Personality Theory

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Personality Theory

• Chapter 16 Genes, Behaviour, and Personality

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Personality Theory and the Genetics of Behaviour

• Personality theories have not recognized the
  genetics of behaviour until recently.
• They have focused on nurture, not nature.
• Even schizophrenia and the affective disorders
  were regarded as products of experience in the
  family.

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An Exploration of the Modern World of Behaviour
Genetics

• What is a gene?
• Genes are segments of chromosomes that carry an
  element of genetic information and determine
  heredity.
• Genes control development and metabolism of
  organisms.
• Paired chromosomes (23 in the human) determine
  the characteristics of the organism
• smooth or wrinkled shape of garden peas, human
  eye and hair colour.

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• The mid-20th century discovery of the genetic
  code by Watson Crick
• Deoxyribonucleic acid (DNA) is the physical basis
  of the genetic code
• Its molecular structure is a double coil or
  helix.
• 2 twined strands of phosphate and sugar groups
  held in place at a fixed distance from each other
  by nitrogenous base pairs.
• The bases are adenine, thymine, guanine, and
  cytosine. Adenine pairs with thymine. Guanine
  with cytosine.

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• Genetic transcription when cells divide the
  coils of the DNA molecule unwind
• The base pairs separate and a member of each pair
  stays attached to each of the strands.
• The nucleus of a cell holds a base, a deoxyribose
  sugar, and a phosphate.
• These are nucleotides

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• Each nucleotide pairs with a base of the unwound
  helices to form a new strand with each of the
  original strands.
• 2 molecules of DNA result, copies of the
  original.
• Mutations or copying errors are rare (1 in
  several million) and usually impair
  survivability.
• An example the Waltzer mouse

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Darwin and the Inheritance of Traits

• The problematical concept of pangenesis
• Gemmules given off by bodily cells create new
  bodily cells like those of parents
• No variation?
• No adaptation to environment?

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Mendels Basic Experiments
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Huntingtons Disease
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Research Methods

• We rely on 3 research methods to study the
  influence of multiple genes.
• Family studies investigate genetic and
  environmental similarity among family members of
  varying genetic relationship.
• An example The trait of Sociable from the
  Thurstone Temperament Schedule

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The Thurstone Temperament Schedule

• Typical items
• You start work on a new project with a great deal
  of enthusiasm.
• YES or NO
• You are often low in spirits.
• YES or NO
• Most people use politeness to cover up what is
  really cut-throat competition.
• YES or NO

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Thurstones Sociable Trait

• Persons with high scores in this area enjoy the
  company of others, make friends easily, and are
  sympathetic, cooperative, and agreeable in their
  relations with people. Strangers readily tell
  them about their potential troubles.

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Correlations for Trait Sociable from 3 Studies
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The Risk of Schizophrenia In the General
Population and Among Family Members
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Twin Studies

• The idea of studying twins was introduced by Sir
  Francis Galton.
• The essence of the method is to compare identical
  and fraternal twins. Genetic influence would make
  identical twins more similar.
• But how do we control for the possibility that
  identical twins may be treated more alike so that
  some, or even most, of their similarity may be
  environmentally caused?

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• 3 methods
• Compare twins who are mislabelled with
  correctly-labelled twins
• Measure the family environment.
• When we do, identical twins are treated somewhat
  more alike, but not in ways that affect their
  abilities and personality.
• Correlate measures of differences between twins
  with measures of similarity of treatment.

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Testing the Equal Environments Assumption
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Some Twin Study Findings of Genetic Influence
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Identical Twin Correlations on Personality
Variables (from the Minnesota Study of Twins
Reared Apart)
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Adoption Studies

• A simple and uncomplicated design
• There are a couple of potential problems to be
  considered, though.
• The representativeness of adopted children and
  families
• Selective placement placing children in families
  resembling birth parents (mother, mostly)

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• Some adoption heritability findings
• Schizophrenia and depressive disorders
• Risk from birth parents.
• Alcoholism
• 22 per cent of adopted-away sons of alcoholic
  fathers were alcoholic.
• Extraversion and neuroticism
• Heritability is smaller than in twin studies.
• This is likely due to non-additive genetic
  variance (combinations of genes have different
  effects).

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Personality Theory and the Genetics of Behaviour

• Genetic influences greatly outweigh environmental
  effects on personality and development.
• Environmental influences are those of the
  non-shared environment (e.g., birth order, peers,
  teachers).
• These findings have shocked personality and
  developmental psychologists, who regarded the
  family environment as critical.

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• There are shared environment effects
• Juvenile delinquency, aggression, and loving
  relationships are examples.
• Personality and developmental psychologists will
  have to make a place for behaviour genetics in
  their theories and research.

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• Behaviour geneticists will have to refine their
  identification of heritable variables.
• An example divorce, which shows a significant
  genetic influence.
• Is there a divorce gene?
• The complexity introduced by genetic-environment
  interaction.

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Take-Home Messages

• Personality theories have not recognized the
  possibility that behavioural traits might be
  heritable.
• Experience in the family has been considered
  crucial.
• Some basics in genetics
• The gene
• Genes are blueprints for the assembly and
  regulation of proteins.
• The genetic material of chromosomes.

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• DNA is the molecular basis of the genetic code.
  Its molecular structure is a double helix.
• Nucleotides pair with the bases of the unwound
  helices and go on to form a new strand with each
  of the original strands.
• Errors of genetic transcription (mutations)
• An example the Waltzer mouse
• Darwin and an unworkable theory of heritability.

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• Mendelian genetics from experiments on garden
  peas
• A Mendelian single-gene example of a human
  disorder Huntingtons Chorea, is due to a
  single, dominant gene.

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• Research methods used to study polygenic
  inheritance
• Family studies We look for family similarity in
  personality characteristics, an absolute
  essential to continue.
• A primer on correlation
• Twin studies We compare identical and fraternal
  twins on the measures were interested in, and a
  greater similarity of identical twins implicates
  the heritability of the trait.

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• We must disentangle genetic and environmental
  effects, and there are ways to test the equal
  environments assumption.
• A review of some twin-studies research
• The special and dramatic case of identical twins
  reared apart
• Adoption studies a very uncomplicated research
  design
• Some adoption-study findings

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• Personality theory and the genetics of behaviour
• When we compare genetic and shared environment
  influences, genetic influences are far more
  important.
• What does this mean?
• Personality theories and developmental theory
  have to adapt.
• Behaviour genetics needs to identify specifically
  what genes transmit.