1
Problem 7
• How does the Big Five change among twins when they are raised together or separately?
• How are the Big Five influenced by nature or nurture?
• What kind of research is used to examine hereditability?
• Why are certain personality traits more biologically/environmentally driven?
• Why do identical twins develop differences?
Behavioural genetics
• Genetic influences on behavioural qualities (personality, abnormalities)
Goals of Behavioural Genetics
1. Finding Percentage of variance: individuals are different from each other (percentage that
are due to genetic & environmental causes)
2. Determining the way in which genes & environment interact & correlate with each other
o Where these effects take place
Heritability index
• Estimate of the role of heritability in accounting for individual differences on the variable
studied
• Heritability (the same as heritability index): statistic that refers to proportion of observed
variance in a group of individuals that can be accounted for by genetic variance
o Describes to which extent genetic differences between individuals causes
differences in observed property (e.g. height, extraversion)
• Formal definition: Proportion of phenotypic variance that is attributable to genotypic
variance
• heritability is an estimate of the percentage of phenotypic differences due to genetic
differences
• Phenotype variance: observed individual differences (e.g. heigh, weight, etc.)
• Genotype variance: individual difference in total collection of genes possessed by each
person
o E.g. 0.5 → 50% of phenotype variance id attributable to genotypic variation
• Heritability index depends on variations in environment in studied society
o Reflects characteristics of a population
→ the more homogenous the environments are, the greater is the heritability index
• No correlation between genetic & environmental factors
• Environmentality: percentage of observed variance in a group that can be attributed to
environmental difference → the larger the heritability, the smaller the environmentality
, 2
Misconceptions about Heritability
• Heritability refers only to differences in sample/ population → not an individual
o Cannot say “Meredith’s height is 90 % heritable”
• Heritability is not stable → if environment changes, then heritability changes
o E.g. heritability can be high in one population (Sweden) & low in another
(Nigerians)
• Heritability is not a precise statistic
o Error of measurement can distort heritability statistics
Nature-Nurture Debate
• Two levels of analysis
o Level of individual
§ There is no issue to debate
§ Individual unique constellation of genes
o Level of population of individuals
§ Disentangle the influence of genes & environment
§ Two sources
Ø Genes
Ø Environment
§ “Which is more important in accounting for the differences?”
Family Studies
• Focus: Correlate degree of genetic relatedness among family member with degree of
personality similarity
o Capitalize on the fact that there are known degrees of genetic overlap among family
members
• Each parent share 50 percent of his genes with each of the children
• If personality traits highly heritable → those with greater genetic relatedness should be more
similar in traits → than with other less genetically related family members
• If personality trait not at all heritable → should be no difference in similarity of trait between
more or less genetically related family members
• Limitation: Genes cannot be studied exclusively → family members share similar
environment
Twin studies
• Method that compares degree of similarity on measures on personality obtained by twins
o Monozygotic/identical (share 100% DNA)
o Dizygotic/fraternal (share 50% DNA)
• If genetics determine personality → monozygotic twins should possess same personalities
o Fraternal twins are just as similar as identical twins on characteristic → characteristic
not heritable
o Identical more similar than fraternal twins on characteristic → characteristic
heritable
• Studies
o Identical twins more similar than fraternal twins on dominance (IT 0.57 & FT 0.12) &
height (IT 0.93 & FT 0.48)
Problem 7
• How does the Big Five change among twins when they are raised together or separately?
• How are the Big Five influenced by nature or nurture?
• What kind of research is used to examine hereditability?
• Why are certain personality traits more biologically/environmentally driven?
• Why do identical twins develop differences?
Behavioural genetics
• Genetic influences on behavioural qualities (personality, abnormalities)
Goals of Behavioural Genetics
1. Finding Percentage of variance: individuals are different from each other (percentage that
are due to genetic & environmental causes)
2. Determining the way in which genes & environment interact & correlate with each other
o Where these effects take place
Heritability index
• Estimate of the role of heritability in accounting for individual differences on the variable
studied
• Heritability (the same as heritability index): statistic that refers to proportion of observed
variance in a group of individuals that can be accounted for by genetic variance
o Describes to which extent genetic differences between individuals causes
differences in observed property (e.g. height, extraversion)
• Formal definition: Proportion of phenotypic variance that is attributable to genotypic
variance
• heritability is an estimate of the percentage of phenotypic differences due to genetic
differences
• Phenotype variance: observed individual differences (e.g. heigh, weight, etc.)
• Genotype variance: individual difference in total collection of genes possessed by each
person
o E.g. 0.5 → 50% of phenotype variance id attributable to genotypic variation
• Heritability index depends on variations in environment in studied society
o Reflects characteristics of a population
→ the more homogenous the environments are, the greater is the heritability index
• No correlation between genetic & environmental factors
• Environmentality: percentage of observed variance in a group that can be attributed to
environmental difference → the larger the heritability, the smaller the environmentality
, 2
Misconceptions about Heritability
• Heritability refers only to differences in sample/ population → not an individual
o Cannot say “Meredith’s height is 90 % heritable”
• Heritability is not stable → if environment changes, then heritability changes
o E.g. heritability can be high in one population (Sweden) & low in another
(Nigerians)
• Heritability is not a precise statistic
o Error of measurement can distort heritability statistics
Nature-Nurture Debate
• Two levels of analysis
o Level of individual
§ There is no issue to debate
§ Individual unique constellation of genes
o Level of population of individuals
§ Disentangle the influence of genes & environment
§ Two sources
Ø Genes
Ø Environment
§ “Which is more important in accounting for the differences?”
Family Studies
• Focus: Correlate degree of genetic relatedness among family member with degree of
personality similarity
o Capitalize on the fact that there are known degrees of genetic overlap among family
members
• Each parent share 50 percent of his genes with each of the children
• If personality traits highly heritable → those with greater genetic relatedness should be more
similar in traits → than with other less genetically related family members
• If personality trait not at all heritable → should be no difference in similarity of trait between
more or less genetically related family members
• Limitation: Genes cannot be studied exclusively → family members share similar
environment
Twin studies
• Method that compares degree of similarity on measures on personality obtained by twins
o Monozygotic/identical (share 100% DNA)
o Dizygotic/fraternal (share 50% DNA)
• If genetics determine personality → monozygotic twins should possess same personalities
o Fraternal twins are just as similar as identical twins on characteristic → characteristic
not heritable
o Identical more similar than fraternal twins on characteristic → characteristic
heritable
• Studies
o Identical twins more similar than fraternal twins on dominance (IT 0.57 & FT 0.12) &
height (IT 0.93 & FT 0.48)
