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.