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April 11, 2018

The “g” in genes

Filed under: behavioral-science,Genetics,Psychology,science — Razib Khan @ 1:12 pm
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Intelligence, or smarts, is once of those words which has many meanings. That’s why we say “street smart” or “book smart.” When psychologists speak of intelligence, however, they are usually referring to something more precise and specific. The image above is a sample of a question from the Raven’s Progressive Matrices test, which is “used in measuring abstract reasoning and regarded as a good non-verbal estimate of fluid intelligence.”

Fluid intelligence “is the capacity to reason and solve novel problems, independent of any knowledge from the past.”

When I was an undergraduate student, my physics professor would often assign problems on the exam which had no explicit corollaries in the problem sets or lecture. During one after-exam review session, one student brought this issue up, and the professor simply offered that given what we learned in the course, we should be able to “derive a method to solve a general class of problem.” I suspect from the distribution of scores that more than half the time a typical student couldn’t derive a method in the allotted period. I know this was often the case with me.

In relation to tests which measure one’s analytic skills or cognitive tasks like memory recall, researchers have found that outcomes are positively correlated. If you do well on one test of this sort, you tend to do well on another such test.

The variable which summarizes these correlations is termed the “general intelligence factor,” often just shortened to g.

When it comes to intelligence, this is what psychologists are really interested in — not the outcome on one specific test. General intelligence is the most distilled and reduced aspect of “book smarts” that psychologists have been able to construct.

So what good is it? More than half of the variation in academic achievement is predicted by variation in g. People in higher status and higher paying jobs tend to have higher general intelligence. And higher g also correlates with a longer lifespan. Because of these correlations it is no surprise that intelligence testing was originally used to identify children who were not performing as well as their peers, and see if they might benefit from special attention.

Not only does general intelligence correlate with many things in one’s life, there is also a correlation between parents and offspring. The most recent work suggests that about 50% of the variation in general intelligence in the population can be accounted for by variation of genes. That is, intelligence is 50% heritable.

Multivariate Gaussian distribution

The implication here is that though parents and children, or siblings, may exhibit a correlation, it is imperfect. The brilliant mathematician Carl Friedrich Gauss came from unexceptional parents, and his numerous descendants are not particularly exceptional. In contrast, the Bernoulli family were a literal mathematical dynasty.

For a complex trait which exhibits a distribution, there are many variables at play, and genes are just one of them. Because so many genes seem to control behavioral and cognitive traits, such as intelligence, until recently, we couldn’t pinpoint any specific region of the genome which impacted variation on these characteristics within the normal range.

With modern genomic methods, which survey variation across the whole genome across huge numbers of people, this is changing. For example, a new paper establishes links to variation in intelligence at over 500 genes! This is still a small number in the grand scheme of things, but whereas five years ago we didn’t know any genes associated with intelligence, today we know hundreds.

A “chip” which asseses thousands of genetic markers

Though indirect methods, such as comparing correlations with and across families, allow us to arrive at a 50% proportion for what is heritable in intelligence, known genomic variation only accounts for a few percent of this heritable component as of this writing. But within the year, it seems likely that the 10% value barrier will be broken, and eventually we may know most of the genetic positions that account for the heritable component of intelligence within human populations.

Then the full story can begin to be told, because once we start to establish the boundaries of the genetic basis of intelligence, we can explore the environmental territory — which accounts for the other 50% our intelligence.

Explore your Regional Ancestry story today.

The “g” in genes was originally published in Insitome on Medium, where people are continuing the conversation by highlighting and responding to this story.

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