Behavioural Genetics

Goal:
– classify observed variation in a personality trait into variation related to related to differences in the
differences among individuals and variation
which they have been exposed to

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
V phenotype
= V genotype
+ V environment

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 personality may not be independent—certain genotypes tend to be exposed preferentially to certain environments b.
interaction: certain combinations of genes and 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
MZ twin
DZ twin brother, sister parent, child grandparent, grandchild uncle, aunt, nephew, niece half-brother, half-sister first cousin second cousin unrelated person
Genetic Similarity
100%
50%
50%
50%
25%
25%
25%
12.5%
6.25%
0%

Family studies
 compare similarities in personality between siblings and between parents and children
 siblings:
 parent-child: typical r = .15
typical r = .25
 problem: family members share both genes & environment; can’t disentangle respective influences on personality

Twin studies



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 h 2 = 2(r
MZ
– r
DZ
)

Twin studies


‘heritability’ : proportion of observed variation in a trait attributable to genetic variation between individuals h 2 = 2(r
MZ
– r
DZ
) proportion of observed variation attributable to shared environmental factors:

2r
DZ
- r
MZ proportion of variation attributable to non-shared environmental factors:
1 - h 2 – (2r
DZ
– r
MZ
)

Twin studies - Components of variance for the Big Five personality traits (Loehlin, 1992)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Ex trav er sio n
Ne ur oti cis m
Ag re eab len es s
Co ns cie nti ou sn es s
Op en es s
Nonshared E
Shared E
Genetic

Twin studies - Components of variance for the Big Five personality traits (Riemann et al ., 1997)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0% self peer self peer self peer self peer self peer
E N A C O
Nonshared E
Shared E
Genetic

Twin studies

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

Familial correlations for adoptive and biological relationships
Relationship
Adoptive parent - child
Adoptive siblings
Biological parent - child
Biological siblings
Average r
.05
.04
.15
.20

Familial correlations for adoptive and biological relationships –
Relationship
Adoptive parent – adopted away child
Adoptive parent – adoptee child
Biological parent - child
Adoptive siblings
Biological siblings r sociability
.03
.06
.13
-.04
.11
r emotionality
-.01
.02
.04
.08
.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

Behavioural Genetics

– 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