PSYCH 2B03 Lecture Notes - Lecture 9: Confounding, Phenotype, Zygote

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10 Jun 2018
Biological Bases of Personality
Lecture 9
Type and trait approach emphasize a biological basis of personality moderately
emphasized by psychodynamic approach (id, ego, superego, archetypes) humanist
approach (physiological motivation, cognitive/aesthetic needs, actualizing tendency)
disappears in cognitive approaches
Factors Involved in Personality
Three variables contribute to personality and personality development
Shared environment environmental and experiential factors. Things that we share w/
others. We share experiences & physical place w/ our siblings in a home. We share the
same geographic environment w/ our neighbours. We share the same teachers & school
environment w/ our classmates
Non-shared environment things that each of us experience individually and don’t share
with anyone. The friends we have are different from our siblings. Differences in parental
o The bigger & more important part of our environmental experience. What makes
us who we are is mostly due to the non-shared environmental experiences, not the
Families that all have the same interests and views on things can be thought to be
attributed to genetics but it could be b/c they all had the same shared experiences
Phenotype & Genotype
Phenotype a collection of observable and measurable characteristics (e.g., height,
weight, ear shape, body build, personality, IQ)
Genotype the specific collection of genes that is inherited from parents
We look at twin, family, and adoption studies to determine how much of our phenotype is
determined by our genotype
We want to know what role the genotype has in determining one’s personality. We study
twins for this reason b/c they have the same genotypes & they are the same age; this
eliminates the confounding variable of age
o Monozygotic 100% identical twins, developed from a single fertilized zygote
that split
o Dizygotic 50% identical, developed from two separate fertilized eggs. They are
no more similar than siblings
Correlations of a variable (IQ) between monozygotic twins that were raised together vs.
raised apart
o Assumes r=1.00 when IQ is 100% determined by genetics
o The closer the correlation is to 1.00, the more likely IQ is determined by genetics
Correlations of variable between monozygotic twins reared together vs. apart compared
to dizygotic twins reared together vs. apart
o Assuming IQ is 100% determined by genetics
MZ: r=1.00 for both reared together & apart
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DZ: r=0.50 for both reared together & apart
Family studies looks at genetic similarity between different members of the family
Adoption studies looks at similarity between adoptive parents and biological parents. If
we assume phenotypic characteristics are determined by genes, there should be 0
correlation b/w adoptive parents and child
Problems with Studies
DZ twins confused as MZ dizygotic twins can be both same or different sex. When
they’re the same sex and happen to resemble each other strongly, they may be mistaken
for MZ twins but their genomes are not identical. Only problem that makes us
underestimate importance of genes
Selective placement when identical twins are placed in different homes (which doesn’t
happen often), they’re placed in homes similar to biological background (same race, SES,
etc.) they’re not reared in uncorrelated environments
o They may be listed as reared-apart twins but they’re being reared by very similar
Differences b/w MZ and DZ twins in environmental similarity MZ twins are often
reared in environments that emphasize their similarity (parents will dress them the same,
enroll them in the same activities, buy them the same gifts) but DZ twins reared to
emphasize their individuality
o MZ twins have increased phenotypic similarity while DZ twins have decreased
similarity d/t the difference in emphasis
o May not be due entirely to genetics, but the similarity of their environment in
which they’re reared
Assortive mating when people mate with those who have similar genes to them (e.g.,
intelligence. People of very high IQ almost never marry those w/ very low IQ & vice-
versa d/t opportunities)
o With random mating, individuals share no genes in common. The child will
receive 50% of genes from mom and 50% from dad
o When there is overlap in genes w/ assortive mating, child will receive genes that
both mom and dad possessed. They will receive 70% of genes from each parent
so they are more genetically similar to their parents
Distributions and Variance
Variance in life experiences (VE) and genetic variances (VG) contribute to differences in
some phenotypic characteristic. VT = VE + VG
Heritability coefficient most widely-used measure for determining the total variance
associated w/ genetic variance in the sample
o h2 = VG / VT (the ratio of genetic variance to total variance)
o Other formulas for calculating the heritability coefficient using:
h2 = rmza used when data consists of correlation between pairs of MZ
twins reared apart (mza). This should yield the highest heritability
h2 = 2rdza used for DZ twins reared apart (dza). Have to double
coefficient b/c impact needs to be doubled since they’re only 50% similar
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h2 = 2(rmz rza) used when data consists of both correlations b/w pairs of
MZ twins and DZ twins; both pairs must be reared in the same way
(together or apart)
o Applies only to groups, not individuals tells us how much of total variability in
some phenotypic characteristic can be attributed to genetic variability within our
sample. Will not work for individuals
It looks at the source of variation between individuals
Ex. Two people w/ differing heights (4ft and 6ft). Assuming 50%
heritability coefficient (height is determined half by genes, half by
environment). The difference in heights (2ft) is half d/t genes and half d/t
o Varies from population to population, over time it is not a constant. It is
different from time to time and across different populations
The heritability coefficient for IQ is higher today than 50 years ago b/c
genetic variability is higher than environmental
There is now less environmental variation b/c everyone is getting the same
healthcare, education, living environments, etc.
Genes are making the same contribution to IQ now and 50 years ago but
it’s not about individuals; it’s about group variance
Heritability coefficient is higher in places where there is more
environmental variability (e.g. countries where there are some people who
are very poor and some who are very rich) or genetic variability (countries
w/ high immigrant populations more genetic variability)
o Valid only if measures used to calculate it are valid depends on how valid your
phenotypic measures
o One’s characteristic behavioural style, particularly those having to do w/
reactivity and energy level
o Consistent response disposition that individuals bring to various situations that
give their behaviour its characteristic form
o Presumably inherited, evident early in life and observable in variety of situations
o Probably influences personality development
Thomas, Chess & Birch (1970) one of the first attempts to define early measures of
o Looked at infants and rated them on 9 characteristics (not all are shown):
Level of motor activity how much time the spent physically moving
around, moving their legs, etc.
Positive response to new objects some infants will cry and others will
laugh w/ new things
Regularity in biological functions some infants wake & sleep at the same
time, regular & predictable in biological functions
Friendliness/good mood some are happy, smiling all the time while
others are grumpy & crying more of the time
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