PSY230H5 Lecture Notes - Twin Study, Heritability
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Lecture 12&14-genetics and envt
- To find out whether any similarities among relatives are due to the environment
or heredity, the two effects are separated and studied
1) Biologically related relatives who do not live in the same household
2) Adoptive relatives who are not biologically related
- Pairs of relatives are used in the study; two brothers, a mother a daughter,
opposite sexed twins.
- They fill out a personality questionnaire, or observer reports are done.
- The results are studied all together and compared. The variance is determined and
the researcher checks to see if variance is due to WITHIN FAMILY variation or
BETWEEN FAMILY variation. This is done through a technique called ANOVA
• Intraclass correlation coefficient: to figure out how similar relatives are,
researchers calculate the proportion of variance due to between-family
- Researchers often use twins to study the nature vs. Nurture debate
- When they are comparing the variance in a trait between two non identical twins,
non twin siblings, or mother and child, then they double the correlation found by
2, because they only share 50% of their genes. This doubling is not done with
identical twins because they share 100% of their genes.
- Even after the doubling, the correlation because non identical twins is usually less
than that of identical twins
• Additive genetic effect: genes working together contribute to personality
traits. Each gene’s effect will either lower or higher the level of a trait. The
combined effect of the genes can be estimated by adding together their
• Non-additive/multiplicative genetic effect: for example let’s say that genes a
and b are very rare, so if a person has BOTH genes, then their level of a
certain trait will be very high, but if they only have a or b, then the level will
not be even remotely high. Both genes need to be present for a increasing
effect to take place.
- Comparing similarity of difference kinds of relatives: study the variations in traits
between identical and fraternal twins raised together.
- Ex: correlation between identical twins was 0.7 and for fraternal twins it was 0.4.
Therefore 70-40=30. Multiply 30 x 2 because of the 50% genetic difference.
Therefore there is 60% genetic influence on a trait. REVIEW PAGE 120.
- There are two types of environments that people living in the same household
• Shared environment: everything that happens in the house that everyone
• Non shared or unique: these are difference experiences that are unique to the
individual. Their separate set of friends, the unique way they are treated by
their parents, etc.
- When the researcher wants to measure the variance in traits as an affect of the
shared environment, they will use pairs of relatives who are not biologically
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Lecture 12&14-genetics and envt
related but live in the same household and measure the variance. Genetic
influence cannot be responsible.
- There are two ways in which we can account for how much effect the unique
environment has on the individual
• Determine how much of the trait has already been influenced by genetics
(additive or non additive) and the shared environment, then the left over is
caused by the unique environment.
• Researcher can measure how much certain aspects of the two individuals
differ, such as parental treatment or group of friends, then see how much this
affects a certain trait
- Studies have shown that genetics have 50% influence on personality (30%
additive and 15% non additive) and the shared environment only accounts for
about 5-10% of variation seen in traits. Error of measurement accounts of 20%.
The remainder is influenced by the unique environment.
- Are relatives personalities really measured independently?
- Measured independently means that the results have not been influenced by other
- For example, if you and your sibling are doing a personality questionnaire, you
may tend to compare yourself to your sibling while answering, and over estimate
of underestimate a trait while comparing yourself.
- Or, if an observer report is being done on two people, the observer may compare
the two and over estimate or underestimate the traits of the two relatives, making
the researcher believe that the two are more different than they actually are.
- This is referred to as “contrast effect” – the tendency to emphasize differences
between related persons
- The opposite of this is “Assimilation effect” – the tendency to emphasize
similarities between two related people
- Experiments done showed that contrast effect is more heavily seen in fraternal
twins than identical twins.
• Assortive mating: there is not tendency for parents to be similar in their levels
of a trait. They are not more similar than two random people.
- The research assumes that there is no correlation between parents for a certain
trait; otherwise the offspring would come out almost identical to their parents in
- The features of the womb environment can constitute aspects both of the common
environment and also the unique environment
• Selective placement: placing an adoptive child into a similar household/family
of that of their biological family.
• Equal environment assumption: assumption that the greater similarity of
identical twins is due to their greater genetic similarity and not due to any
greater similarity of their environment.
- Similarity in personality of identical twins is NOT due to similar treatment. See
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