Unit 6: Intelligence and General Cognitive Ability
Unit 6: Intelligence and General Cognitive Ability
“What Is intelligence?… Twin, Adoption Studies, and The Heritability of General Cognitive Ability… Heritability and The Malleability of General Cognitive Ability… Gene-Environment Interplay In Studies of General Cognitive Ability… Why is General Cognitive Ability Heritable? – UPDATED… Genetics of Intellectual Disability – UPDATED… Supplemental – The Replication Crisis…”
Module B: Twin, Adoption Studies, and The Heritability of General Cognitive Ability
Module C: Heritability and The Malleability of General Cognitive Ability
Module D: Gene-Environment Interplay In Studies of General Cognitive Ability
Module E: Why is General Cognitive Ability Heritable? - UPDATED
Module F: Genetics of Intellectual Disability - UPDATED
Module G: Supplemental - The Replication Crisis
Module A: What Is Intelligence?
Likely to be more behavioral genetic research on intelligence than any other phenotype around.
A lot of interest in intelligence partly because it is a very important contributor, not determiner, to important outcomes in life.
Three key questions:
How do we define intelligence?
Is intelligence unitary or are there multiple aspects to intelligence?
Can intelligence be measured?
Defining intelligence:
Every 20 years, the American Psychological Association puts out a definition of intelligence. The last one in 1996 was defined as the ability to understand and use complex ideas, learn from experience etc.
One problem: it does not define intelligence but what intelligent individuals do!
Most scientists define intelligence as a theoretical construct (to be fair, not unlike the idea of Gravity, and Gene).
Unitary or Multiple:
Four ability tests: verbal ability, numerical ability, spatial test, and a mechanical test.
People who do well (or poorly) on one tend to do well (or poorly) on the other.
For some, the positive correlations suggest general intelligence or general cognitive ability.
For others, that the correlations are not perfect suggest distinct abilities.
Most behavioral genetics research is on general cognitive ability.
Hence, one number cannot summarize all of a person’s talent but it does tell us something important.
Measured:
IQ tests were developed to predict academic success – sampled skills across intellectual domains and the scores were summarized as a single average score.
IQ tests measure general cognitive ability.
Normally distributed:
Mean of 100 (2/3 of population fall between 85 and 115)
~ 2.3% above 130 – gifted
~ 2.5% below 70 – intellectually disabled
Interestingly we can see it is not a perfect bell curve – there are slightly more people in the lower tail (below 70) than upper tail (above 130).
Reiterate: IQ score does not summarize a person’s talents and is not the most important thing but it is an important aspect.
Module B: Twin, Adoption Studies, and The Heritability of General Cognitive Ability
Genetic influences are strong e.g. General Cognitive Ability (GCA) correlations for MZ twins are 0.86, DZ 0.6, full siblings 0.47, parent-offspring 0.42, half siblings 0.31, and cousins 0.15.
GCA correlations for reared apart MZ twins are also in the ~0.70 range, and confirm genetic influences.
At the same time, GCA correlations for reared together relatives are higher than reared apart, suggesting strong shared environment factors (further confirmed in studies of non-genetically related individuals reared-together).
Some studies show for example that heritability/genetics is ~50%, shared environment about 39% and non-shared environment about 14%.
What is significant is that the shared environment (such as the common family environment) seems to have a higher influence than non-shared, which is unlike many psychological traits.
Module C: Heritability and The Malleability of General Cognitive Ability
Flynn Effect: average IQs have increased ~3-5 points per decade, over successive cohorts.
Like height (see earlier units and modules), the trait is highly heritable but has increased over the decades.
Some of the increase is likely to be because we have become better at taking tests, but more importantly it is also likely due to a higher level of collective intelligence due to better public health, better education, more education etc
Important to keep in mind that heritability and malleability are not always linked.
Next module: Gene Environment interaction in general cognitive ability.
Module D: Gene-Environment Interplay In Studies of General Cognitive Ability
Shared environment accounts for 35% of individual differences in general cognitive ability.
(The non-shared environment component accounts for 15% of individual differences in general cognitive ability, but about 10% of individual differences or variants in general cognitive ability is really just a measurement error).
Adoption studies have helped us to understand the influence of the shared family environment:
Adoption children have higher general cognitive ability than non-adopted siblings (because it usually means moving to a better rearing environment);
The difference is greatest when they are in socioeconomically advantaged homes (IQs are improved 10 years after adoption);
Malleability – the social class of adoptive and birth parents matter.
Heritability – 50% of variance due to genetic factors but heritability is diminished in poor and working class homes; advantaged homes are able to create more opportunities
Another study looks at reading ability – the genetic effect is higher when teacher quality/effectiveness is higher because they are creating opportunities for their students.
Module E: Why is General Cognitive Ability Heritable? – UPDATED
General Cognitive Ability is heritable because of:
Neurogenetics;
Specific genetic factors
Neurogenetics
DNA does not code for brain directly but codes for proteins that would be expressed as intervening brain phenotypes (e.g. brain structure and/or brain processes).
Three criteria to determine the important brain phenotypes:
Heritable e.g. total brain volume;
Correlated with general cognitive ability e.g. total brain volume does not have overwhelming correlation but is statistically significant;
Correlation with general cognitive ability due to shared genetic factors e.g. gray matter in total brain volume.
Specific genetic factors
Candidate gene approach has not been successful;
GWAS of general cognitive ability did not uncover any significant links (could be the size was too small);
But GWAS of related phenotypes (obtained a college degree, and number of years of education) found three significant effects but each effect is small.
Most of heritability still missing and we might find more if we have even large sample sizes.
