Welcome back. [SOUND] So, last module, what I did is we went through, how biometricians, use information on phenotypic similarity among relatives, and mostly we focused on twins, monozygotic and dizygotic twins. How they can use that information, to get an estimate of heritability, where heritability is the proportion of variance. In a phenotype that's associated with, genetic factors. Because they need to make assumptions in order to, estimate the heritability, we recognize that the heritability estimate itself, is probably, at best, an approximation, of some true value. So we need to be cautious, when, looking at particular, heritability estimates, not to over interpret them. What then, is the utility of estimating heritability? Well, I'm going to try to explain the utility of estimating heritability is, first by telling you, what I don;t think heritability is. These are, things are, problems that have come up over the years. How I think, heritability coefficient. Has been misinterpreted. And at the end, I'll try to, give you reasons why I think it's useful, to know, at least approximately, the extent to which genetic factors, might contribute to individual differences in a behavioral trait. The heritability coefficient. So, the first thing I'm going to talk about, is the heritability is not an index of whether or not. A phenotype can be changed by intervention. This is a common mistake that's made, over and over again. But I'm going to give an old example of this. This, it's kind of an interesting example. And this is a quote from an article that was published oh, quite a few years ago. In, the Times of London. And, what, the article was about, was a very large twin study, I think it was a registry based twin study, it was done in Europe. In which, they did a twin study of income levels. And what they observed was, not surprisingly hopefully, to you at this point, is that monozygotic twins were more correlated in their income levels. Than dizygotic twins. So that, income levels were heritable. And they, in the study, actually I don't, I, I can't recall exactly what the heritable estimate is. It's really not, that important at this point. But the, the study reported that, a heritability estimate for your income level. And, the way the Times of London interpreted. This very large study of twins is that, the findings, the finding, what's the finding here? That income level is heritable. The findings are significant matter of social policy, because it implies, that we can't make society more equal. That is we cant' equalize income levels, because they're somehow coded in our genes. So, the, the finding that income levels were heritable, meant, that we couldn't change, income levels. That they're somehow predestined in our genes. Not the case. We might think well, this is a journalist making kind of a rookie mistake. This is not something that really sophisticated, scientists would ever do. Well, here's a quote from a very prominent, and, and deservedly prominent psychologist that actually accompanied the Times article. Hans Isaac is one of the most famous individual differences psychology of the 20th century. He made many, many, important contributions to the field, but look at what he says. It really tells the royal commission, on the distribution of income and wealth, that they might as well pack up. Income is heritable, therefore, nothing we can do about it. It's in your genes. Not the case. [SOUND]. And to illustrate it's not the case, let's take something that almost nobody, would argue is not heritable. Right. Here's a study, from actually a friend of mine. A well known, Finnish geneticist, Karri, [LAUGH] I think I'm going to have a hard time pronouncing his last name, Silventoinen. And Karri has collected, from various countries, a very large sample. Of twins, and he's measured their similarity, for height, and he's, here, I'm just taking from his article here. The estimates of, these are added genetic variant estimates of height for men and women, across these various countries. And in, in the point here, obviously, is that height is not surprising is a very highly heritable trait. Looks to be, 75 to even, maybe as much as 90% heritable. Height is highly heritable, but if we take the logic from the previous slide, does that mean height is something that's fixed, that our environments can't change? Well, no. One thing, that's been happening in all of these European countries, is that height has been going up. And this is from an article actually, published in 2013, a review article. A very nice review article if you're interested in height, of increases in the average height. These are mostly based on males. And the reason they're based on males, the same thing is going on with females, but the reason they're based on males is, the data come from consc, usually often from conscript boards. And, usually only males are conscripted, going into the military. And you can see for country after country, this is spanning quite a, a large historical period. But for country after country, the average height of a, of an adult male has been increasing. In fact, over the last, 100 years, the average male. In Europe has grown, by 11 centimeters. That's a phenomenal increase, in height. Height is a highly heritable trait. Nonetheless, height has been changing over time. Highly heritable, does not mean fixed. It doesn't mean it's unchangeable. Height has been changing. And it's been changing not because, our genomes. Our genomes don't change that fast. Over the past, 100 years. It is because, our environments have changed. Better public health. Better control of infection. Better diet has, has led to, the increases in height. So, high irritability is not, in, incompatible with, with things being environmentally manipulative. Something that we'll talk about again, in particular when we talk about IQ. This is, I think, a mistake that, comes back over and over again, when people talk about, the heritability of IQ. As you will see, IQ is a heritable trait, but it doesn't mean that IQ is not changeable, although some people have argued that. So the first, thing that heritability is not, it's not an index of whether or not you can or cannot change a, a trait. The second thing it's not, it's not a biological constant, that is. The heritability applies, to a certain population, at a certain period of time. It can change if the situation changes. And I'm going to illustrate this with a study, a study a kind of like about, this is the heritability of reading. So it's whether or not, how well children are reading, and in this case they're looking at kindergarteners so they're about five years old. And they're looking at three different countries, and these are three different large twin studies. One in Australia, one in the US, one Sweden. And what they've done is they've applied, the Faulkner model, to the MZ and DZ, correlations and generated, an estimate of additive genetic effect. The a estimate and shared environmental effect. And of course, there's a non shared environmental effect, but I'm just not giving it here, because every, all the important information is here. Look at the heritability estimates in the different countries. What is the heritability of reading? Well clearly the heritability of reading is different, in Australia. Than it is in Sweden. So there's not, one inheritability estimate here, and also actually, the shared environmental effect, differs, from across the three countries. So, these various components. There aren't, fixed biological constants. They actually can vary, they can vary over region. They can vary historically. They can vary developmentally. Now I'm going to come back to this example in a couple minutes, because it's actually a beautiful, I think a beautiful example, of why the heritability estimate is a worthwhile statistic. [BLANK_AUDIO] What is heritability used for? I gave you the quote I think in the last module, from the Nature Review's genetics article that said, this is a very important, statistic. That, even though we have, challenges in estimating it, we really need to pay attention to it. Why do geneticists, [INAUDIBLE] geneticists feel that. First, it gives us an approximate sense, of the importance, of genetic factors to individual differences in a particular point in time in a particular, environment. Secondly, and this is why, there's been somewhat of a resurgence in the interest in heritability. Because now as you'll see as we get later into the course, we can actually begin to get down at the genome level, to identify this specific genetic effects, that might be underlying individual differences in, in, in, behavior. Heritability, is an index that, actually molecular geneticist are interested in, because it tells us. Something about, whether or not they're likely to identify those, genetic effects. The third reason I think, heritability is useful, is by comparing that, it's not a biological constant actually, can be very informative. And to illustrate this, I'd like to back to that reading study. The reading study was actually a longitudinal study, and I've given you the citation if you want to look at the study. It's actually, I think a very interesting and nice study. They, what they did, is they came back in first grade, and they redid the reading achievement measure, now. The twins are, in grade school, first grade. And look at, what happens to the heritability estimates now, they don't vary across the countries. And heritability estimates are actually quite high, they're all about 80%. And the shared environmental effects are all now consistently low. Well what was going on here? Why were you getting the differences here, but all of the sudden not here? Why are the heritability estimates here high, and the shared environmental estimates low, but larger shared environmental effects here? Well, what happens, it turns out, relates to, how children are treated in the different countries. In, Australia and the US. Kindergarteners, there is a curriculum for reading in kindergarten. So, all of the twins when they're going to, kindergarten, are getting the same standard, curriculum for teaching them how to read. In Sweden, there is no such, curriculum for reading in kindergarten. They dont think it's important for a five year to read. So they don't try to teach them, in, kindergarten, whether or not to read. Consequently whether or not, an individual in Sweden, a five year old learns to read, is going to be dependant upon his or her parents, what happens in the home. But, for people in Australia, or five year olds in Australia, in the US, they're not dependent on their home. They're, they're going to learn to read at school. So what happens is, the heritability is high here because it doesn't depend upon what's going on at home. It's low here, and the shared environmental effect is high because whether or not they read, depends upon whether, how they're being treated at home. When they come back in first grade, now Sweden, as well as the US and Australia, have a standard curriculum, that every child, will get, in order to learn to read. Now, whether or not a Swedish six year old reads, doesn't depend upon. How he or she is treated at home. They're all going to learn, how to read, by what they get in, their, their curriculum at school. So, the inheritability estimates are high, the shared environmental effects are low. So, by looking at the comparisons of the inheritability, they're beginning to give us insights, as to what's going on with reading and the, the relative contribution, of what's going on at home. Versus at school, and, how those impact share environmental effects and heritable effects. Next time, we're going to look at, something that, we actually assumed doesn't exist in, generating the heritability estimates, a very important concept, gene-environment interaction. [SOUND]. [BLANK_AUDIO]
