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Lecture 7

PSYC 2P25 Lecture Notes - Lecture 7: Prenatal Development, Trait Theory, Guppy

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Michael Ashton

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Heredity and Environment- continued
Conclusions from h2 studies:
1. MZ twins have quite similar personalities, mainly due to heredity (mostly additive)
2. Overall, DZ twins and/or nontwin siblings are only modestly similar in personality, but the
similarity is mostly due to additive genetic influences not the shared (common) environment
parental SES, values, child-rearing style, etc. seem to have little effect on personality traits!
3. Differences between siblings are due to…
a. Additive genetic influences (not much nonadditive), and non-shared environmental
influences (i.e., ones that affect siblings differently this can include random events in
prenatal development).
Results indicate that common environment has a weak effect on personality.
However, this is influence of normal differences in the family environment on individual
differences in personality within a population or community.
If entire community has high (or low) level of some variable
(e.g., % young people, % single mothers, crime rate, income inequality, etc.)…
This might influence the personality trait levels of of persons who grow up in that community.
Some Possible Within-Family Environment Effects
Birth Order
Sulloway (1996) reported rs of .30-.40 between “birth order” and Openness to Experience:
o i.e., younger siblings liberal, nonconforming, risk-taking, have wide interests;
Origin (?):
o older siblings tend to identify with authority/ parents/status quo, but younger siblings
want change
o also, older siblings bigger/stronger/smarter, so younger siblings need to try diverse
activities, or go to new places.
Historical Findings (as claimed by Sulloway):
o Scientists with “radical” theories tend to be younger siblings
o Political/religious revolutionaries tend to be younger siblings
o (But how to decide who is radical/revolutionary?)
However, recent results show modest effects...
Jefferson et al. (1998) obtained self-, peer-, and spouse-reports of people’s personalities..
In peer ratings (but only peer ratings), later-borns were slightly more Extraverted, Agreeable and
Open to Experience than were first-borns, but rs not large (in .20s).
Partly consistent with Sulloway’s theory, but weak and inconsistent effect not much link
between birth order and personality.
Different Treatment by Parents & Different Peers
Two other variables that differ for siblings are (1) parental treatment, and (2)
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