Genetic and environmental basis of intelligence Flashcards
What are inherited characteristics? (examples)
Controlled by biological structures (genes) passed to us by biological parents
eg:
- blood type
- hair colour
- eye colour
- intelligence(?)
What is a genotype?
genetic component, coded in DNA inherited from parents
What is a phenotype?
outward manifestations of the individual
What is heritability? (ref to phenotypes)
The proportion of phenotypic variance due to genetic differences.
100% = differences in intelligence is entirely inherited
0% = differences in intelligence not related to heritability
How can family environment be a significant environmental influence?
Children raised in the same home will be more similar. However, even children raised in the same home will have different experiences growing up
What are some examples of environmental influences?
- Socioeconomic status
- prenatal health
- birth order
- family size
- culture
- education
What did Galton (1865) study?
the relationship between eminence and families
boys adopted by eminent men and raised in privilege were less likely to be eminent than natural-born privileged sons of eminent men - genetic link?
What did Galton conclude?
Intelligence is genetically determined and fixed at birth
What is an example of early animal research into intelligence?
Tolman (1924) and Tyron (1940) selectively bred two strains of rats
- “maze bright” - quickly run around maze and find food
- “maze dull” - much slower at learning to find food in maze
What are the 3 main methods for investigating the influence of genetics/environment on intelligence in humans?
- twin studies
- adoption studies
- family studies
How are family studies used in the context of intelligence?
Investigate the similarity of family members in respect of intelligence
What is the genetic similarity of first-degree relatives? (parents, siblings, children)
50% genetic similarity
What is the genetic similarity of second-degree relatives? (grandparents, grandchildren, aunts, uncles, nephews, nieces)
25% genetic similarity
What is the genetic similarity of third-degree relatives? (great-grandparents, great-aunds/uncles, 1st cousins)
12.5% genetic similarity
What is the hierarchy of similarity?
- siblings
- aunt-niece
- cousins
- stepmother-stepchild
What are monozygotic twins?
identical twins
identical genotype
100% genetic similarity
What are dizygotic twins?
Fraternal twins
Genetically no more alike than other siblings
50% genetic similarity
What is the assumption of twin studies?
Environments are matched but MZ more genetically similar than DZ
- if environment is most important, MZ and DZ will be equally similar
- if genetics are most important, MZ will be more similar than DZ
What are the correlation coefficients for MZ and DZ twins (Eysenck, 1994)? What does this tell us about heritability of intelligence?
MZ = 0.87
DZ = 0.53
Genetics are important but not the only factor of intelligence
What is the issue with environment for MZ and DZ twins that makes it hard to make conclusions about the influence of genetics?
MZ twins are often treated in a more similar way than DZ twins (dressed the same, play more together, spend more time together) so their environments are more similar than DZ - cannot conclude that genetics are the main influence
How could the influence of genetics be investigated more accurately?
Control genes - manipulate environment of MZ twins (adoption studies)
Who was Cyril Burt, what was he interested in and how did investigate it?
- British educational psychologist
- Interested in heretidy of intelligence - attributed difference in intelligence to genetics
- Investigated via study of MZ twins reared apart
Why did Burt’s work cause controversy?
- Claimed he had fabricated his data and research assistants
- raw data could not be found
What is a criticism of Burt’s work related to correlations?
Too much similarity in his correlations
1943 - 15 sets r=.770
1955 - 21 sets r=.771
1966 - 53 sets r=.771
would expect to see greater variance in correlations as additional twin sets were added.
AND - was it possible to find that many MZ twins reared apart? no other study managed to identify as many
What are some recent research supporting Burt’s conclusions?
Bouchard and McGue (1981) MZ twins reared apart r=.72
Ridley (1999) MZ twins reared apart r=.76
What was the Minnesota study?
100 sets of DZ and MZ twins/triplets separated early on in life, reared apart and then reunited as adults
General intelligence strongly influenced by genetics = MZ twins r=.70
What are some limitations fo separated twin studies?
- Some MZ twins actually found to be living in different branches of same family
- ppts difficult to recruit
- selective placement (adoption agencies try to place children in an environment that matches as closely as possible to birth environment)
- what about prenatal environment? - Majority of twin studies focused on middle class samples
- are twins representative of general population?
How could researchers control the environment and manipulate genes?
same age unrelated siblings - virtual twins
- no genetic link but reared in same environment
- 21 pairs of virtual twins tested, reared together in same home before age of 1 yr
- mean correlation between children .17 - lower than MZ and DZ twins and siblings who all share proportion of genetic similarity
What was the Fullerton Virtual twin study?
- adopted by 1yr old
- sibling age difference 9mo or less
- same grade at school
- absence of any birth events that may affect development
- current age of 4+yrs
- same and mixed sex pairs
Completed subscales from WISC-III
Correlations .30 at time one and .11 at time 2 - modest relationship to no relationship
Importance of shared genes over shared environment increases over time.
What was the Texas adoption study?
300 families who had adopted children from a home for unwed mothers
Initially, correlations between adopted parents and children similar to correlations with biological children
- environment influences intelligence when children are small but as they develop, the influence of environment decreases and genetics increases
