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Video transcript
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.