PSYCH 3F03 Lecture Notes - Lecture 5: Twin, Twin Study, Heritability

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25 Jan 2013

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Intelligence = general cognitive ability; Spearman’s g, the substantial
covariation among diverse measures of cognitive ability; the general
factor that typically accounts for about 40% of the total variance of
diverse cognitive tests
More is now known about the genetics of individual differences in
intelligence than about any other trait, behavioral or biological
Heritability = the proportion of observed trait variance in a group of
individuals that can be accounted for by genetic variance
At the level of the individual there is no nature-nurture debate; only at
the population level can we disentangle genetic and environmental
Indexed by heritability quotient range from 0 to 1. 1 means trait is
influenced solely by inheritance and 0 means trait is not influenced by
Twin Studies
Out of 83 births, approx. 1 pair of twins are born.
Are identical (MZ) twins more similar to each other than fraternal (DZ)
If twins are no more similar than fraternal twins, then genetics does
not play a significant role in heritability and so on.
Heritability = 2(rmzrdz)
One limitation of twin studies is their assumption of equal
Adoption Studies
To whom are adopted children more similar — their biological or
adoptive parents?
If a child resembles adoptive parent more than biological parent than
there is an environmental role in the determination of an individuals
Adoption studies circumvent the assumption of equal environments
One limitation of adoption studies is their assumption of unequal
In studies of more than 10,000 twin pairs:
MZ twin r = .86
DZ twin r = .60
MZ twins remain alike whether they are raised together or raised
Heritability = 50%
Shared environment is important, too:
Parent-Offspring r = .19
Sibling-Sibling r = .32
Environments produce similarities b/n people with no genetic similarity
The Nature of Nurture
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