Heritability (Behavior) Video Lecture - Personal Learning

OK.
So I ended the last video,
the one on twin and adoption
studies, with some
pretty specific language.
I said that through
those studies,
psychologists could estimate
the extent to which variation
of traits, or attributes,
for any given population--
the extent to which
that variation can
be attributed to
differences in genes.
And I used that language,
that specific language
because I was actually defining
the term "heritability."
So a textbook might define
the term heritability
as the proportion of
variation-- so a percentage,
because a proportion is just a
percentage-- among individuals
that we can attribute to genes.
So I'll write "due to genes."
And so I know that
even at this point,
that definition is
probably not very clear.
And we're going to spend the
entirety of this video just
trying to clarify this concept.
And so assume that we're
talking about intelligence.
And we say that heritability
of intelligence is 50%, so IQ.
And we've said that the
heritability is 50%.
And so what we're not saying
is that the intelligence
is 50% genetic.
That's what we're not saying.
Really, what we are saying
is that the difference
in intelligence can
be accounted for
or is 50% attributable to
the genetic differences.
So we're talking about the
differences in these traits.
Because really to ask how
much of our intelligence
is mandated by our
genome as opposed
to instilled in us
by our environment
is completely inappropriate.
And that's what we learned in
that tea and hot water example.
But what can be answered and
what those twin and adoption
studies started to show
us is the percentage
of the different, or
variance, right here
that's attributable to
the differences in genes.
So I know I've said
that several times.
And let me move to an example.
One of my favorite
examples of heritability
actually comes from Mark Twain,
the famous author and comedian.
And he said that suppose you
raised four boys in barrels
until they were 12 or so.
So I've drawn some barrels here.
And we've got four
boys inside of them.
And we'll feed
them through a hole
so that they get the
same diet and such.
And we'll remove all
of their waste products
on the same schedule.
And their environments
will be 100% controlled,
almost sadistically controlled,
but 100% controlled.
By the age of 12, their IQs
would probably be pretty low
compared to the
population around them.
But the kicker is
that they're probably
not all going to
be the same, right?
So the differences
in their intelligence
couldn't be attributed
at all to the environment
because we've absolutely
controlled their environment.
And we would say that their IQ
differences were nearly 100%
heritable because their
environments were nearly 100%
the same.
So h2 is just short
for heritability.
And we give it a lower
case because we're
talking about one
specific trait,
in this case intelligence
as opposed to many traits.
But the heritability
would be close to 100%.
And so that was Mark Twain's
example of heritability.
But alternatively, you
could say that maybe you
have four identical quadruplets
with exactly the same genome.
And you raise them in crazy,
different environments.
And so maybe one is raised in
the rain forest as a tribesman.
And then maybe one is raised
in the desert as a nomad.
And then maybe one is raised
in a North American family
as maybe an industrial worker or
at least in an industrial kind
of blue-collar family.
And then maybe one we'll just
say is raised in outer space.
He's raised up
here near the Moon,
maybe on the International
Space Station.
But the heritability of the
intelligence of these four boys
would be quite a bit lower
because now their environments
are counting for a bigger
percentage of the differences
in their intelligence.
And I've said here that the
heritability is actually
0% because we've
established that they're
identical quadruplets.
So genetically they're the same.
And their environments
are completely different.
So we would say that
all of the differences
in their intelligence
must be attributed
to their environments.
So maybe a nice, oversimplified
way to think about this idea
is that as the
environment becomes
more and more controlled,
like in the example
of the boys in the
barrels, differences
in behavioral traits are more
closely tied to heredity.
And the heritability of that
trait is therefore higher.
And then maybe
another thing that
might increase
heritability would
be increased genetic
variation that
leads to different phenotypes.
So if there was more genetic
variation in these boys,
say maybe if they were
fraternal quadruplets instead
of identical
quadruplets and they
had different genotypes and
more genetic variation leading
to different phenotypes or just
the expression of their traits,
those new variations would be
more related to their genes.
So in this case, again, the
heritability would increase.
And so heritability
is either increasing
because genetics are
contributing more to the genes
or because nongenetic
factors, like the environment,
are contributing less.
But what matters is
that we're talking
about the relative
contribution of genes
to the variation in
behavior or traits.
And so you might
have just caught
on that heritability
then becomes necessarily
dependent on the
population that's studied.
And so think about
one last time,
the population of boys
inside the barrels.
Their heritability
is much higher--
or the heritability of their
IQ rather is much higher
than it would have
been if we had instead
studied the quadruplets.
So one last time,
heritability of a trait
is the extent to which variation
can be attributed to genes.
And it's very, very
dependent on the populations
and the environments
that we study.
But hopefully,
that just gives us
a quick context for
what we mean when
we use this word in the future.