It was with great delight that Plomin got his hands on microarrays that could detect 500,000 genetic markers--hundreds of times more than he had previously used. He and his colleagues got cheek swabs from 7,000 children, isolated their DNA, and ran it through the microarrays. And once more the results were disappointing.I had blogged about this study when it came out. I repeat my comments from 2006 which are still valid today:
“I’m not willing to say that we have found genes for intelligence,” Plomin declares, “because there have been so many false positives. They’re such small effects that you’re going to have to replicate them in many studies to feel very confident about them.”
It appears that the hunt for genes affecting intelligence is not going well. I can't say that I'm surprised, because I have always maintained that intelligence is an emergent property of a set of co-operating genes during development in a particular environment and I don't anticipate that the geno-centric approach will take us closer to understanding it.I have expressed the same view in the recent entry on genome-wide association studies:
Intelligence, and -I believe- other complex traits are like complex dishes with many ingredients. The ingredients themselves (e.g., salt, lettuce, or chicken) are themselves unremarkable, but it is the way that they are put together and turned on and off by internal and external stimuli (the pot, the temperature, time, etc.) that makes a good dish.
This Lego-block paradigm is based on the notion that most of our alleles are commodity "building blocks"; if they are brought together harmoneously, they produce positive results. The occasional allele may have a large effect, and some alleles fit better together than others. Yet, most of the success or failure of a construction depends on how the components fit together, and not what they are.From the Carl Zimmer article:
Researchers have made images of their developing brains once a year, and Shaw has focused much of his attention on what the pictures reveal about the growth of the cortex, the outer rind of the brain where the most sophisticated information processing takes place.
In all children the cortex gets thicker as new neurons grow and produce new branches. Then the cortex thins out as branches are pruned. But in some parts of the cortex, Shaw found, development took a different course in children with different levels of intelligence. “The superclever kids started off very thin,” Shaw says. “They got really relatively thicker, but in adolescence they got thinner again very quickly.”
I had blogged about this study in 2006; check out that blog entry to see the thickness curves of cortex in development.
At the dawn of the genetics era, physical anthropologists' ideas that intelligence was correlated with the brain's observable properties were often ridiculed. And, yet neuronatomical correlates are pretty much the only game in town when it comes to giving a prediction (admittedly a very coarse one) of a person's IQ
That doesn't mean that genes don't play a role in intelligence; they do, and it's a sizeable one. But that role is hidden in a gene-gene and gene-environment interaction web of thousands of factors, where the individual components aren't really important, but the way they are put together are.
This realization also leads one to question genetic fetishists' conclusions about environmental influences on IQ.
It is true that scientists have looked at a lot of possible environmental influences on IQ and have come up short on significant environmental factors that can boost a person's IQ. There is simply very limited evidence that any particular environment can achieve this --sort of really bad influences such as malnutrition or some infectious diseases in childhood. And, yet we know that part of the variation of IQ is due to environmental influences. What gives?
What scientists have looked at are recognizable, "obvious", environmental influences (parenting style, schooling, etc.), which are analogous to the "common variants" in genetics.
Just as a microarray-based genome-wide association study has no clue about the rare family-level gene complexes and disease factors, so studies of environmental influences have no clue about the rare family/school/peer group micro-environments affecting a person's development.
Thus, the failure to find strong environmental influences on IQ doesn't strengthen the nature side of the nature-nurture divide, just as the failure to find strong genetic influences on IQ doesn't strengthen the nurture side.
The truth is, that Intelligence is an emergent property of a complex web of genetic and non-genetic interactions.
A human being is like a black box with zillions of inputs, some of them genetic, others environmental. We know that the box's output, e.g. its IQ score on a test is related to its inputs; but the relationship isn't linear and tidy: you can try different inputs from here to eternity, but you won't be able to figure out what the output is.
As I wrote in my post on height and body mass index, real progress will come about only when we finally look into the box:
Real progress will only come about with more developmental and functional studies, i.e. studies that actually look at what genes do in the body.
Figuring out how humans "work" is easier said than done. But, I believe, there is no shortcut.