Biological Approaches
Kretschmer (1925)
Sheldon (1942)
Somatotype – degree of body type
1. Endomorphy – plump, soft,
2. Ectomorphy – slender, bookworms
3. Mesomoprhs – muscular, large-boned
Sheldon’s research
• 200 men, assessed personality and body
type
• Found:
endomorphs – relaxed, social, warm
Mesomophs – competitive, bold, energetic, assertive
Ectomorphs – restrained, inhibited, avoidant
• Problems?
Role of inheritance in personality
• Behavioral genetics – study of genetic influences
on behavioral qualities; also environmental
influences
• “Nature and Nurture”
– Nature = genetics
– Nurture = all environmental factors
Prenatal nutrition, exposure to drugs
 Friends, neighborhood
 Parenting, etc.

Methods to study behavioral
genetics
1. Selective Breeding
• Thoroughbred horses – winners
• Dog breeds
• Mice – qualitatively different responses to alcohol
Methods to study behavioral
genetics cont’
2. Twin Studies
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monozygotic twins – one egg is fertilized, then zygote
splits into two, share same genetic material (MZ)
Dizygotic twins – two eggs are fertilized with separate
sperm; share about 50%; same as siblings (DZ)
Can compare MZ & DZ twins raised separately and
together
Heritability estimate = amount of variance accounted for
by inheritance for the trait under study
Twin Studies
• Similarity in twins reared together is due to
•
genes and environment
Similarity in twins reared apart is due to genes
only because they don’t share experiences.
Minnesota Twin Study
8000 pairs to twins 1936 – 1955
1200 pairs of twins 1971 – 1981
8400 pairs of twins 1983 - 1990
Twin Studies of personality
• Estimates that 41% of individual
differences in extraversion due to genetics
• Estimates that 31% of individual
differences in neuroticism due to genetics
• Averaged personality: 30 – 50%
Twin Studies
• 40% of Big Five due to
genetics
• 25 % of Big Five due to
nonshared environment
• 25% of Big Five due to
shared environment
Genetics
Shared
Enviro
Nonshared
Nonenviro.
Personality
40 -50%
010%
40- 50%
Intelligence
50 –
70%
0–
10%
20 – 30%
Twin Studies
**assumption: co-twins who are raised
together experience similar environments
But MZ twins may be treated more similarly
than DZ twins
Also, some separated twin pairs are raised
by family members (i.e. overlap in
environment?)
Temperament
Buss & Plomin (1984)
Temperaments = inherited personality traits that are present early in childhood
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activity level
Sociability
Emotionality
Larsen & Diener (1985, 1987)
4.
Impulsivity/aggression
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Caution: possible rater bias
Good description of general style, but not strongly predictive of specific
behaviors – probably need more specific traits
• These findings may overemphasis gender
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differences. There are many similarities in what
both genders seek in partners. Both genders
look for:
good sense of humour
pleasing personality
agreeableness
emotional stability
kindness
lovingness
Inhibited and uninhibited children
Kagan (1994, 1999)
high-reactive: crying, arching back,
unhappy responsive to novel stimuli
(20%)
low-reactive: calm to novel (40%)
Aggression
• Rushton, Fulker, Neale, Nias and Eysenck (1986)
• 500+ MZ and DZ twin pairs
• altruism, empathy, nurturance, assertiveness and aggressiveness
•
•
•
and
found high correlations for MZ twins for each personality variable
Estimates of 50% heredity for aggression
• Monozygotic twins were found to be more similar than dyzygotic
twins on aggressiveness (for MZ twins, r = .40 and for DZ twins, r
= .04).
• nearly 50% of the variance for each personality variable was due to
hereditary causes.
Summary
1. Personality traits: MZ around .5, about
2.
3.
4.
5.
.3 for DZ
Twin studies and FFM
Parallels between temperaments and
FFM
Shared envinronment
Nonshared environment
**importance of
Interaction between genes and
environment
1. Same environmental experience has different
effects on individuals
2. Individuals with different genetic make-ups
evoke different responses from the
environment
3. Individuals with different genetic make-ups
select and create different environment