Wednesday, 24 September 2008

Common genetic variants and intelligence

The New York Times has run an article on genetic research by Dr. David Goldstein of Duke University. His main finding is that most human diseases with a genetic basis are not due to common alleles. They are apparently due to rare alleles that have not been eliminated by natural selection. This seems to argue against the common variant theory of disease, i.e., natural selection has caused many modern diseases by favoring genetic variants that keep us going as long as we can reproduce and then let us fall apart once we’re reproductively useless.

Dr. Goldstein has also found that common genetic variants do not explain variation in IQ, at least not among different human populations:

He says he thinks that no significant genetic differences will be found between races because of his belief in the efficiency of natural selection. Just as selection turns out to have pruned away most disease-causing variants, it has also maximized human cognitive capacities because these are so critical to survival. “My best guess is that human intelligence was always a helpful thing in most places and times and we have all been under strong selection to be as bright as we can be,” he said.

This is more than just a guess, however. As part of a project on schizophrenia, Dr. Goldstein has done a genomewide association study on 2,000 volunteers of all races who were put through
cognitive tests. “We have looked at the effect of common variation on cognition, and there is nothing,” Dr. Goldstein said, meaning that he can find no common genetic variants that affect intelligence. His view is that intelligence was developed early in human evolutionary history and was then standardized.

The finding itself is not surprising. The human brain is a complex organ with more than a trillion nerve cells. Clearly, a lot of genes are brain-related. If natural selection has caused one such gene to vary from one human population to the next, the same selection pressure has probably caused others to vary as well. Thus, in the event that human populations differ genetically in cognitive performance, the overall difference should reflect an accumulation of small differences at many gene sites—often too small to measure.

But what about g? Doesn’t g imply that one gene accounts for most genetic variation in intelligence? Perhaps. Alternately, g may correspond to a large number of brain genes that co-vary because they lie next to each other on the genome. In any case, the chances are not good that we will find g by trolling through the common variants we have discovered so far. The genome is a big place. Such a random search would be like looking for a needle in a haystack.

11 comments:

Tod said...

If Dr. Goldstein was correct in believing that (many different) rare varients are causing one illness across a population, it means that each clearly identifiable disease is usually caused by different genes in different people. In that case it is astounding that a disease appears in more or less the same form in most people.

It has long been suggested schizophrenia is not one disease. I expect this is because the medical geneticists familiar with the familial studies have long been saying that there is no evidence for it's genetic causation.

Dr Goldstein bases his conclusion of no significant IQ differences on the testing of volunteers "of all races", but those who volunteer for such testing would tend to be the more intelligent. He needed to add 2000 of the typical non-volunteers to the study to be justifyed in drawing any conclusion.

His negative results may be due to the study being across races. Surely studying the IQ and genetic variation within families would reduce the size of the "haystack".

Peter Frost said...

I suspect that most diseases do not have a genetic basis. They are probably chronic low-grade infections that co-exist with the human organism without triggering an immune response.

This is, after all, what evolutionary theory would predict. It's not in the germ's interest to kill its host. It just wants to use him or her as a milk cow. Eventually, this low-grade infection will harm the host, but by then the germ may no longer be detectable. It may even be gone.

I agree with your second point. He was probably using foreign students at American universities -- hardly an unbiased sample. In any case, you would have to use large samples to obtain any measurable effects. There may be one big gene that accounts for most variation in IQ, but most of the other IQ-related genes probably have very small effects.

Anonymous said...


He says he thinks that no significant genetic differences will be found between races because of his belief in the efficiency of natural selection. Just as selection turns out to have pruned away most disease-causing variants, it has also maximized human cognitive capacities because these are so critical to survival.


I am surprised that someone who understand natural selection could make such a stupid statement.

Different human environments require different survival skills and different amounts of cognitive skills. Cognitive skills are largely required in complex social environments. Until 200 years ago, Aboriginal Australians did not live in socially demanding environments, unlike a great many people in, say, China and Europe.

Natural selection optimizes for the local environment. If cognitive skills are not needed, selection is not going to select for them, given how expensive the brain it.

This is evolution 101. Get real.

Billy Buzz said...

Doesn’t g imply that one gene accounts for most genetic variation in intelligence? Perhaps.

No, not "perhaps". How can seven standard deviations of continuous phenotypic variation, that is almost entirely heritable be the result of one allele? Variation can't be "additive" if there is nothing to add.

Peter Frost said...

Billy Buzz,

Why couldn't it be due to genes that are physically close to each other on the same chromosome?

Billy Buzz said...

You said "one gene", not many genes close together.

Peter Frost said...

Billy Buzz,

My writing style may have confused you. I was putting into words what many people believe when I wrote: "Doesn’t g imply that one gene accounts for most genetic variation in intelligence?"

Personally, I don't know.

Billy Buzz said...

Many people believe that? Who?

And again, it's a logical statement. Correlated intelligence factors "implies" one gene is responsible for intelligence differences by what logic?

(It definitely implies different cognitive abilities like math, memory, and spatial skills have broadly overlapping genetic pathways. But I just don't get how it possibly would suggest the thing you say)

How could seven standard deviations of heritable, strictly additive intelligence score differences all be due to one gene? I don't think that is even possible.

Peter Frost said...

First, I don't believe that a single gene accounts for most variation in IQ. But I've heard that argument made (and the word 'most' should be underlined). A quick search turned up this web article:
http://home.ipoline.com/~hhk/JQ-issue.htm

"Intelligence, being such a complex combination of attributes and cognitive abilities would obviously be related to a large number of different genes. A surprising recent finding was that a single gene could also influence numerous other abilities."

Billy Buzz said...

"But I've heard that argument made (and the word 'most' should be underlined). A quick search turned up this web article:"


Oh, come on! This was covered in my last comment. There is a difference between pleiotropy (one gene having multiple effects in the same organism) and one gene accounting for the entirety of population variation in a continuous trait.

The former IS very strongly implied by g, the latter is completely incoherent. The latter requires a large number of different alleles by definition.

Differences can't be explained by the same thing: by definition they must be explained by different things.

Peter Frost said...

Billy Buzz,

"one gene accounting for the entirety of population variation in a continuous trait."

This seems to be the source of our misunderstanding. The hypothesis is that one gene may account for most genetic variation in performance on IQ tests. There are three things to keep in mind:

1. There are still many other IQ-linked genes in the picture.

2. Around 40% of variation in IQ is non-genetic.

3. This gene may not exhibit a simple dominant/recessive mode of expression.

So you could still get a smooth normal distribution of IQ even though most of the genetic variation is being generated by one gene.

Having said all of that, I am skeptical. We may be looking at several genes that are close to each other on the same chromosome. Or the single-gene hypothesis may be wrong.

On a final note, while I disagree with this hypothesis, I don't try to misrepresent it.