Heritability and Individual Differences
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Extraversion?
Neuroticism?
Depression?
Alcoholism?
Only genes are
inherited
Genotype – Genes
Phenotype – Traits
The Human Genome
Contains between 30,000 and
40,000 genes
 All are located on 23 pairs of
chromosomes
 The body contains roughly 100
trillion copies of the human genome
 The Human Genome Project
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Different genotypes → Same phenotype
Same genotype → Different phenotype
Context (other genes, cellular, etc.) is
critical
No specific genetic markers for
personality
What is Heritability?
The proportion of observed variance
in a group of individuals that can be
explained or “accounted for” by
genetic variance.
What is Heritability?
The proportion of phenotypic variance
that
is attributable to genotypic variance
H of 20 = 20% GV and 80% Environmental
What is Heritability?
Misconceptions
It can be applied to a single
individual
 It is constant or immutable
 It is an absolutely precise statistic
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Behavioral Genetics Methods
Selective Breeding
Thompson
Assess rat ability to traverse maze
Designate rats as maze-bright or
maze dull based on performance
Breed maze-bright with maze-bright
and maze-dull with maze dull
Assess maze running ability
Maze bright strain far superior to
maze dull strain
Cooper and Zubek
Simultaneously manipulate genes and
environment
Genes:
Obtain offspring of maze-bright and
maze-dull strain
Environment:
Raise in normal, enriched, impoverished
environment
Enriched
Enriched
Normal
Normal
Restricted
Restricted
MazeBright
111
117
170
Maze-Dull
120
164
170
Genes matter in some
environments but not others
Environment can override effects of
genes
Behavioral Genetics Methods
Twin Studies
 Monozygotic
Twins (100% shared
genes)
 Dizygotic Twins (50% shared genes)
If MZ twins are more similar than DZ twins,
this provides evidence of heritability
Heritability = 2(rmz – rdz)
0.7
0.6
0.5
0.4
0.3
DZ
MZ
0.2
0.1
0
-0.1
-0.2
Emotionality
Activity
Sociability
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
DZ
MZ
Crime
Alcoholism
Homosexuality
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
MZ
MZ Verified
DZ
Random Pair
MZ
MZ
Verified
DZ
Random
Pair
Trait
h2
Height
Weight
IQ
.80
.60
.50
Conservatism
Religiosity
TVviewing
.30
.16
.20
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1. Are the results valid? Do twin
studies overestimate heritability?
2. What are the mechanisms that
explain heritability, especially complex
behaviors?
Generalizability – Small, self-selected,
samples
Biased Ratings – Influenced by
expectations
MX:DZ ratio > 2:1
(Note: genetic ratio is 2:1)
Possible reasons:
1. Environment
MZ treated more similarly
MZ interact more frequently
2. Genetic nonadditivity
Twin studies with additional
controls
Adoption studies
Twin/Adoption studies
Sweden – use twin registry
Measure: I, E (from Eysenck),
Interaction Frequency
Results:
MZ > DZ for I, E, and Interaction
Frequency
MZ > DZ for I, E, controlling for
Interaction Frequency
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
-0.05
-0.1
Adopted
Parents
Biological
Parents
Adopted Sibs
Bio. Sibs
Sociability
Activity
MZ Twins
Apart
Together
DZ Twins
Apart
Together
Extraversion
Introversion
Openness
Conscientiousness
Agreeableness
.30
.25
.43
.15
.19
.54
.41
.51
.41
.47
.04
.28
.23
.03
.10
.06
.24
.14
.23
.11
Emotionality
Activity
Sociability
.37
.27
.20
.49
.38
.35
.04
.00
.19
.08
.18
.19
Mean
.27
.44
.11
.15
Many demonstrations of heritability,
even for complex traits (e.g., crime)
No gene(s) for complex phenotypes
How can they be inherited?
Genotype
High Emotionality
High Activity
Ignored
Punished
Low self-esteem
Aggressive
Delinquency
Phenotype
Crime
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Selective Exposure – Genes influence types of
environment to which one is exposed
Differential Susceptibility – Genes influence
how one reacts to environment