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Lecture 11

lecture 11 notes - proper!


Department
Psychology
Course Code
PSYB30H3
Professor
Marc A Fournier
Lecture
11

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LECTURE 11- CONTINUITY AND HERITABILITY contd
behavioural genetics, in terms of psychology, is the study of Heritability
the extent to which given behavioural traits are heritable
has very little to do with specific genes (thats molecular genetics) this si
concerned with Heritability
the earliest methodology to study behavioural genetics = family studies
Family Studies
correlation of degree of personality similarities with the degree of genetic overlap
between family members (degree of shares genetics b/t family members)
capitalizes on the fact that we know how genetically similar family members are due
to the various relations
parents each share 50% of their DNA with their offspring
siblings on avg. share 50% of their DNA with each other
grandparents share about 25% of their DNA with their grand kids
first cousins share 1/8 of their DNA with each other
logic: if we know a given trait is highly heritable then we should see more of an
overlap in personality of that trait b/t 2 members of a family that are related the
closest (eg, siblings will be more similar than first cousins on that heritable trait)
if a given characteristic is low in Heritability (genes play a smaller role) than there is
no reason to expect that the siblings will resemble each other more closely than first
cousins will on that trait
confound: family members who share more genes in common also share more similar
environments
siblings not only resemble each other b/c of their genetic similarities but maybe
because of their environmental similarities
in family studies, these two can never be disentangled, its a major flaw so no
reliable conclusion can come from these studies
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a better methodology is the Twin Studies
Twin Studies
1 out of 83 births is a twin birth which is good for these studies (more data)
There are 2 types of twins:
1) monozygotic (MZ) which are identical (1/3 of all twins)
2)dizygotic (DZ) which are fraternal twins (2/3 of all twins)
othey share 50% of their DNA with each other just like any other siblings
r(mz) is the correlation of the extent to which the MZ twins resemble each other on a
certain trait
r(dz) is the correlation of the extent to which DZ twins resemble each other on a
certain trait
if the construct is highly heritable then one would predict that MZ twins would be
resemble each other more on that trait than DZ twins [r(mz) is higher than r(dz)]
if construct is low in Heritability then theres no reason to believe that MZ twins
resemble each other more than DZ twins on that trait and r(mz) is approx the same
as r(dz)
the more difference there is between r(mz) and r(dz), the more heritable the trait is
H = 2*[r(mz) r(dz)] know this formula for the exam!!
Assumption of twin studies:
1)Equal Environments: assumes that the environments of the MZ twins are the
same as DZ so none of the heritability can be accounted to environment;
environment is constant
This is not true! (eg parents dress MZ twins more similar than DZ twins; MZ
twins have more friends in common than DZ twins)
So now the question is to what extent would the high similarities in environment
of MZ twins make us concerned about the HE we obtain
Evidence suggests that even though there are few environmental similarities
that MZs have that DZs dont, theres nothing to prove that these small
differences cause huge inflation to the MZ HEs we get
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So it seems like a reasonable assumption to make even though it isnt met all the
time
2)Representativeness: twins are like everyone else. There is nothing that
distinguishes twins from non-twins other than the fact that they are born at the
same time
This isnt always true as well they are more likely to be born underweight,
underage (prematurely) compared to non-twins
Are these differences large enough to make twins unrepresentative of non-twins?
Evidence suggests that twins are reasonably representative of non-twins despite
these small differences
Adoption Studies
The most powerful method with which to study heritability.
This study is also a question of comparing correlations.
They track down children in adoptive families and measure the personality
characteristics of those children
They also measure those same personality characteristics in their adoptive AND
biological parents
They then calculate two correlations
One: between children and their biological parents
Two: between children and their adoptive parents
If there is a positive correlation between children and their adoptive parents, then
there is evidence of an environmental influence on personality
If there is a positive correlation between children and their biological parents, then
there is evidence of a genetic influence on personality
To whom are the adopted children more similar their biological or adoptive
parents?
Adoption studies circumvent the assumption of equal environments
Biological parents are not providing the environment, the adoptive parents are.
There is no confound in this design between biology and environment, the two
sources of influence are separate from each other
Potential problems:
Representativeness
We have to assume that adoptive parents are reasonably representative of the
general population of parents
In terms of socio-economic status, personality, education level, adoptive
parents look like everybody else. So they are a good representation of the
rest of the population of parents. No striking differences to affect
reliability
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