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Which genes are responsible for intelligence?

Over the past decade, there have been several studies suggesting potential gene variants that may be linked to IQ. 

Where, oh where, does intelligence come from?

Genes seem a sure bet--or at least a significant player in a pool of important factors.  After all, scientific studies have long suggested that intelligence, as measured by intelligence quotient (IQ) tests, is heritable.  If one of your parents has a high IQ, you are more likely to have a high IQ.  If you are a member of a pair of identical twins, you are more likely to have matching IQ scores than your fraternal counterparts.  Your IQ is also more likely to be on par with your siblings than your cousins.  IQ, like height or eye color, seems to pass through the ancestral bloodlines. Thus, there must be a few genes helping propagate those smarts.

Over the past decade, there have been several studies suggesting potential gene variants that may be linked to IQ.  Specific gene variants of APOE, a gene associated with Alzheimer's disease, seemed likely candidates.  Single nucleotide polymorphisms (SNPs) from the brain-derived neurotrophic factor (BDNF), a cholinergic receptor (CHRM2) and cathepsin D (CTSD), a gene linked to schizophrenia, also showed some promise.  All in all, about an even dozen different SNPs have popped up over the years as being significantly correlated with IQ scores.  But a new study, led by Chris Chabris and David Laibson, suggests that none of these are linked to intelligence when you have a statistically powerful sample size.  The group used three independent data sets with thousands of participants--and none of the findings were replicated.

So why look for intelligence genes at all? Chabris, a psychological scientist at Union College, believes that they can help identify interesting molecular pathways and neural circuits in the brain.

"I don't view finding genes as an end in itself.  It's not that interesting to have a list of random letter and number acronyms listed next to some trait.  That doesn't add much to our knowledge," he says.  "But once we know something about the gene, then we can connect intelligence at the psychological level to biology in a new and potentially more interesting way."

He argues that by identifying the genes associated with a trait like intelligence, scientists can then ask probing questions:  Which biological systems or pathways are those genes involved in?  Where in the brain are their proteins produced?  What is the evolutionary history of those genes?  It can offer scientists, he maintains, a whole new way of understanding the biology of the mind.

Which is why it was a disappointment--and a bit of a surprise--that none of the previously identified genes could be linked to IQ scores. 

Chabris is quick to point out that the results of his study do not mean that the previous studies were flawed:  rather, intelligence is probably a lot like height and may involve a variety of different genes.

"The way height seems to work--why some people are taller and shorter than others within a normal range--seems to be the additive effect of hundreds or even more individual genes, each of which just add or take away that one millimeter," he says.  "There's probably something similar at work in intelligence and other psychological traits."

So what will it take to further our genetic understanding of intelligence?

Chabris' answer is fairly simple:  larger sample sizes.

"Social science traits are complex.  There is no single gene causation like you see in something like Huntington's disease," he says.  "So there are probably multiple or even hundreds of genes at play.  We need to increase our sample sizes by a large degree of magnitude, 100,000 subjects or even more, so we can find the genes that only have these small effects.  Once we nail those down, then we can look at what I think is the coolest question, which is how can we further understand how the brain works by looking at these genes at the molecular level."

He concedes it's a daunting task.  "It's probably going to be a lot more complicated than we hoped.  If it's a thousand genes, well, that's quite a lot to wrap your mind around.  But the field as a whole is probably capable of figuring it out."

What do you think?  What role do genes play in intelligence?

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