PSY210H1 Lecture Notes - Lecture 5: Twin, Twin Study, Heritability
Lecture 5 PSYA02 Notes
•Intelligence = general cognitive ability; Spearman’s g, the substantial
covariation among diverse measures of cognitive ability; the general
factor that typically accounts for about 40% of the total variance of
diverse cognitive tests
•More is now known about the genetics of individual differences in
intelligence than about any other trait, behavioral or biological
•Heritability = the proportion of observed trait variance in a group of
individuals that can be accounted for by genetic variance
•At the level of the individual there is no nature-nurture debate; only at
the population level can we disentangle genetic and environmental
•Indexed by heritability quotient range from 0 to 1. 1 means trait is
influenced solely by inheritance and 0 means trait is not influenced by
•Out of 83 births, approx. 1 pair of twins are born.
•Are identical (MZ) twins more similar to each other than fraternal (DZ)
•If twins are no more similar than fraternal twins, then genetics does not
play a significant role in heritability and so on.
•Heritability = 2(rmz – rdz)
•One limitation of twin studies is their assumption of equal environments
•To whom are adopted children more similar — their biological or
•If a child resembles adoptive parent more than biological parent than
there is an environmental role in the determination of an individuals
•Adoption studies circumvent the assumption of equal environments
•One limitation of adoption studies is their assumption of unequal
•In studies of more than 10,000 twin pairs:
•MZ twin r = .86
•DZ twin r = .60
•MZ twins remain alike whether they are raised together or raised apart!
•Heritability = 50%
•Shared environment is important, too:
•Parent-Offspring r = .19
•Sibling-Sibling r = .32
•Environments produce similarities b/n people with no genetic similarity
•The Nature of Nurture
•When used as outcome measures in genetic research, measures of the
environment consistently show some genetic influence (genes shape the
•Why? Because our genes influence how we react and interact with the
environment (genotype-environment correlation)
•Genes influence our traits which influence our behaviors which have an
influence on our environments.
•Environments are systematically correlated with our traits.
•Example: genetic influences account for about 40% of the variance in
scores from the Home Observation for Measurement of the Environment
•The heritability of g increases during development:
•Adulthood (≈ 80%)
•Heritability quotient is not a fixed identity and the heritability of a
trait varies over age.
•Why Does Heritability Increase?
•Completely new genes come to affect intelligence as more
sophisticated cognitive processes come online during
•Small genetic effects snowball across the life span, as individuals
select or create environments that foster their genetic
•Genetic factors account for about half of the observed variation in
general cognitive ability
•Environments matter too, and show some genetic influences (genotype-
•The heritability of intelligence increases across the life span, from 20%
in infancy to 40% in childhood to 80% in adulthood