CHAPTER SIX – Genetics
Genotype: genetic makeup that codes for a specific trait
Behavioural genetics: the study of the genetic and environmental contributions to
individual difference in personality and behaviour
Nature and Nurture as Allies
Certain environments have different effects on people depending on their specific
genetic makeup = genotype-environment interaction
People change environments and environments change people, making it hard to find
the impact of genes on environment = genotype-environment correlation
Phenotype: manifestation of genes, the observable physical or psychological trait that is
coded by genes
Phenotype = genotype + environment + gene-environment correlation + gene-
Heritability (h ): amount of observed individual differences in some characteristic that
can be accounted for by genetic differences. Refers to differences across a
group/population, not an individual.
Refers to the inheritance of a particular trait in a particular environment at a
Environmentality (e ): estimates the extent to which observed individual differences can
be traced in any way to individual differences in environment
As a general rule: the greater the heritability of a characteristic, the less the
environmentality. And vice versa.
There are higher heritability estimates in equalizing environments (environments that
are the same for everybody) and lower estimates in more variable environments. This is
because, in equalizing environments, everybody has access to resources in rich
environments there is little variance in the effect that environment can have. Shared and Nonshared Environments
Shared environments: aspects of the family environment that are generally the same for
all children in the household, including physical/psychological/social aspects.
Physical: type of dwelling; number of computer or books; etc.
Psychological: home atmosphere; parenting practices; psychopathology; etc.
Social: socioeconomic status; family structure; etc.
Nonshared environments: includes experiences that relatives have which make them
different from one another.
For personality, most of environmental influence is of the nonshared variety. When
family members resemble each other, it is due more to heritability than to shared
Children growing up in the same family are not more similar than children
growing up in different environments.
Family environments may actually make children different from one another or maybe
researchers just haven‟t identified specific aspects of the environment that are shared.
Still, parents may handle children differently based on their personality, creating unique
environments for each child.
Monozygotic (MZ): identical twins, exact genetic duplicates of each other.
Dizygotic (DZ): fraternal twins, occurs when two zygotes develop in utero at the same.
They are really no more alike than any other sibling, sharing 50% of their genes.
One measure of heritability is to calculate the correlation (r) between twins on a given
trait and compare the correlation between MZ and DZ twins.
h = 2(r mz – dz)
Another way is to compare identical twins who have been raised in separate
environments. = MZA twins: monozygotic twins raised apart.
h = r mza
This method does have limitations. Selective placement: If the identical twins are
placed in similar environments, this may increase the similarity between them,
artificially inflating the heritability estimate. Also have to assume that families that adopt aren‟t different than families that
don‟t adopt. If there‟s something special about adoptive families, and
environmental effect will get labeled as a genetic effect.
Equal environments assumption: assumes that people have not treated MZ twins more
alike than DZ twins. If MZ twins are treated more alike, they may score more similarly
on a particular trait, artificially inflating the heritability estimate.
Applies only to similar treatment that is related to the specific characteristics
Assumption of representativeness: assumes that the twins are typical of the population
Heritability of Common Personality Characteristics
The variance in personality traits typically breaks down as:
Observed differences in personality traits = 40% genetics + 0% shared
environment + 40% nonshared environment + 20% error
Identical twins are very similar in the five-factor traits, even when raised apart =
moderately strong genetic component to the traits.
The heritability of the five factors is in the 0.41 to 0.50 range, indicating that 41-
50% of the variation in these traits is due to genetic factors.
Identical twins that were raised together have the highest correlations (even fraternal
twins who are raised together look a little similar). This suggests that there is a
moderate effect of the environment. The environment (shared and nonshared) accounts
for about 47-53% of the variance in the trait. The shared environment accounts for only
about 8-17% of the variation in the traits.
Science of genetics
Alleles: different forms of the same gene
Mendelian inheritance: inheritance patterns where one trait dominates over another
99.9% of the human DNA sequence is