Personality Psychology: BIOLOGICAL APPROACHES

MBB2

Comte’s ‘hierarchy of sciences’

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Biological approaches

• Explanations of the biological basis of personality operate at several levels, from “distal” to “proximal”

  • Genetics

  • Brain functioning

    • Neural systems

    • Neural structures

    • Neurochemicals

  • Hormonal factors

Genetics of personality

• Is personality inherited?

• DNA as source of our similarities & differences

  • ~20,000 protein-coding genes

  • ~3,000,000,000 DNA base pairs

  • Most DNA is shared between people: ~99.6% is identical between any two people

• To what extent does this DNA variation underpin variations in personality?

Research designs: Family studies

• Examine resemblance between family members as a function of genetic relatedness

  • 50%: child, parent, sibling

  • 25%: grandchild, grandparent, half-sibling, uncle, aunt, nephew, niece

• Greater resemblance for closer relations implies genetic contribution

• BUT: genetic contributions are confounded with shared environmental contributions

Research designs: Twin studies

• Compare resemblance between monozygotic (MZ) & dizygotic (DZ) twins

• MZ twins are 100% related, DZ twins 50% related

• Greater resemblance for MZ twins implies genetic contribution

• Environments are equally similar for both kinds of twin, so environmental factors are not confounded

• BUT: possibility of more similar environments for MZ twins, & perhaps twins are unrepresentative

Correlation

‘Resemblance’ is indexed by the correlation coefficient

For example …

Recent twin study of singing

Summary of 2748 twin studies (1958-2012)

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Research designs: Adoption studies

• Compare resemblance of adopted children to adoptive parents (APs) & biological parents (BPs)

• APs are 0% related but supply environment, BPs are 50% related

• Degree of resemblance to APs & BPs shows environmental & genetic contributions

• BUT: adoption must occur early; problem of selective placement; biological mother provides prenatal environment as well as genes

Variance components

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Example (IQ)

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Heritability

• Behavioural genetic studies yield estimates of heritability = proportion of variance in the trait accounted for by genes • e.g., .8 for height, .7 for weight, .5 for maths aptitude '

• Most personality attributes show heritabilities from .3 to .5

• This is even true for apparently purely learned attributes (e.g., political attitudes, vocational interests)

Example correlations (Loehlin, 1992)

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Estimating heritability

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Important caveats

• Substantial heritability of personality does not mean that parents and children are always similar in their personality traits

• Heritability relates to variation within a population: it says nothing about genetic contribution to any individual’s personality

• Heritability does not imply that personality is fixed

• A substantial heritability can coexist with substantial environmental contributions to personality

The role of the environment

• Behavioural genetics clarifies the role of the environment

• ‘Shared’ environmental influences (those that make siblings more similar) tend to be weak

  • e.g., parental education, neighbourhood, class, ethnicity, diet

• ‘Non-shared’ environment influences (those that make siblings more different) tend to be much stronger

  • e.g., illnesses, friend groups, differential treatment by parents

• Environmental factors can themselves be genetically influenced

  • e.g., susceptibility to accidents & other life events

Genetic & environmental contributions to the Big 5 (from Riemann et al., 1997)

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Shared versus non-shared environment

• “salient environmental effects do not make siblings similar. That is, they are not shared by children growing up in the same family.”

• “theories of socialization had assumed that children’s environments are doled out on a family-by-family basis. In contrast, the point of non-shared environment is that environments are doled out on a child-by-child basis

Specific personality-related genes

• Heritability says nothing about specific genes or genetic mechanisms

• Several specific genes have been identified in candidate gene studies, but they do not replicate

• e.g., Novelty-seeking & dopamine sensitivity

• More recent research, surveying the entire genome with huge samples, finds few replicable personality genes

• Most traits appear to be influenced by hundreds of genes, each with very small effect

Dimensions, again

What you get when you add many small influences is a phenotype that looks like this …

Brain functioning: Systems

• Eysenck’s theory

• Neuroticism & limbic system reactivity

• Gray’s theory

Eysenck’s theory

• Extraversion & low cortical arousal

  • Leads to desire for stimulation (e.g., novelty, excitement)

Neuroticism & limbic system reactivity

• Leads to greater autonomic NS arousal to threat & stress

Gray’s theory

• Impulsivity & “behavioural activation system” (BAS)

  • Linked to sensitivity to reward & pleasure

  • Associated with a tendency to approach rewards

• Anxiety & “behavioural inhibition system” (BIS)

  • Linked to sensitivity to punishment & pain

  • Associated with a tendency to avoid punishments

Brain functioning: Structures

• Some links have between found between Big 5 & the size (volume) of brain structures

  • Extraversion with a region involved in processing reward information

  • Agreeableness with regions that process information about other people’s intentions and mental states

• However, a recent meta-analysis suggests these links are questionable

• A more promising direction is exploring patterns of functional activity and connectivity in the brain

For example …

• Neuroticism

• Extraversion

Neuroticism

• Associated with high resting activity of the amygdala

• Greater neural response to errors in dorsal anterior cingulate cortex (involved in conflict monitoring)

Extraversion

• Greater neural response to experiences of reward in frontal brain region

Brain functioning: Chemicals

• Personality factors may be associated with neurotransmitter activity in the brain

• Extraversion & dopamine levels

  • Exploration, approach & incentive motivation

• Neuroticism & norepinephrine levels

  • Negative emotion, vigilance for threat, cautiousness: ‘neurobehavioural warning system’

• Agreeableness & opioids

  • Attachment processes

• Constraint & serotonin levels

  • Inhibition of emotional response, low impulsiveness; low serotonin → aggression & emotional instability

Hormonal factors

• There is evidence that personality is influenced by prenatal exposure to sex hormones

• Ratio of 2nd (index) to 4th (ring) finger (2D:4D) is associated with testosterone exposure

• Lower ratio in men than women, especially on right hand

• i.e., men tend to have longer ring finger than pointer finger

• Among men, lower 2D:4D ratio correlates with:

  • Physical aggression

  • More stereotypically ‘masculine’ career interests (realistic & enterprising)

  • Less stereotypically feminine gender role

• In women, lower 2D:4D ratio correlates with:

  • More indirect aggression (spreading rumours, malicious humour, excluding people)

  • More stereotypically ‘masculine’ interests (enterprising, less social)

Example study (Benderlioglu & Nelson, 2004)

• Examined reactive aggression in women

• Participants were asked to raise money for a fictitious charity by making calls

• Calls went to kind but non-donating or hostile confederates

• Hostility assessed by how hard phone was put down & by tone of follow-up letter

• Women with lower 2D:4D were more hostile