Genetic and Environmental Influences on Personality Flashcards
To what extent is personality variation caused by heredity or by the environment?
Nature versus Nurture
To what extent is personality variation caused by heredity or by the environment?
Genes and environment are acting together
What proportion is due to hereditary differences and what proportion is due to environmental differences?
Step 1: Examine the similarity of pairs of relatives
Step 2: Compare different types of relatives
Step 1: Examine the similarity of pairs of relatives
Measure a certain personality trait in a large number of pairs of relatives
Calculate how much variance there is altogether in the trait
Calculate the proportions of variance that is
- due to within-family differences
- due to between-family differences
Similarity due to common genes or due to shared environment
Step 2: Compare different types of relatives
Relatives who have their heredity in common but not their environment
Relatives who have their environment in common but not their heredity
Identical or monozygotic (MZ) twins raised apart
- 100% of their genes in common
- correlation for a given trait is the proportion of variance due to genetic influence
Dizygotic (DZ) twins or non-twin siblings raised apart
- 50% of their genes in common
- correlation reflects the effects of 50% of the genes
-> Difficult to find many relatives who have been adopted
Comparing similarities between identical twins and
between fraternal twins raised together
(Same environment)
Any extra similarity of MZ twins beyond that of DZ twins -> due to the additional genetic similarity of MZ twins (= 50% extra genetic similarity)
rMZ = h2 + c2
rMZ: correlation between MZ twins
h2 : heritability estimate
c2: shared environment estimate
rDZ = ½ h2 + c2
rDZ: correlation between DZ twins
h2 : heritability estimate
c2: shared environment estimate
h^2 = 2(rmz - rdz)
h2 = estimate of the average proportion of variance for a given personality trait in a population
that are due to genetic variation across this population
Similarity of MZ twins is exactly twice as great as the similarity of DZ twins
BUT, similarity of MZ twins could also be less or more because of
a) non-additive genetic influences
b) shared environment
Additive and non-additive genetic influences
Combined effects of two or more genes on personality
Additive genetic effects
each gene contributes separately
Non-additive (multiplicative) genetic effects
influence of a particular gene depends on the presence or absence of the another gene
Non-additive effects affects both MZ twins in the same way because they always have the same combination of the genes
DZ twins will have the same combination of two genes about ¼ of the time
Consequence: When non-additive genetic influences on a trait are involved
MZ twins will be more than twice as similar as fraternal twins
Common environment influences = between-family environmental influence
Unique/non-shared environmental influences = within-family environmental influence
Similarity of MZ twins can be less than twice as similar as fraternal twins if the level of trait also depends on aspects of the environment in which one is raised
Environmental influence can also be estimated by measuring the similarity between adoption
Unique environment influences
Siblings experience different environments, potentially influencing their personality
After estimating the heredity + common environment influence…
The variance that is left over can be attributed to unique environment influence
German twin studies: comparing identical twins raised together with fraternal twins raised together for the Big Five factors
(Riemann, Angleitner, & Strelau, 1997)
With self-report data:
- the MZ twins mean correlation = .54
- DZ twins = .27
- Heritability of personality characteristics = .54
- > Additive effect and no common environmental effect
With observer report data:
- MZ twins = .45
- MZ twins = .20
- Heritability of personality characteristics = .50
- > Almost all additive effect and no common environmental effect
Combining self- and observer reports:
- Heritability of personality characteristics = .65
- > higher levels of heritability than one of these methods is used alone
Very similar results for self- and observer reports
Combination of both indicate higher heritability
Methods used alone underestimate the heritability
Loehlin (1989)
Comparing identical with fraternal twins raised together for Extraversion and Neuroticism (Eysenck) in three countries
Most of the score indicated that 40-50% of personality can be explained by genetic
Minnesota Twin Study (Bouchard & McGue, 1990; Tellegen et al., 1988)
45 sets of MZ twins reared apart and 26 sets of DZ twins reared apart
High correlations: average twin correlation = .54
Religiosity:
Abrahamson, Baker, and Caspi (2002):
650 adopted and non-adopted adolescents
- Correlations above .40 for both adopted and non-adopted relatives
- Low heritability: .12
- Impact of shared environment, no genetic influence
Impact of shared environment tend to diminish (decrease) during adulthood
Koenig, McGue, Krueger, and Bouchard (2005)
Religiosity levels of fraternal twins
= .60 during adolescence
= .40 in their 30s
Religiosity levels of identical twins
= .70 during adolescence
= .60 in their 30s
-> Genetic tendencies of identical twins help to maintain similar religiosity levels
Political attitudes:
Eaves et al. (1997): ~7000 twin pairs
MZ twins were more highly correlated than DZ twins
Conservatism levels of fraternal twins
~ .50 during teenage years
~ .40 after the age of 20
Conservatism levels of identical twins
~ .50 during teenage years
~ .60 after the age of 20
Shared environment has a strong influence during adolescence, but this influence weakens after the age of 20
Genetic influence strengthens after the age of 20
Religiosity and political attitudes suggests personality is…
Shared environment: strong influence in adolescent
When they get older: the genetic strengthened
Behavioural manifestations of personality dimension…s
such as sensation seeking, extraversion, neuroticism?
Smoking: Dutch study with 1572 twins in smoking
Vink et al, 2005
Heritability of smoking initiation is .44
44% heritability to begin smoking behaviour
Meta-analysis in smoking
Li et al. 2003
Showed that heritability of smoking persistence is .53
53% due to genetic that once people have started, they continue
Studies on the heritability of alcohol drinking
- show moderate heritability ranging from .21 to .56
Studies on alcoholism show heritability estimates of .50 or greater, up to .71
This means that drinking behaviour can strongly explained by genetic factor
Massive genetic influence on personality traits and influence behaviour
The Assumptions
Participant’s personality is measured independently
There is little or no “assortative mating”
Are relatives’ personality really measured independently?
Non-independence could inflate or deflate the similarity between relatives
-> Biased heritability estimates
For example,
Siblings completing the same questionnaire may want to emphasise the differences between them
instead of comparing themselves with people in general
Observers (e.g. parents) may compare their children with each other instead of with children in general
= Contrast effect
Contrast effect
Overestimate the difference when comparing the two with each other instead of the others
Assimilation effect
Overestimation of the similarity
Siblings completing the same questionnaire and considering themselves as very similar
Observers perceiving the siblings as very similar may have a tendency to emphasise the similarity
How to overcome contrast and assimilation effects?
Obtain observer reports from someone who knows only one of the individuals
But does not know the individual’s relative very well
Direct observations of participants’ behaviour
Subsample (300 pairs) of the German twins took part in a variety of activities while being observed and video-recorded
(Borkenau, Riemann, Angleitner, & Spinath, 2001)
Stronger similarity in personality when assessed by observations then with self- or observer reports
DZ twins were more than half as similar to each other as were identical twins
-> Evidence for common environment effect
Findings support idea that there is a contrast effect for DZ twins for self-report personality
Is there really no assortative mating for personality?
Researchers assume that there is no tendency for parents to be similar in their personality traits
They are similar to each other like any two people chosen at random
If mothers’ and fathers’ levels of a trait were highly correlated
then DZ twins will share more than 50% of their genes with each other
So comparing MZ with DZ twins would lead to an underestimation of the heritability
Studies have shown that parents are not so similar in any personality characteristic in ways that would distort the results of heritability studies
But beliefs, attitudes, and cognitive abilities are still more likely to pass on the children from parents
Assumptions of twin-based heritability studies
Objections to two assumptions of studies with twins raised apart:
1. Twins may share some features of their early environment
- Adopting households may differ in important ways from households in general
Objection to an assumption of studies with twins raised together:
3. Identical twins raised together might be treated more similarly than fraternal twins
Are twins’ early environments really separate?
The twins in adoption studies are separated at a very young age (~ 5 months)
However, those few months together, sharing the same environment, may contribute to their similarity
Studies have compared early-separated with later-separated twins
but found no evidence that the shared environment during those months together would influence the similarity between twins
What about common womb environment?
The womb as a unique and a common environment for twins and siblings
Non-twin siblings:
Consistencies across pregnancies -> common
Changes across pregnancies -> unique
Some of the similarity of twins (or non-twin siblings) raised apart may be attributable to the common womb environment
Womb effect has not yet been addressed in heritability studies on personality traits
Are twins’ adoptive households really very different?
Socioeconomic status: higher income and educational levels
Parenting style: more gentle and attentive
Limited variation among adopting households may tend to limit the estimated effect of common environment
As the fact that adoptive children may have common environment
McGue et al. (2007): adoptive parents compared to non-adoptive parents
Somewhat lower levels of antisocial behaviour and drug and alcohol abuse
Slightly higher socioeconomic status
McGue et al. (2007): antisocial behaviour of adoptive parents was also completely unrelated to antisocial behaviour of the adopted children
Restricted variability among adopting parents does not necessarily lead to an underestimation of shared environment effects on behaviour
-> the environment influence no significant effect on the result of genetic component in personality
Selective placement: selecting adoptive parents similar to the biological parents
May underestimate the environmental influence when comparing identical twins raised apart
e. g., same race background
- > Studies have shown that there is very little selective placement
Are identical twins really treated differently by others? Equal environment assumption?
Identical (vs fraternal) twins become more similar due to more similar environments
Same clothes, activities,… lead to more similar treatment which may affect their personality
Loehlin and Nichols (1976): 300 identical twin pairs from Texas
Similar vs. different treatment:
similarity of treatment had little effect on similarity of personality
-> show that genetic influence does play a role in personality
Scarr and Carter-Saltzman (1979): parents who misidentified DZ and MZ twins = effects of perceived versus actual genetic similarity
Personality similarity is greater for MZ believed to be DZ
-> show similarity of identical twins is not due to similar treatment
Effects of the unique environment on personality:
Parental treatment and peer groups
Loehlin (1997): 800 pairs of twins
Personality characteristics, different parental treatment, differences in peers
Size of difference in personality was weakly correlated with amount of differences in parental treatment and in peer groups
= unique environmental effects (.15)
But personality differences may also cause differential treatment and differences in peer groups
Burt, McGue, and Iacono (2009): 450 pairs of twins at age 14 and at age 17
Longitudinal relations between differencesbetween the two members of a twin pair in delinquent (Externalising) behaviour and differences in having deviant friends (“Peers”)
- Twins who engaged in more delinquent or externalising behaviour at age 14 tended to have more deviant friends at age 17
- > Behaviour (reflection of personality) tends to influence peer groups more than peer groups influence behaviour (and personality)
Peer groups tend to have a strong impact on adolescents leads to differences between groups, cultures, or generations
e.g., clothing, music choice,… also norms regarding drug use, sexual activity, delinquency
In terms of individual differences in personality trait levels among people, the influence of groups of friends seems to be rather modest
Birth order:
Alfred Adler
First-borns become insecure after loosing their favourite position when sibling(s) arrive(s)
Last-borns tend to be spoiled
Middle-borns less insecure and less spoiled
Birth order:
Sulloway (1995, 1996)
Later born siblings: higher levels of Agreeableness and lower levels of Conscientiousness,
but especially higher levels of Openness to experience (creative)
They need to be more creative and unconventional to impress their parents
Studies asking respondents to compare siblings provide support for higher Openness levels among later-born siblings compared to first-borns
Studies assessing personality traits with self- and observer reports show virtually no difference for Openness to experience
-> contrast effect
Difficulties in separating the effects of heredity and environment
Genotype-environment correlations
Three different types: 1. Passive 2. Active 3. Reactive
Passive genotype-environment correlation
Environmental experience is influenced by parents’ genetic predispositions, which are also inherited by those children
e.g., athletic parents create sports-oriented home environments
Active genotype-environment correlations
Children select their environment as a function of their own genetic tendencies
e.g., athletic children actively chooses to practice sports
Reactive (evocative) genotype-environment correlations
Children experience different environments depending on other people’s reaction to the children’s different genetic tendencies
e.g., athletic children may be provided with more sport opportunities such as signing up for sport team by parents
Same environment may influence people’s levels of a given personality trait in different ways, depending on their genetic characteristics
e.g. Overprotective parenting style
Some may be genetically inclined to like this, whereas others may not
-> become risks-averse, dependent person versus independent person
Such genotype-environment interaction effects of environment may decrease similarities between relatives
Lower heritability correlations
NO lower heritability estimates based on differences between correlations (comparing MZ with DZ twins)
Caspi et al (2003): do genes and environment interact and leads to depression? (influence of life stress, gene on depression)
Test of association between depression and a) stressful life events, b) gene characteristics (5-HTTLPR) and c) their interaction
People differ from each other in their sensitivity to stressful life events
Risk of depression after a stressful event depends on genetic factors: high versus low genetic risk
-> Genetic vulnerability
Specific genes may also worsen or buffer the effects of stressful life events
Gene involved in the serotonin system: 5-HTT gene
Results for depression symptoms and diagnosis of major depression
- Main effect of stressful life events
- Marginally significant or no main effect of 5-HTTLPR
- Stressful life events clearly predicted depression symptoms and diagnosis of major depression
Results for probability of suicide ideation or attempt
Main effect of stressful life events
No main effect of 5-HTTLPR
Interaction: Stressful life events clearly predicted suicide ideation or attempt among s allele carriers but not among those without s-allele
Results for probability of major depression
Main effect of childhood maltreatment (abuse)
No main effect of 5-HTTLPR
Interaction: Childhood maltreatment predicted adult depression only among individuals carrying an s-allele
