This video, we're going to be talking about heredity, and a really important concept here is going to be heritability, which is basically how much variation in phenotype across people is due to differences in genotype. And what that basically means is that when we are seeing different behaviors or mental qualities across people, we are asking how much of that variation seems to be due to genetics, basically. And so we actually calculate heritability as a proportion between 0 and 1, and we call this a heritability estimate. So, if we have a heritability estimate of 0, then what we are basically saying there is that genes do not contribute to individual differences on that trait at all. In contrast, if we had a heritability estimate of 1, we are basically saying that only genes contribute to individual differences on that trait.
Now, I would encourage you within the field of psychology, at least to kind of think about 0 and 1 as kind of theoretical limits. They almost never come up in psychological research. Typically, the values that we're going to see in psychological research are going to range from about 0.3 to about 0.6. So, just to give you a more concrete example of a heritability estimate, let's say we ran a study and found, you know, for IQ, we have a heritability estimate of 0.5. What we are basically saying there is that 50% of the variability that we see across people in IQ is due to genetics, and about 50% of the variability seems to be due to the person's environment.
So that's kind of the idea with heritability estimates. Now, in psychology, we do not often study actual genes. We're not geneticists, we're psychologists, right? So psychologists have come up with some pretty interesting study designs to kind of help us tease apart if genes or environment seems to be playing a bigger role. So we tend to study heredity in 3 major ways.
So, first, we very often study pairs of twins. So, when we are thinking about twins, of course, you can either have monozygotic twins or identical twins, which we are just going to abbreviate as MZ twins, and MZ twins share 100% of their genetic material. Okay, so these twins share all of their genes and if they are being raised together, they also share all of their environment as well, right? Now, in contrast, you can have dizygotic or fraternal twins, which we're going to abbreviate as DZ twins. And DZ twins, like any other sibling pair who share parents, share 50% of their genetic material.
So, assuming that they are being raised together they share half of their genes and all of their environment. So we can do studies basically contrasting MZ twins and DZ twins to see if genes or environment seem to be playing a bigger role in things like behaviors and mental qualities. So, for example, if we find that MZ twins are more similar on aggression scores compared to DZ twins, then that is evidence that there's some kind of genetic component for aggression. That's how we would do those types of studies. Now we can take this one step further and we can also study MZ twins who have been separated at birth, and this gives us a really unique situation where you have 2 people who share 100% of their genetic material but almost none of their environment, maybe like a basic cultural environment, but aside from that, none of their environment is shared, and so you can basically try and see if these twins are similar to each other or different from each other.
You can also compare MZ twins who were separated at birth with DZ twins who were separated at birth, to see if they are more or less similar to each other. One really famous study that did this found that monozygotic twins who had been separated at birth, their IQ scores correlated at 0.7, which is really, really high compared to DZ twins who had been separated at birth, and their IQ scores correlated at, like, 0.3. So that's significantly lower. Right? So that gives us pretty strong evidence that there seems to be a strong genetic component to IQ because we saw how similar those MZ twins were and they, of course, share their genetic material.
So, we can do twin studies in kind of 2 different ways. We can also study adoptees and their birth and adopted parents. So with adoptees, what you have is that they're going to be sharing their DNA with their birth parents but none of their environment, and then they're going to share their environment with their adopted parents but none of their DNA. So we can look to see if they are more similar to their birth parents or to their adopted parents to get a sense if genes or environment seem to be playing a stronger role in their behaviors or mental qualities. So the big idea behind all of these study designs is basically to help us figure out what traits are more influenced by genetics and which traits seem to be more influenced by environment.
That's kind of what we're getting at here. And we are always acknowledging that there's always going to be an interaction of genes and environment as well. Alright. So that is how psychologists study heritability, and I will see you guys in our next video. Bye bye.