Genes and Personality
Within each nucleus in each cell in the body are two complete sets of the human genome, one
from the mother and one from the father.
Still must figure out what the role of gene sequence play in the human body, mind, and
behavior!
Junk DNA, “junk’ DNA, and Riboswitches.
Each individual has similar characteristics and individual characteristics at the same time.
Controversy in the study of the human gene has lead to incredible headlines (novelty seeking
genes) and the biggest worry of all called eugenics.
But what do the findings really mean about genes and behavior?
The goals of behavior genetics are to reveal the contribution (percentage of variance) due to
environmental and genetic causes. It is also to reveal how genes and environment interact.
Heritability is a statistic that refers to the proportion of observed variance in a group of
individuals that can be accounted for by genetics. It describes the degree to which genetic
differences between individuals cause differences in an observed property such as height,
extraversion, or thrill seeking.
These observed individual differences are called phenotypic variance.
Genotypic variance refers to individual differences in the total collection of genes possessed
by each individual.
The environmental component is simply the proportion of phenotypic variance that is not
attributable to genetic variance.
Environmentality (the opposite of heritability) is the percentage of observed variance in a
group of individuals that can be attributed to environmental differences.
As heritability effects increase/environmentality effects decrease (and visa versa).
 Heritability effects are differences in populations not individuals.
 Heritability is NOT a constant. If an environment changes, heritability can change, too.
 Heritability is not a precise statistic.
The Nature-Nurture Debate
At the individual level vs. population level
It does not occur at the individual level but is an issue at the population level.
Behavioral Genetics Methods
 Selective Breeding (choosing or breeding for a specific characteristic)
o May be unethical to conduct on humans
 Family Studies (studies that correlate degree of genetic overlap among family members
with degree of personality similarities
o (Each parent shares 50% of their genes with their offspring)
o If a personality characteristic is highly heritable, then family members with greater
genetic relatedness should be more similar to each other than are family members
with less genetic relatedness.
o But they also share the same environment.
o Violated equal environment assumption (below)
 Twin studies (looks at the heritability of identical twins that share 100% of their genes or
fraternal twins that share 50% of their genes
o Identical twins separated at birth often reveal amazingly similar characteristics and
personalities
o We can look both at the influence of genes and heredity in twin studies
o There are two important considerations
 Equal environments assumptions (the twin method assumes that the
environments experienced by identical twins are no more similar to each
other than are the environments experienced by fraternal twins.
 Some concern about the validity of this assumption
 Adoption Studies (a powerful approach that examines the correlations between adopted
children and their adoptive parents with whom they share no genes. If one finds a
positive correlation between adopted children and their adoptive parents, then this
provides strong evidence for environmental influences on the personality trait in
question)
o Avoids the equal environments assumption
o There are concerns however about the assumption that adoptive children and
adoptive parents are representative of the population in general.
o There are also issues with selective placement of children in homes of parents that
are similar to the birth parents on some specific trait.
Twins reared apart is a strong test of environmental influences but is extremely rare.