General intelligence, known as g or IQ, has long been the focus of cognitive genetics research.
But what about more specific cognitive abilities, the building blocks that make up IQ?
A sweeping new meta-analysis of over 740,000 twins surprisingly suggests:
- Specific cognitive abilities (SCA) like reading, math and processing speed are just as heritable as IQ itself, around 50-60% on average. Some SCA are even more heritable.
- Unlike IQ, SCA heritability does not increase with age. Most stay stable or even decrease across lifespan.
- Strikingly, SCA independent of IQ are still substantially heritable, about 50% on average. Genes influence specific abilities separate from overall intelligence.
Source: Intelligence 2022
Studying the Genetics of Specific Cognitive Abilities (SCAs)
Psychologists have proposed that cognitive abilities are hierarchically organized, with general intelligence or g at the top.
Below g are broad domains like verbal, spatial, memory and processing speed abilities.
These “specific cognitive abilities” or SCA are in turn comprised of narrower aptitudes measured by individual tests.
Research has shown that about 50% of IQ differences between people can be attributed to genetic factors.
The rest is due to environment. But how do these numbers break down for SCA?
Do some specific abilities have stronger or weaker genetic influences than others?
And does the heritability of SCA change across the lifespan, like it does for IQ?
To find out, researchers conducted an exhaustive meta-analysis of over 740,000 twin pairs from 77 studies on 11 SCA domains.
The massive dataset allowed them to estimate average heritability of each domain.
It also let them break down SCA genetics by age group, from early childhood to old age.
Surprisingly, the meta-analysis overturned several assumptions about SCA and genetics:
Specific Cognitive Ability Heritability on Par With IQ
Across all SCA domains, genes accounted for 56% of ability differences on average.
Almost identical to IQ’s 50% heritability.
Some SCAs More Heritable Than Others
Heritability varied widely between domains, from 39% for auditory processing to 64% for math ability and processing speed.
Some SCA are significantly more heritable than others.
Surprisingly, acquired knowledge SCA like vocabulary and general knowledge were most heritable at 58%.
Fluid reasoning, considered closer to IQ, was substantially less heritable at just 40%.
SCA Heritability Stable Across Lifespan
Unlike IQ, SCA heritability is mostly flat.
The meta-analysis found no clear rising trend from childhood to adulthood for any domain, countering assumptions.
After early childhood, most SCA heritability hovers around 50-60%.
Some SCA like memory and visual processing showed no age trend at all.
Substantial Heritability Beyond IQ
Most surprisingly, SCA stripped of IQ influence were still about 50% heritable on average.
Genes affect specific abilities separately from overall intelligence.
The findings suggest that genes influence more narrowly defined aptitudes above and beyond general cognitive ability.
There appears to be genetic specificity even within the inherent correlations between mental abilities.
Why are some SCA far more heritable than others?
And how do genes affect specific abilities independently of IQ?
The meta-analysis results pose intriguing questions for cognitive genetics.
Differential Heritability Between SCA
The wide range of heritability differences between SCA domains is puzzling.
Some possibilities researchers identified:
- Reliability – Less reliable tests create lower ceilings for heritability estimates. For example, short-term memory had the lowest heritability but also concerns about reliability.
- Cognitive complexity – More complex abilities may involve more genetic influence. For example, processing speed was far more heritable than reaction time. However, this does not explain why fluid reasoning was less heritable than processing speed despite seeming more cognitively complex.
- Multivariate models – Analyses that model correlations between SCA may clarify their genetic architecture and explain differential heritability.
Specific Cognitive Ability Heritability Independent of IQ
It’s surprising SCA stripped of IQ variance remain highly heritable.
Some hypotheses:
- Phenotypic correction – Regression-based SCA scores remove environmental as well as genetic IQ variance. Cholesky model SCA scores still include total variance.
- Additive genes – If SCA genes have mostly additive effects, they can be picked up in GWAS and polygenic scores. Non-additive gene interactions are harder to detect. Twin study results suggest largely additive SCA gene effects.
Developmental Trends in SCA Heritability
Why does IQ heritability increase with age but SCA heritability does not?
Possible factors:
- Schooling – Early education could reduce environmental variance for school-taught SCA like literacy and math but not IQ. However, untaught SCA also did not show increasing heritability.
- IQ amplification – Since IQ represents shared SCA variance, its developmental amplification may not be visible in individual domains.
- Data limitations – Differences in measures and samples across ages make developmental trends difficult to discern reliably.
Future Directions for Specific-Cognitive Abilities (SCA) Research
The meta-analysis results raise more questions than they answer about the genetics underlying specific cognitive abilities.
Some promising future research directions:
- Expanding SCA data – Fill gaps for untested domains and ages. Use consistent measures across development.
- Multivariate modeling – Model correlations between SCA and extract general factors to clarify genetic architecture.
- SCA GWAS – Conduct large GWAS directly on specific abilities and SCA independent of IQ.
- SCA polygenic scores – Build polygenic predictors of ability profiles based on SCA GWAS results.
- Interventions – Leverage SCA polygenic scores to identify and enhance cognitive strengths independent of IQ.
The genetics of specific cognitive abilities turn out to be more complex than previously assumed.
But new findings are setting the stage for exciting research to further unpack the inherited basis of human mental abilities.
Stay tuned as scientists build on these discoveries – we may one day understand enough to boost abilities across the full spectrum of cognition.
References
- Study: The genetics of specific cognitive abilities
- Authors: Francesca Procopio et al. (2022)
