Genotypes: The genetic makeup that codes for a specific trait.
Behavioural genetics is the study of the genetic and environmental
contributions to individual differences in personality and behaviour.
Nature and Nurture as Allies
We call the combination of nature and nurture a genotype-environment
What if people change environments and environments change people, so
that we really can’t separate the impact of genes on environment? This is
called a genotype-environment correlation.
In fact, some researchers advocate an even more complicated equation for
understanding the manifestation of complex human traits – a concept known
Phenotype = Genotype + Environment + Gene-environment correlation + Gene-
Genes and Environment as Co-actors
Heritability (h ) is the amount of observed individual differences in some
characteristic that can be accounted for by genetic differences. Heritability
refers to differences across a group or population of people- not to a specific
Heritability therefore, refers to the inheritance of a particular trait in a
particular population at a particular time, so sometimes heritability
estimates differ depending on the exact sample and methods used by
Environmentality (e ) estimates the extent to which observed individual
differences can be traced in any way to individual differences in
Shared and Nonshared Environments
One way of thinking about environmental influences on characteristics such
as height or personality is to identify aspects of the environment that are
shared and unshared among relatives living together in a household. Shared environments includes aspects of the family environment that are
generally the same for all the children in the household including physical,
psychological, and social aspects.
In contrast, the nonshared environment includes experiences that relative
have which make them different from one another. This may include unique
experiences within the family, or outside the family. When it comes to
personality most of the environmental influence ends up being of the
One measure of heritability is to calculate the correlation (r) between twins
on a given trait and compare the correlation between MZ twins and DZ twins.
The exact formula is to double the difference between these correlations or
h = 2(r mz– rdz
A second way of estimating heritability is to compare identical twins who
have been raised in separate environments. We call these kinds of twins MZA
twins (monozygotic twins raised apart). If such twins score similarly in a
trait such as Extraversion, then we know that Extraversion has a strong
genetic component. Studies of MZA twins are particularly powerful in
disentangling the effects of gens and environment because they have
identical genes but different environments. Here is a second formula for
h = r MZA
The double-the-difference method assumes that twins were reared under
equal environments. That is, it assumes that people have not treated MZ
twins more alike than DZ twins.
This equal environments assumption applies only to similar treatment that is
related to the specific characteristics under study. For example, people often
dress their twins alike in identical sailor suits or matching dresses. This
probably happens more often to MZ twins than to DZ twins. Of we were
studying something like fashion sense, then this would violate our equal
environment assumption. But unless wearing matching outfits, affects a
specific personality characteristic like shyness, then this assumptions still
The double-the-difference formula also assumes that twins are typical of the
population. We call this the assumption of representativeness.
Twins may be more alike on a certain characteristic due to selective
placement during the adoption process and not their genetics. Selective
placement makes it impossible to see the effect of genetics apart from the
effect of environment because it confounds the two.
Research Methods Illustrated: Correlational Designs I: The Logic of Adoption and
Twin Studies When two variables are related, there are always at least three possible
explanations of the findings. First, it’s possible that one variable causes the
other, like being around the same people all the time cause people to develop
a similar sense of humor. Second, it’s also possible that the second variable
causes the first one, like people who have a similar sense of humor spend
more time hanging around each other. Finally, it’s also possible that some
third variable – like genetics – causes people to hang around each other and
to find the same jokes funny.
Heritability of Common Personality Characteristics
A solid finding in the research – one that has been well replicated across
many samples and for both self-report and other report – is that virtually all
individual differences in human behaviour including cognitive abilities,
personality, social attitudes, psychological interests, and psychopathology
are moderately heritable.
The variance in personality traits typically breaks down like this:
Observed differences in personality traits = 40% Genetics + 0% Shared environment
+ 40% Nonshared environment + 20% Error.
Then and Now: The Science of Genetics
People believed that inherited characteristics- of people and plants-were
blended. That is, the offspring of a pea plant with wrinkled peas and a plant
with smooth peas would have moderately wrinkled peas. But that is not what
Mendel found. Instead, he found that the next generation was all smooth.
What happened to the genetic information for wrinkly peas? Mendel
reasoned that the trait must still be present in the genotype even though it
was not expressed in the phenotype. If this was true, then the genotype ought
to be passed on to the next generation. Indeed, about 75% of the plants in the
next generation were smooth and 25% were wrinkled.
From such observations, Mendel developed two hypotheses, part of what we
now consider his first law of inheritance.
First, each parent plant passes on one form of the gene (which he called
element) for a given characteristic to its offspring, who get two forms of the
gene, one from each parent (note that different forms of the same gene is
called alleles). These two alleles can either be the same or different. When
the alleles are different, one characteristic will be dominant over the other.
However, both alleles will be passed on to the next generation.
This concept of dominance explains the pattern of seeds Mendel observed in
successive generations. In fact, such inheritance pattern where one trait
dominates over the other is called Mendelian inheritance and Model is now
known as the founder of modern genetics. Epigenetics: The study of how the environment changes the function of
genes without changing the genes themselves.
We know that a gene is a sequence of DNA that codes for a specific trait.
Genes are composed of coding regions called exons and noncoding regions
called introns. Of the 3.3 billion base pairs of DNA in the human genome, only
about 2 to 3% are functioning genes. The remainder of the DNA – nearly 2
meters of it – was once thought to do nothing because it occurs outside
genes. This so-called junk DNA is actuall