PSYB30H3 Chapter 6: Chapter 6
Genetics and Personality
The Human Genome
● genome
○ refers to the complete set of genes an organism possesses.
● The Human Genome Project is a multibillion-dollar research endeavor
dedicated to sequencing the entire human genome—that is, to identifying the
particular sequence of DNA molecules in the human species
○ the manner in which human genes get decoded into proteins turns out
to be far more variable than in other species.
■ may account for the complex differences we observe between
rodents and humans
○ these protein-coding genes, making up roughly 2 percent of the human
genome, are only part of the story.
■ Many parts of the other 98 percent of the DNA in the human
chromosomes used to be chalked up as “genetic junk”
because scientists believed that these parts were functionless
residues that served no purposes.
■ Genetic researchers are discovering that this “junk DNA” is not
junk at all. Rather, parts of these chunks of DNA have an impact
on humans, potentially affecting everything from a person’s
physical size to personality
● Most of the genes within the human genome are the same for each individual
on the planet. That is why all normally developing humans have many of the
same characteristics: 2 eyes, 2 legs, 32 teeth, 10 fingers, a heart, a liver, 2
lungs, and so on.
○ Some of the genes that differ from individual to individual influence
physical characteristics, such as eye color, height, and bone width.
Some genes that differ across individuals influence the behavioral
characteristics that define human personality.
Controversy About Genes and Personality
● Part of the reason for the controversy is ideological. Many people worry that
findings from behavioral genetics will be used (or misused) to support
particular political agendas.
○ If individual differences in thrill seeking, for example, are caused by
specific genes, then does this mean that we should not hold juvenile
delinquents responsible for stealing cars for joy rides?
● Another part of the controversy concerns the idea of eugenics.
○ Eugenics is the notion that we can design the future of the human
species by fostering the reproduction of persons with certain traits and
by discouraging the reproduction of persons without those traits.
■ Many people are concerned that findings from genetic studies
might be used to support programs intended to prevent some
individuals from reproducing or, even worse, to bolster the
cause of those who would advocate that some people be
eliminated in order to create a “master race.”
● Knowledge is better than ignorance, scientists argue
● If people believe that hyperactivity, for example, is caused by parenting
behaviors when in fact hyperactivity turns out to be primarily influenced by
genes, then attempts to influence hyperactive behavior by altering parental
practices could cause frustration and resentment on the part of the parents
● Finding that a personality characteristic has a genetic component, for
example, does not mean that the environment is powerless to modify that
characteristic.
Goals of Behavioral Genetics
● “What causes some individuals to be tall and others to be short?”
● One of the central goals of genetic research is to determine the percentage of
an individual difference that can be attributed to genetic differences and the
percentage due to environmental differences.
● In the case of height, both environmental and genetic factors are important.
○ Clearly, children tend to resemble their parents in height—generally,
tall parents have taller than average children and short parents have
shorter than average children.
○ The environment, which contributes 10 percent to individual
differences in height, is far from trivial. In the United States, average
adult height has increased in the entire population by roughly 2 inches
over the past century → due to increase in nutrition
● even though some observed differences among people can be due to genetic
differences, this does not mean that the environment plays no role in
modifying the trait.
What Is Heritability?
● Heritability
○ statistic that refers to the proportion of observed variance in a group of
individuals that can be accounted for by genetic variance
○ describes the degree to which genetic differences among individuals
cause differences in an observed property, such as height,
extraversion, or sensation seeking.
● Heritability has a formal definition: the proportion of phenotypic variance that
is attributable to genotypic variance.
○ Phenotypic variance → refers to observed individual differences,
such as in height, weight, or personality.
○ Genotypic variance refers to individual differences in the total
collection of genes possessed by each person.
○ Thus a heritability of .50 means that 50 percent of the observed
phenotypic variation is attributable to genotypic variation.
○ A heritability of .20 means that only 20 percent of the phenotypic
variation is attributable to genotypic variation.
● In these examples, the environmental component is simply the proportion of
phenotypic variance that is NOT attributable to genetic variance.
○ A heritability of .20 means that the environmental component is .80.
● environmentality.
○ the percentage of observed variance in a group of individuals that can
be attributed to environmental (non genetic) differences
○ the larger the heritability, the smaller the environmentality and vice
versa.
Misconceptions About Heritability
● One common misconception about heritability is that it can be applied to a
single individual.
○ IT CAN’T
■ For an individual, genes and environment are inextricably
intertwined. Both play a role in determining height, and they
cannot be separated. Thus, heritability refers only to differences
in a sample or population, not to an individual.
● Another common misconception about heritability is that it is constant
○ Heritability is a statistic that applies only to a population at one point in
time and in a particular array of environments.
○ If the environments change, then heritability can change.
● A final common misconception is that heritability is an absolutely precise
statistic
○ Nothing could be further from the truth. Error or unreliability of
measurement, for example, can distort heritability statistics
** In sum, heritability is best regarded merely as an estimate of the percentage of
phenotypic differences due to genetic differences. It is not precise. It does not refer
to an individual. And it is not eternally fixed
Nature–Nurture Debate Clarified
● nature-nurture debate
○ the arguments about whether genes or environments are more
important determinants of personality
