Transcript Behavior genetics

The GENETICS of PERSONALITY
Behavioural Genetics
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Goal:
– classify observed variation in a personality
trait into variation related to genetic
differences among individuals and variation
related to differences in the environments to
which they have been exposed
Levels of analysis in behavioural genetics
1.
Overall division of variation in personality
into variaion related to genetic differences
and variation related to ‘non-genetic’ (i.e.,
environmental) differences
Vphenotype = Vgenotype + Venvironment
Levels of analysis in behavioural genetics
2. Further subdivision of genetic and
environmental effects
a. Genetic effects
i. Additive genetic effects
ii. Non-additive genetic effects
e.g., dominance
Levels of analysis in behavioural genetics
2. Further subdivision of genetic and
environmental effects
b. Environmental effects
i. Shared environment: environmental factors shared by
family members that contribute to their similarity
(e.g., common neighbourhood, SES, parents)
ii. Non-shared environment: environmental factors not
shared by family members that do not contribute to their
similarity
(e.g., differential parental treatment; different friends, peers,
teachers; accident & illness)
Levels of analysis in behavioural genetics
3.
Correlations & interactions between
genotype and environment
a.
correlation: genetic & environmental influences on
b.
interaction: certain combinations of genes and
personality may not be independent—certain
genotypes tend to be exposed preferentially to certain
environments
environments may lead to idiosyncratic effects—the
effect of genotype on personality may vary with
environment or vice versa
Levels of analysis in behavioural genetics
4.
Underlying causal mechanisms of genetic
and environmental influence
 sociobiological
 developmental
 molecular genetic
Starting point: known degrees of genetic relatedness
Relationship
Genetic Similarity
MZ twin
100%
DZ twin
50%
brother, sister
50%
parent, child
50%
grandparent, grandchild
25%
uncle, aunt, nephew, niece
25%
half-brother, half-sister
25%
first cousin
12.5%
second cousin
6.25%
unrelated person
0%
Family studies
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compare similarities in personality between
siblings and between parents and children
 siblings:
 parent-child:
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typical r = .15
typical r = .25
problem: family members share both genes &
environment; can’t disentangle respective
influences on personality
Twin studies
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MZ twins share 100% of genes; any differences
between them must be due to environmental
influences
observed difference in resemblance between MZ and
DZ twins on a personality trait constitutes ½ the
effect of genes on that trait
‘heritability’: proportion of observed variation in a
trait attributable to genetic variation between
individuals
h2 = 2(rMZ – rDZ)
Twin studies
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‘heritability’: proportion of observed variation in a
trait attributable to genetic variation between
individuals
h2 = 2(rMZ – rDZ)
proportion of observed variation attributable to
shared environmental factors:
2rDZ - rMZ
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proportion of variation attributable to non-shared
environmental factors:
1 - h2 – (2rDZ – rMZ)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
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Ag
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Nonshared E
Shared E
Genetic
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Percentage of Variance
Twin studies - Components of variance for the Big Five
personality traits (Loehlin, 1992)
Twin studies - Components of variance for the Big Five
personality traits (Riemann et al., 1997)
100%
Percentage of Variance
90%
80%
70%
60%
Nonshared E
Shared E
Genetic
50%
40%
30%
20%
10%
0%
self
E
peer
self
N
peer
self
A
peer
self
C
peer
self
O
peer
Twin studies
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Limitations:
– assumes equal environments
– assumes additive genetic effects
– assumes genetically random mating
Adoption studies
genetically related family members who are
adopted apart—do not share family
environment;
 genetically unrelated family members adopted
together—do not share genes;
 resemblance between relatives adopted apart =
direct test of influence of genetic factors on
personality;
 resemblance between unrelated individuals
adopted together = direct test of the influence
of shared environment on personality
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Familial correlations for adoptive and biological
relationships
Relationship
Average r
Adoptive parent - child
.05
Adoptive siblings
.04
Biological parent - child
.15
Biological siblings
.20
Familial correlations for adoptive and biological
relationships – Colorado Adoption Project
Relationship
rsociability
remotionality
Adoptive parent – adopted away child
.03
-.01
Adoptive parent – adoptee child
.06
.02
Biological parent - child
.13
.04
Adoptive siblings
-.04
.08
Biological siblings
.11
.04
Separated twins
correlations on personality variables
between monozygotic twins reared
together (MZT) tend to be high (.50 to
.70);
 correlations between monozygotic twins
reared apart (MZA) tend to be similar,
suggesting little influence of environment;
 suggest that shared genes account for
roughly 50% of variance in personality
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Behavioural Genetics
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In sum…
– 30 – 50% of variance in personality questionnaires
can be attributed to genetic differences among
individuals
– environmental factors account for a majority of the
variance, but they do not contribute substantially to
similarity between family members
– shared genes rather than shared environments are
responsible for resemblances in personality between
members of the same family