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[♪ dramatic music ♪]
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This double helix is DNA,
the genetic code that contains
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instructions for
all living things.
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But how much of the way we
think and behave is stored
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in this sequence of genetic information,
and how much do we receive
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from the environment
around us?
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Most of our traits are
influenced by many genes.
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How tall you are, for example,
reflects the size of your face,
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vertebrae, leg bones, and so forth,
each of which may be influenced
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by different genes interacting
with your environment.
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Complex traits, such as intelligence,
happiness, and agressiveness
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are similarly influenced
by groups of genes.
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Thus our genetic predispositions,
our genetically influenced traits,
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help explain both our shared
human nature and our human diversity.
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>>Heredity is what we get
when our parents
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shuffle their gene decks
and deliver a hand to us,
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and it turns out, in many, many ways
to be very important.
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So our temperament,
our personality, our intelligence
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are all powerfully
influenced by our genes.
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>>It is the pursuit of
behavioral genetics
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to tease apart what is
genetically programmed
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from what is influenced by the environment
around us, and how the two interact.
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One of the classic techniques to
scientifically tease apart the
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influences of environment and heredity
is to study identical and fraternal twins.
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>>Twin studies are a valuable tool
of behavior geneticists,
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and what they enable us to do is
compare genetic clones, identical twins,
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to see how similar they are
compared to fraternal twins,
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who also are born at the same
time and raised in the family.
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And it turns out that identical twins are
more alike in so many different ways
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than are fraternal twins.
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>>Studies of thousands of twin pairs
in the USA, Sweden, Finland, and Australia
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provide a consistent answer.
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Measuring traits such as
extroversion and neuroticism,
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identical twins are much more
similar than fraternal twins.
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In a seminal experiment beginning
in 1981 by Thomas Bouchard
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at the University of Minnesota,
researchers located and studied
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80 pairs of identical
twins reared apart.
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Participants were given tests
to measure personality traits,
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intelligence, and abilities, occupational,
leisure interests, and social attitudes.
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>>Despite their different rearing,
they're often strikingly alike,
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and that's a powerful illustration
of the power of genetic influences.
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>>Another way to study behavior
genetics is through adoption studies.
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>>Studies of adopted children, comparing
them to their biological parents,
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with whom they share genes,
and their adopted parents,
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with whom they share
a nurturing environment,
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also reveal the power of genes.
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>>[Michael Lyons] The idea is to see,
does this offspring resemble
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the biological parents,
or resemble the adoptive parents?
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>>Twin studies and adoptive family studies
do seem to throw the focus on genetics,
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but to what extent is
our behavior genetic?
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To try to figure this out,
behavioral geneticists
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can mathematically estimate
the heretability of a trait.
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Heretability refers to the extent
to which variations among people
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within a group are
influenced by genes.
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In an example looking at the
heretability of intelligence,
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the slope of this graph shows
the correlation between the
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average intelligence of parents
and their offspring.
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If we have calculated that
the heretability of intelligence
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in this case is 50 percent,
this does not mean that
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your intelligence is
50 percent genetic.
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Rather it means that genetic
influences explain 50 percent
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of the observed
variation among people.
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>>[Lyons] So take a concrete example,
I think that's the clearest way
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to try to explain it, if we wanted
to look at height and see to what extent
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the genes and the environment influence
individual differences among height.
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>>If the heretability of
height is 90 percent,
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this does not mean that a 60 inch tall
woman can credit her genes entirely
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for 54 inches and her
environment for the other 6 inches.
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Whether it is height, personality,
or intelligence, we can never say
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what percentage of an individual's
traits are accounted for by their genes.
