Now, let's look at how nature and nurture affect IQ. Identical twins share 100%
of their genetic make up. They're genetically identical. Fraternal twins share
only 50% of their genetic makeup. This is also true for siblings and parents.
So, you share 50% of your genetic makeup with each one of your parents. So 50%
with your mom and 50% with your dad. And you share 25% of your genetic makeup
with your grandparents. Aunt, uncles, nieces and nephews. And people you are
not related to? Well, you guessed it, you share 0% of your genetic make up with
that. So we can turn these percentages into numbers ranging from 0 not related
to 1 being identical twins, this is called genetic relatedness. Now,
environment has to vary too. Typically, biological parents and children live in
the same household. But that doesn't necessarily mean they have the exact same
environment, but it is very similar. Now of course, there's also cases where
biological parents and siblings are raised completely separate from each other.
For example, when people are adopted. Now, let's look at a graph to show how IQ
is related to different types of nature and nurture. Here are our IQ
correlations. Remember, 0, 1, 2, 3, not very good. They're not really related.
But high correlations mean they're really similar. And here, we'll see if they
share an environment or not. Meaning if they live together or if they don't. So
first, let's look at identical twins. They have a one on genetic relatedness.
Here we can see that identical twins who were raised together have a 0.85
correlation between IQ scores. So because they're genetically identical, and
they were raised in the same environment, their IQ scores are really similar.
But we know nature matters because identical twins who also have a one in
genetic relatedness because they're genetically identical, but were raised
apart. So they didn't share the same environment, have a 0.75 IQ correlation.
Meaning they're still really similar, but not as close as if they were to live
together. Now, let's look at genetic relatedness of 0.5. Meaning sharing 50% of
your genetic makeup. So fraternal twins share 50% of their genetic makeup.
Those who were raised together, so they shared an environment have a
correlation of about 0.6. So that's pretty related. However, if they're raised
apart, it goes just below .4. Showing that environment does play a big factor
in IQ. Siblings again, who you share 50% of your makeup with. Those who are
raised together with a shared environment have just below a .5 correlation. So,
it's not very related. However, if the siblings were raised apart, so they
didn't share an environment. This significantly drops to just above a 0.2,
which is a really low correlation. And parents who raise their children, so
sharing an environment with them again have less than a 0.5 correlation.
However when they didn't share an environment, again it drops to about a 0.2.
These show the importance of environment. Now, let's look at adoption. So these
people have 0 genetic relatedness but they share the same environment. So
adopted siblings are just above a .3 in their IQ correlation and adopted
parents are just below a .2 in their correlation. Even thought they shared the
same environment, they didn't have a very strong correlation among IQ scores.
That shows us that genetics are an important factor too. The formal conclusion
of these studies says about 50% of the differences between people's
intelligence is due to genetic influence. In about 40%, its due to
environmental effects. That leaves 10% being unexplained. So we just don't know
yet.
