Beyond General Intelligence: The Genetics of Specific Cognitive Abilities
What your genes say about how you think
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Cambridge University Press has just finalized deals for French and Spanish translations of my book The Ape That Understood the Universe. Coming soon to a French or Spanish bookstore near you!
This Week’s Essay: Beyond General Intelligence
When it comes to intelligence, psychologists have a favorite letter: g. Short for “general intelligence,” g is the mental common denominator of all cognitive tasks. More than a century ago, psychologist Charles Spearman noticed that people who excel at one kind of mental task tend to excel at others: Those who ace math tests, for instance, also tend to do better at solving puzzles, remembering lists, or making sense of prose. This led him to suggest that all these mental feats draw on a single, general-purpose cognitive ability, which he labeled g.
As Spearman himself realized, however, g isn’t the whole story. Alongside this broad intellectual capacity, there are various specific cognitive abilities: things like numerical ability, language ability, spatial ability, and the like. Two people with the same overall g can have very different specific strengths and weaknesses. To understand people’s performance on cognitive tests, we need to factor in both g and the specific abilities. g explains why people’s scores on different tests tend to rise and fall together; the specific abilities explain why the correlations are far from perfect.
The Nature and Nurture of Intelligence
If there’s one thing everybody knows about intelligence, it’s that it’s not distributed equally: Some people are luckier than others in their cognitive endowment. This raises one of the oldest and most important questions in psychology: Where do the differences come from? To what extent are they due to nature, and to what extent to nurture?
For g, we’ve got a clear answer. We know that, averaging across the lifespan, the heritability of g is around 50%, meaning that roughly half the variation among individuals in general intelligence is due to genetic differences. And we also know that, bizarrely enough, the heritability of g increases with age: It’s around 20% heritable in infancy, 40% in childhood, and 60% in adulthood.
What about specific cognitive abilities? Here, the picture is hazier. For many decades, these abilities have been cast as side characters in the story of intelligence, overshadowed by g’s starring role. But this might be about to change. A massive new meta-analysis by Francesca Procopio and colleagues suggests that the specific abilities deserve a larger share of the spotlight.
A Mega Meta-Analysis
Pooling data from more than 747,000 twin comparisons across 77 studies, Procopio and her team examined 11 specific cognitive abilities drawn from the Cattell-Horn-Carroll model of intelligence: quantitative knowledge, processing speed, reading and writing, and more.
They were interested in five main questions:
How heritable are the specific abilities overall?
Are some abilities more heritable than others?
Do genetic effects on the specific abilities add up in a simple, linear way, or do they involve exotic interactions among genes?
Are specific abilities heritable over and above the heritability of g?
Does the heritability of the specific abilities rise with age as it does for g?
The answers were fascinating - and in several cases, the reverse of what Procopio and colleagues had expected.