Psychology - Behavioral Genetics

[♪ dramatic percussive music ♪]

>>[Narrator] This double helix is DNA,
the genetic code that contains

instructions for
all living things,

but how much of the way
we think and behave is stored

in this sequence of genetic information,
and how much do we receive

from the environment
around us?

Most of our traits are
influenced by many genes.

How tall you are, for example,
reflects the size of your face,

vertebrae, leg bones, and so forth,
each of which may be influenced

by different genes interacting
with your environment.

Complex traits, such as intelligence,
happiness, and aggressiveness

are similarly influenced
by groups of genes.

Thus our genetic predispositions,
our genetically influenced traits,

help explain both our shared
human nature and our human diversity.

>>[David Myers] Heredity is
what we get when our parents

shuffle their gene decks
and deliver a hand to us,

and it turns out, in many, many ways
to be very important.

So our temperament,
our personality, our intelligence

are all powerfully
influenced by our genes.

>>[Narrator] It is the pursuit
of behavioral genetics

to tease apart what is
genetically programmed

from what is influenced by the environment
around us, and how the two interact.

One of the classic techniques
to scientifically tease apart

the influences of environment and heredity
is to study identical and fraternal twins.

>>[Myers] Twin studies are a valuable
tool of behavior geneticists,

and what they enable us to do is
compare genetic clones, identical twins,

to see how similar they are
compared to fraternal twins,

who also are born at the same
time and raised in the family.

And it turns out that identical twins are
more alike in so many different ways

than are fraternal twins.

>>Studies of thousands of twin pairs
in the USA, Sweden, Finland, and Australia

provide a consistent answer.

Measuring traits such as
extroversion and neuroticism,

identical twins are much more
similar than fraternal twins.

In a seminal experiment beginning
in 1981 by Thomas Bouchard

at the University of Minnesota,
researchers located and studied

80 pairs of identical
twins reared apart.

Participants were given tests
to measure personality traits,

intelligence, and abilities, occupational
and leisure interests, and social attitudes.

>>Despite their different rearing,
they're often strikingly alike,

and that's a powerful illustration
of the power of genetic influences.

>>Another way to study behavior
genetics is through adoption studies.

>>Studies of adopted children,
comparing them to their biological parents,

with whom they share genes,
and their adopted parents,

with whom they share
a nurturing environment,

also reveal the power of genes.

>>[Michael Lyons] The idea is to see,
does this offspring resemble

the biological parents,
or resemble the adoptive parents?

>>Twin studies and adoptive family studies
do seem to throw the focus on genetics,

but to what extent is
our behavior genetic?

To try to figure this out,
behavioral geneticists

can mathematically estimate
the heritability of a trait.

Heritability refers to the extent
to which variations among people

within a group are
influenced by genes.

In an example looking at the
heritability of intelligence,

the slope of this graph shows
the correlation between the

average intelligence of parents
and their offspring.

If we have calculated that
the heritibility of intelligence

in this case is 50 percent,
this does not mean that

your intelligence is
50 percent genetic.

Rather it means that genetic
influence explains 50 percent

of the observed
variation among people.

>>[Lyons] So take a concrete example,
I think that's the clearest way

to try to explain it, if we wanted
to look at height and see to what extent

do genes and the environment influence
individual differences among height.

>>If the heritability of
height is 90 percent,

this does not mean that a 60 inch tall
woman can credit her genes entirely

for 54 inches and her
environment for the other 6 inches.

Whether it is height, personality,
or intelligence, we can never say

what percentage of an individual's
traits are accounted for by their genes.

>>Another related kind of
phenomenon is what's called

the 'gene environment interaction,'
and that is cases in which our genes

determine how sensitive
we are to environmental influences.

>>Today, behavior geneticists
have progressed beyond

asking if genes
influence behavior.

Fundamentally, we
know they do.

So the question is, what are the
specific genes that influence behavior,

and how can we
identify them in populations?

The molecular approach to
behavioral genetics

addresses this question
from the bottom up,

and has the power to reveal
at-risk populations

for some of the most
prevalent diseases today.

In labs worldwide, geneticists
team up with psychologists

to pinpoint genes that put people at risk
for such genetically-influenced disorders

as learning disabilities, depression,
schizophrenia, and alcohol dependence.

It is the study of chromosomes
using the powerful tools of

DNA scanning and
genome sequencing.

The most powerful potential for
DNA sequencing in this capacity

is to predict risk so that steps can be taken
to prevent problems before they happen.

The molecular approach also opens up
the door to a realm of ethical dilemmas.

As we begin to identify
more specifically the genes

that influence a range of faculties,
could future parents have the opportunity

to screen a developing fetus
for these genetic traces?

Should we enable parents
to screen their fertilized eggs

for health, and for brains, or beauty?

>>Bipolar disorder, if you could
wave a magic wand and eliminate

the risk genes for bipolar disorder,
would you do that?

Well bipolar disorder, for example,
is associated in many interesting ways

with creativity. So many of our greatest
works of art might not exist

if these genetic variants that were
associated for risk of bipolar disorder

didn't exist in the gene pool.

>>The pursuits of molecular
behavioral genetics have the potential

to relieve a great deal of human suffering
and to give us insights into the

delicate relationship between
nature and nurture,

but we must keep an open dialogue about the
ethical implications of what is possible.