Unit 6: Intelligence and General Cognitive Ability

MOOC Summaries - Introduction to Human Behavioral Genetics - 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…”
(Source)

Summaries

  • Module A: What Is Intelligence?
  • 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.
Chop Chop MOOCs’ summary of https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=67

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.
Chop Chop MOOCs’ summary of https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=69

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.
Chop Chop MOOCs’ summary of https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=87

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.
  • Heritability is not constant!
Chop Chop MOOCs’ summary of https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=89

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.
Chop Chop MOOCs’ summary of  https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=163

Module F: Genetics of Intellectual Disability – UPDATED

  • Definition of intellectual disability: low cognitive function, low IQ score
  • Intellectual disability is associated with IQs less than 70
  • Most people that have an intellectual disability have a mild form of it.
  • Intellectual disability is an X chromosome-linked disease.
  • In general, males are at higher risk for nearly all neurodevelopmental disorders.
Chop Chop MOOCs’ summary of  https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=159

Module G: Supplemental – The Replication Crisis

  • Sometimes research studies generate false-positive results, and this is not peculiar to genetics or psychology.
  • Factors that make research susceptible to false-positive findings:
    • Samples are small;
    • Effect sizes are small;
    • Large number of potential relations;
    • Flexibility in design, definitions and outcomes;
    • Many research teams competing;
    • Potential financial or political interests.
Chop Chop MOOCs’ summary of  https://class.coursera.org/behavioralgenetics-002/lecture/view?lecture_id=161

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