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Chapter 6

PSYB30H3 Chapter 6: PSYB30-Chapter 6 Notes

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Lisa Fiksenbaum

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Chapter 6:Genetics
Genotypes: The genetic makeup that codes for a specific trait.
Behavioural genetics is the study of the genetic and environmental
contributions to individual differences in personality and behaviour.
Nature and Nurture as Allies
We call the combination of nature and nurture a genotype-environment
What if people change environments and environments change people, so
that we really can’t separate the impact of genes on environment? This is
called a genotype-environment correlation.
In fact, some researchers advocate an even more complicated equation for
understanding the manifestation of complex human traits a concept known
as phenotype.
Phenotype = Genotype + Environment + Gene-environment correlation + Gene-
environment interaction
Genes and Environment as Co-actors
Heritability (h2) is the amount of observed individual differences in some
characteristic that can be accounted for by genetic differences. Heritability
refers to differences across a group or population of people- not to a specific
Heritability therefore, refers to the inheritance of a particular trait in a
particular population at a particular time, so sometimes heritability
estimates differ depending on the exact sample and methods used by
Environmentality (e2) estimates the extent to which observed individual
differences can be traced in any way to individual differences in
Shared and Nonshared Environments
One way of thinking about environmental influences on characteristics such
as height or personality is to identify aspects of the environment that are
shared and unshared among relatives living together in a household.
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Shared environments includes aspects of the family environment that are
generally the same for all the children in the household including physical,
psychological, and social aspects.
In contrast, the nonshared environment includes experiences that relative
have which make them different from one another. This may include unique
experiences within the family, or outside the family. When it comes to
personality most of the environmental influence ends up being of the
nonshared variety.
One measure of heritability is to calculate the correlation (r) between twins
on a given trait and compare the correlation between MZ twins and DZ twins.
The exact formula is to double the difference between these correlations or
h2 = 2(rmz rdz)
A second way of estimating heritability is to compare identical twins who
have been raised in separate environments. We call these kinds of twins MZA
twins (monozygotic twins raised apart). If such twins score similarly in a
trait such as Extraversion, then we know that Extraversion has a strong
genetic component. Studies of MZA twins are particularly powerful in
disentangling the effects of gens and environment because they have
identical genes but different environments. Here is a second formula for
estimation heritability:
h2 = rMZA
The double-the-difference method assumes that twins were reared under
equal environments. That is, it assumes that people have not treated MZ
twins more alike than DZ twins.
This equal environments assumption applies only to similar treatment that is
related to the specific characteristics under study. For example, people often
dress their twins alike in identical sailor suits or matching dresses. This
probably happens more often to MZ twins than to DZ twins. Of we were
studying something like fashion sense, then this would violate our equal
environment assumption. But unless wearing matching outfits, affects a
specific personality characteristic like shyness, then this assumptions still
The double-the-difference formula also assumes that twins are typical of the
population. We call this the assumption of representativeness.
Twins may be more alike on a certain characteristic due to selective
placement during the adoption process and not their genetics. Selective
placement makes it impossible to see the effect of genetics apart from the
effect of environment because it confounds the two.
Research Methods Illustrated: Correlational Designs I: The Logic of Adoption and
Twin Studies
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