r=.76). Thus, across an 18 year period from age 3 to age 21, the criterion-related validity of the IQ scores obtained at age 3 degraded by only .08.

p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 8.0px Times}

The Fagan et al. article is this one:

steve hsu said...

What is "corrected for unreliability"?

MtMoru said...

I remember hearing reporter Aaron Brown describe himself as a late bloomer. He was concered that China's college admissions in particular favored early bloomers.
I wonder how many early bloomers fade early like Armin Hary and how many late bloomers win the race like Carl Lewis.

JLOV said...

steve hsu said...

Yes you can correct for that if you have a sample. How does that help the parent of a *single* 5 year old who receives a noisy score on an IQ test?

Steve said...

There are a handful of "highly gifted" programs with Los Angeles public schools for students with, I believe, tested IQs of 142 on up. Some get into the program based on IQ scores in 2nd grade, some in 8th grade. Once you are in, you are in through high school. In high school, it's noticeable that some of the kids who got in on the HG track due to their 2nd grade scores aren't really scary smart, while everybody who got in based on scoring 142+ in 8th grade really are very smart. 

steve hsu said...

Here's what I have from fig 4.7 in Eysenck's Structure and Measurement of Intelligence. This is using data in which the IQ was tested *three times* over the interval listed and the results averaged. A single measurement at age 5 would probably do worse than what is listed below. Unfortunately there are only 61 kids in the study.

age range                 correlation with adult score (approx. value read from graph)

42,48,54 months           .55
5,6,7                            .85
8,9,10                          .87 
11,12,13                       .95
14,15,16                       .95

The results do suggest that g is fixed pretty early and the challenge is actually in the measuring of it as opposed to secular changes that occur as the child grows up. That is consistent with the Fagan et al. paper cited above. But it doesn't remove the uncertainty that a parent has over the eventual IQ of their kid when he/she is only 5 years old.

JLOV said...

Yes, disattenuation of course does not increase the validity of the actual test. However, as you say, intelligence does seem to become relatively "fixed" at a young age, which means that gifted programs for young children do make sense in principle. Eysenck's data suggest that one could significantly increase the reliability of IQ tests for young children by making them take the test several times.

David Coughlin said...

I'm not trying to make her anything.  If I could make her do anything it would be to put her shoes on the right feet.  I'm not even especially worried about what she will be like when she is 13, 18 or 25.  Mostly I want a good estimate of where she is right now.

Blog Archive