PSYB30H3 Lecture Notes - Lecture 7: Heredity, Extraversion And Introversion, Twin
ProfessorMarc A Fournier
Lecture 6 - Feb 10!
Continuity of personality - do individual differences in some attribute predict individual
differences in that same attribute at some later point in time (X1 —> X2)!
!- if you’re stupid when you’re young will you be stupid when you’re older?!
Coherence of personality - do individual differences in some attribute predict individual
differences in some other attribute at some later point in time (Y1 —> Z2)!
!- if you’re stupid when you’re young will you be unsuccessful when you’re older?!
Consistency of individual differences - ones relative position or rank order within a population of
individuals over time!
Differential continuity is independent of normative changes in trait levels ex; height, intelligence!
!- kids who are tall when 5 will grow up to be taller compared to the average!
How early in life can we see personality consistency? When does personality become set in
!- Freud age 5-6!
!- James age 30!
Roberts and DelVecchio - meta analyzed ﬁndings from 152 longitudinal studies involving a total
of 3217 consistency coefﬁcients and 50, 207 participants. !
1. consistency increases as a function of age. The older you get the more stable you are.
Holding time interval constant at 6.7 years consistency coefﬁcients equaled:!
.31 in infancy!
.43 in childhood!
.54 in college!
.64 in 30’s!
.74 in 50’s!
ex; there is a .74 consistency relationship between how you are at age 45 and 51.6!
2. consistency decreased as a function of time interval. The more time you let pass, the more
evidence of change you’ll see. Holding age constant at 20 years, consistency coefﬁcients
.55 over 1 year!
.52 over 5 year!
.49 over 10 year!
.41 over 20 year!
.25 over 40 year!
ex; From age 20 to 21, there would be a correlation of .55. From age 20 to 30, it’s .49.!
The differential continuity of traits emerges in childhood and peaks around the age of 50 but not
at a level high enough to conclude that ours traits at some point become set in stone. The later
in life you look, the more solidiﬁed they become BUT even then there is room for change.!
Coherence of personality - the inferred consistency of some underlying attribute despite
observed change in its behavioural expression. The way the trait appears and what you need to
measure may be different at different points in life. The investigator must have a theory of how
the diversity of behaviour observed across the life course can be said to belong to the same
equivalence class. !
Berkeley guidance study!
-a study initiated in 1928 with every third birth in Berkley over a period of one and a half years!
-Mothers were interviewed when subjects were ages 8-10; life outcomes were collected when
subjects were ages 30 and 40!
Moving against the world: life course patterns of explosive children!
-Caspi, Elder and Bem: frequency and severity of temper tantrums in late childhood predicted:!
-in men: deterioration in SES, erratic work lives and divorce!
-in women: marriage to men of lower SES, divorce and inadequate parenting!
Jim springer and Jim lewis were identical twins separated at birth who met for the 1st time at 39
years of age!
-their striking similarities in terms of personality and life outcomes raise the intriguing question-
to what extent is personality genetically determined?!
-both Jims were 180 pounds, 6 ft tall, married twice, ﬁrst wives named Linda and second wife
named Betty, both had a son named James, both part time sherif.!
-one was a better writer, one was a better speaker, one combed their hair forward the other
-they shared no common environment yet they had some sort of overlap which must be
Genome - complete set of genes that an organism possesses!
-human genome contains 20,000 and 25, 000!
-genes are located on 23 chromosomal pairs, 1 pair from each parent!
Heritability - the degree to which the genetic differences between individuals cause the
individual differences on a given characteristics !
-the proportion of observed variance in a group of individuals that can be accounted for by
Misconceptions about heritability!
1. They are precise. No they aren’t. There’s no reliable indicator. As part of estimating
heritability, you have to measure peoples traits. Measurements are never perfect. Try to
make sure there is minimal error but there will always be error so there will always be error
when measuring the heritability.!
2. Stay the same over time. No they don’t. They can be stronger at some times and weaker at
other times. !
3. Can be applied to a single person. No they can’t. Heritability is a sample level characteristic
and can not be applied at the level of the individual.!
Nature vs nurture debate!
At the level of the individual, there is no debate between genes and environments. A person can
not exist without both. Individuals can not exist in the absence of the environment or without a
genome. Only at the population level can we disentangle genetic and environmental effects.!
-correlate the degree of genetic overlap with the degree of personality similarity!
-capitalize on the fact that there are known degrees of genetic overlap among family members!
-confound: family members who share more genes in common may also share more similar
environments. Genes and environment are correlated; the more genetic overlap, the more
likely you are to live with them!
-if personality characteristic is heritable, then family members that have more genetic overlap
should be more similar to each other.!
Twin studies - are identical twins more similar to each other than fraternal twins?!
heritability = 2(rmz - rdz)!
-Assumption in twin studies; equal environments but difference in genetic makeup. !
Adoption studies - to whom are adopted children more similar? !
-adoption studies circumvent the assumption of equal environment!
-potential problem: representativeness (assuming that the people under studies are
representative of the general population which is NOT true because they have to be very well
off, smart etc), selective placement!
Family studies < Twin studies < Adoption studies (strongest) < Adoption studies of twins
The MZ correlation is almost twice the DZ correlation!
MZ are usually more then twice as large as DZ correlation!
Less genetic inﬂuence on personality - upper limit of heritability estimates 30% for extraversion,
15% for neuroticism!
Less environmental inﬂuence on personality - correlation between adoptive parents and their
adopted children tend to be around zero. !!
Identical twins are more similar to each other than expected - more then twice as similar as
fraternal twins (double the genetics but more then double the similarity)!
Biological siblings are less similar to each other than expected. Slightly more similar than
In nonlinear/nonadditive terms (curved logarithm)!
-a heredity estimate of 100% is more than twice as large as an estimate of 50%!
-a heredity estimate of 50% is in fact not much larger than an estimate of 0%!
If the heritability of personality traits is 40% what accounts for the remaining 60% of the trait
-measurement error is 20% so the remaining 40% the trait variance should be environmental