Most human traits are not single gene traits.
They're polygenic. There's multiple genes involved
and we now have a state of understanding that we know that genes aren't always on or
always off.
Environmental influences can influence our actual gene structures
and they may activate or deactivate at different times
for different reasons. That's kind of poorly understood what
reasons at what times in which genes, but we now realize it's not just a
static
code that sits there and emits and there's a bunch of dead material in there, right,
a bunch of
of redundant useless DNA material.
So anytime we say something is genetic, you should automatically now
as non-laypeople in this subject go, it's probably more complicated than that.
So one thing we see that has one of the strongest genetic
influences is intelligence. There
is evidence for a pretty big genetic contribution to intelligence
although, I would argue it's not nearly as big as some people would make it out
to be.
John Locke on the flip side, if you know of the tabula rasa
right, Locke in, some of you know, political science, right,
philosophy holds that we just get basically get bored
and we're a blank slate, and everything that we know and do is
environment, it's environmental influence. It's not that either,
it's not all one or all the other. It's more complicated than that. So
Just to look at it, you see the generic similarity studies help us determine
relative contributions.
In a very general sense, identical twins have a hundred percentage genetic
similarity.
Fraternal twins have fifty percent genetic similarity, and unrelated people have
0 percent. Well of course they have some genetic similarity because we all have
the same structure when talk about breaking that
that code right, understanding the genome.
We all have ninety-nine point nine something percent similarity.
That's what makes us a species. That's what differentiates us from other species
who also share large portions of that DNA in the sense that they're
mammals or that they're fish or that they're whatever.
But, as an individual you are unique
in the sense that what makes you you is still your genome.
It's not the human genome. It's the human individual's genome
that we're talking about here, and that's where people get a little bit off-base.
So if we look at it that way we can now look at correlations.
Now, correlations are not causal, we know this. But we do see relationships and we
can make
some inferences from the these, right? Keep in mind correlations are under inferential
statistics.
There's others of them. If intelligence was due solely to heredity, then averages between intelligence
scores should decrease as genetic similarity decreases,
that is the case, that's what happened. However, results
also support environmental influences. So if I'm looking at identical
twins
reared together, so they are virtually identical, but you know that they're not
identical exactly.
They're not the same soul, they're not the same human being.
Most of you know of Mary-Kate and Ashley Olsen,
right, on the Full House, and they made millions of dollars pretending to be the
same human being right,
because they looked identical,
they sounded identical and they can act identical
right, and that worked for them for a long time, but when they became adults,
even though they were making big money, they took a lot of pains to show how
different they really are.
Right? If you know identical twins you might have a hard time telling
them apart from appearance,
but you know they're different people. Being of the same genetic
variety does not make you an identical soul. These are two different people.
So, what we have is two people who look identical who share
virtually all of their genetic material who are now in a shared environment.
So, if we raise them together, the correlation is .86 or so,
that's pretty high alright, that's like wow man you don't ever see
correlations that high in human affairs, so if you are not sophisticated with
your statistics you go, look, man, this must be all genetic because that's a really high
correlation.
It's almost one, and we know one's a perfect correlation.
But, if you raise them apart it falls to
roughly .72.
Now, the unsophisticated would go, well what's the difference? Right? .8 something, .7 something.
That's virtually the same thing.
But it's not the same thing. To understand shared variance how much
is overlap, and we don't know which part of the overlap is which.
To see the shared variance you square that number. You square the correlation to see
how much
should be from very similar influences
in your outcome. Our outcome in this case is intelligence test score, right?
We already know all the issues about the intelligence testing itself,
right, and what we see is that drop equates to this.
If it's .86 squaring it means seventy-four percent of the variance is
shared
and that seems like a pretty strong,
it's a pretty strong argument that there's a heavy genetic influence here, right?
But, you drop down it down to .72,
and that shared variance goes down to fifty-two percent.
Fifty-two percent meaning at best
you got fifty percent from genetic similarity, because now we're rearing them
in two different environments right?
But what, what else is the same about them?
Their looks; do we treat people differently because of how they look in
the society or any society historically?
Uh, no because of their skin color, their hair color, their eye color,
their attractiveness levels, their whatever, height?
You ever see somebody who is really tall and you're like, "hey what basketball team do you play for?" And they go,
"I don't play basketball."
Right, because people make giant assumptions about appearances, they
act on those assumptions as though those assumptions are valid.
So here's the thing. Even though we've got identical twins
in different environments, we're at best we're looking at fifty percent now
shared variance in their outcome scores. One thing that they
still have in common is,
for better or for worse, fair or unfair,
they're going to be treated similarly based on an appearance
because they look the same. Alright, and so there is still shared environmental
influence
although, it's much harder to nail down, and people are very quick to point out
similarities. They're like,
look, look, look, these two have never known each other, and they got together and they
like the same beer,
right? They watch the same same sports team.
But, it's really easy to pick out a couple things that are the same and go,
make a big deal out of that. Like, if you went out and saw a license plate and it said
572KJH you went, "look at that."
Out of all the license plates I could've seen today, 572KJH,
wow, and ignore all the difference's that occurred the rest of the day.
Right? So, what is making a big deal out of the few things that are the same do?
It ignores all the differences and up plays the similarity to give you the
illusion that there's more than there might be to this. So yes,
genetics count, but we're looking at less than fifty percent of the variance accounted
for.
So they count less than half, they still count,
but be careful what kinds of assumptions you make based on these
stereotypes that we have about people.
So that's just a chart there, and different studies find different things, you know.
Same person tested twice, genetic similarity is a hundred percent cause it's you,
and you do pretty much what you did last time. What does that show?
If you can retest somebody, it shows reliability, right? You would expect, but it's not
perfect is it, because there's some measurement error.
On a different day you do different performance so, it shows there is variability even in
your own performance over time.
Now we've got identical twins reared together; identical twins reared apart.
Genetic similarity, a hundred percent, but what do we got?
Reared together, a very high correlation, and then we drop it
when we rear them apart, and you square it and
you find out what your variance is, and then you get fraternal twins.
Well, those are people who have two different eggs and two different sperm.
No different than a brother or a brother
pair or sister pair who are born a year apart, right? Their genetic similarity
is the same. It's random, which egg which sperm
which 23 chromosomes from the mother, which 23 chromosomes from the father.
So, they share that genetic link of mother and father if they're fraternal twins,
and what we see is, you know, they have fifty percent similarity.
Reared together, you got a much higher correlation than reared apart.
Here's biological siblings reared together but differentiated now by
age, right, so different ages bring different effects on them, right? At
different times they'll be doing different things.
The environmental will treat them differently as a function of age, and what you see is the further up
you go, the less similarity you're going to find, even in the same environment,
which is what you would think it is. It's just giving you pause to realize
that heritability is not what a lot of people want to make it out to be.
It's not written in stone. It doesn't have definitive implications about what
you'll see.
You've got genotype and then you've got phenotype. You've got what you have the potential for genetically and
then what you have in actuality
is what's been expressed, what's manifested,
which is a wide range beyond what you might
kind of naively assume would be the case with things like eye color, or hair color,
or skin color.
People are like, "man, you don't look anything like you mom or your dad, what's wrong with that?"
Nothing! Why would you assume that they would?
How many brothers or sisters, or brothers and brothers, or sisters and sisters do you
know that don't reallylook alike?
Different heights, different hair color, different eye color, different skin tone,
and then you want to get something that's so complex as
intelligence and assume there's a 1 to 1 relationship between a mother and a
father and an offspring, it doesn't make sense
if you're more sophisticated in your processing of information on this.
Degree of variation of a trait within a population that's due to heredity for
intelligence,
most research suggest it's thirty percent to fifty percent at most
of the variation is due to heredity.
Differences and outcomes, because it's not one hundred percent this means that
heredity and environment
interact to determine intelligence.
Nature and nurture. Not nature
or nurture, and then we have reaction range:
genetically determined limits for an individual's intelligence where the
quality of the environmental experiences determines
where the individual will fall within this range.
So this is for anything. Look at my height for example.
There's a reaction range of height.
If I'd eaten more protein, and exercised more or, more other nutrients growing up,
I might've been 6'1 or 6'2, I'm 6'0.
Right? If I'd had worse nutrition, worse exercise,
I might've been 5'8 or 5'9,
but I wouldn't have been 5'0 or 7'0,
right, because the genetic program within me is programming for a range.
I don't know what I could've gotten too or I don't know what I would have fallen to
under other circumstances,
but we see in intelligence when people are abused or neglected
they don't get what you would predict in terms of intellectual abilities.
When they are deprived of stimulation, when they are deprived of love, when
they are deprived of educational opportunities,
probably what you get is something in the lower echelon of their range.
But given great opportunities, great support, great love,
all kinds of infusion of support and
resources you might get the upper end of their range,
but for a variety of reasons you might see it go one way or the other. So the range
simply says that
you're not guaranteed to hit a point on a test that we already know has a little
bit of measurement
issues with it, but that you'll go somewhere in that neighborhood probably
in your lifetime, for various reasons. That's all you need to know for your
next test, ya'll.
Study hard, no good luck cause luck never predicted good performance on a test.
Knock it out, have a good weekend. I'll see you next Wednesday. Kyle will be with you
on Monday.