Okay. Welcome back. This is module B, the second module in the seventh unit of this course. And in this module, what I'm going to try to bring in, something that we really haven't considered much in this course up to this point, but it's terribly important. Is the notion of development that we develop over time, and the impact of that notion on behavioral genetic findings. Last time, in the first module, we talked about the shared environment from behavioral genetic perspective. And the finding from twin adoption And although we didn't review them is the finding from we had a part twin study showing that same thing that in general, the shared environment doesn't appear to be a major source of individual differences and behavior. That's unexpected, it's not as if the environment is unimportant, right? The environment is very important, but appears to be the non-shared environment rather than the shared environment. I left off last time by noting that although behavioral geneticists draw this general conclusion that the shared environment doesn't seem to be all that important for most psychological and psychiatric traits. There is certainly three domains for which there is behavioral genetic evidence for important shared environmental effects. General cognitive ability, rule break, rule breaking behavior, and social attitudes. And we left off last time just talking about, well, maybe we get shared environmental. Maybe we should expect to get shared environmental influences there because this is really, these are the domains that parents really focus on when they're socializing the children they rear. I'm going to actually ultimately talk about each of these in turn during this module, but I want to focus primarily on general cognitive ability, because there's much more research here. And I think we'll see a very general pattern emerge from the general cognitive ability of literature that I'll then try to extend to the other domains. If we look from last week at biometrical estimates from based on family data, twin data, adoption data on general cognitive ability. You may recall that the estimate of genetic influences is about 50% heritability. Shared environmental influence is about 35%, non-shared environmental influence about 15%. One thing that I didn't point out, but should be pointed out now, is that the majority of data upon which those estimates are based is derived from children or adolescents. That not much of the data is actually based on adults. And it may be that the magnitude of these biometrical components, the heritability, the influence of the shared environment, maybe those change developmentally. And in fact they do appear to change developmentally. For general cognitive ability, there are multiple lines of evidence of research, that all converge on the same conclusion. That with age, the shared environmental influence, which overall count for 35% of the variance. But the, that shared environmental influence declines with age, while the genetic influence or heritability increases. So again, on average, maybe 35% shared environment, 50% heritability. But if you bring age into the equation, it appears that that 35% declines with age in the 50% increases. I've highlighted four major areas of research that underlie this general conclusion. I'm not going to go through all four here. I'm just going to try to highlight two of these, just to give you a flavor of the behavior genetic research supporting this conclusion. And the first I'm going to talk about cross-sectional studies of reared-together wins. That the heritability of general cognibility increases while the influence is shared environmental influences decreases with age as has been shown by multiple cross-sectional studies of reared-together twins. I'm just going to focus on one that was published recently with in this publication down here. I'm going to focus on it because it's a nice, large sample. It actually involved combining twin studies from six different investigators, and we were fortunate to be one of the, the studies involved that represented four countries. Although like unfortunately, a lot of the behavioral genetic research, there's a Western bias in these countries. The U.S. was one and the other two were in Europe, and the fourth was Australia. In total across those six studies though, we have 10,000 pairs of twins. And what we can look at, at given that large twin sample is we could classify the twins by the age of which they're assessed. And when we did that in this study, this is what we found. It might be a little hard to see the graph, but I'll try to step through it here. A, C, and E, hopefully you remember those letters by now. A is the added genetic effect, C is the shared environmental, E is the non-shared environmental. These estimates are derived from using essentially the Faulkner model. Here are the three age periods that we divided these 10,000 pairs of twins up into. This is the additive genetic effect, or the heritability. Note that it is about 40% in childhood, but increases to about 70% or 65% in adulthood. As a child ages or as an individual ages, I should say, the importance of genetic factors increases in terms of their general cognitive ability. Here is the line for shared environmental influences. That with age decreases, especially as they go from childhood to adolescents, and maybe even a little bit more in young adulthood. This is young adulthood, what if we went out and got middle age people or older people. At least with middle age, it appears this continues to increase while this continues to decrease. So yes, general cognitive ability is an exception to the general rule that you don't find shared environmental influences. But at the same time, finding shared environmental influences on general cognitive ability is conditioned on developmental stage. You tend to find them early in life. They appear to dissipate with age. That's coming from twin studies. Is there another way to show the same thing? Well, there're actually ma, many other ways that might lead to the same conclusion. Again, I'm going to only highlight a second one here. In this case, I'm going to look at those adopted siblings again. Both cross-sectional and longitudinal studies of adopted siblings. This is a recent from a recent review article by Tom Buchard. Again, it's a little bit complicated perhaps. What he's plotted here are the correlations for adopted sibling pairs. Recall what adopted sibling pair is. A pair of individuals who grow up together, but are not genetically related to one another. If they're similar, they're similar because of that shared environment. These are their correlations for general cognitive ability, each dot a different study. And what he, he's putting here the numbers are the sample size of the various studies. On average, I guess there's about 12 studies in childhood. On average, the correlation for adopted siblings in childhood is 0.26, which is not too different from that 35%. So it's, it's consistent with that. A, a good chunk of the variance here in general cognitive ability is associated with growing up in the same home. But this is childhood. What happens in adulthood? If I show you the rest of the graph, he's plotted the correlations now for adopted siblings after they've left their varying home and they're on their own. The lines connect longitudinal studies, ones that aren't connected with lines are different studies. But now the average correlation is no longer point 0.26, but point 0.04. Yes, there's a shared environmental cognitive effect on general cognitive ability. Yes, adopted siblings are similar for general cognitive ability, but only when they're living together, not when they've grown up and gone their separate ways. So, multiple lines of evidence suggests that general cognitive ability has a shared environmental influence, but that influence dissipates, declines with age. What about those other two domains? Do we see a similar pattern there? And I'm only going to give one study in each case. But what evidence exists, suggests that for political I'm sorry for social attitudes and for rule breaking behavior, the pattern we see for general probability holds. So let's first look at social attitudes. Here I'm going to just take one study. It's a nice large study of over 7,000 pairs of twins from the United States, age nine to 75. It's a study done by Linda [UNKNOWN], who's a British researcher, but now at Virginia Commonwealth University in the United States. And these 7,000 pair of twins took what's called the Wilson-Patterson scale. This is a very standard assessment for your political ideology, whether or not you're liberal or conservative. And it's involves, these are sample items, how you feel about abortion, the death penalty, immigration, there are 28 items. And you get a total score. And the total score gives a quantitative measure of how liberal versus conservative you are. So, much like we did with the general cognitive ability study, they divided up their twin pairs as a function of age. Again, they had a nice, broad age range. And here's its a little bit hard to see the graph, but here's a plot of both the MZ and its almost impossible to distinguish in here. The MZ and DZ correlations up to age 20. And its impossible to distinguish the two because the MZ and DZ correlations are virtually identical. If the MZ and DZ correlations are identical, there's no heritable effect. Everything in terms of the, the twin similarity is due to the shared environment. So up to age 20, it's all the shared environment and the non shared environment. Right? Genetics is completely unimportant. The two curves don't diverge. But look what happens to the MZ correlation up here in the DZ correlation. once you hit age 20, they move out on their own, now the MZs are more correlated with the DZs. The heritability increases, now there is heritability of whether not you're politically conservative or liberal. And the shared environment declines, just like we see with general cognitive ability. Rule-breaking behavior, there's a lot of studies here, I'm going to highlight just one. AC&E estimates again, additive genetic shared environmental and non-shared environmental estimates for daily consumption of cigarettes. How many cigarettes you smoke a day, if you do smoke. This is a large sample also coming out of Virginia Commonwealth University by a very prominent psychiatric geneticist named Ken Kenler. What he did in this case, they're middle aged individuals, and he asked them to report how many retrospectively report how much they smoked at different ages And what he's plotted here is the proportion of variance associated with genetic factor or the heritability, the a component in red, shared environmental factors in light blue, nonshared environmental factors in dark blue. With age, the heritability of how much you smoke increases. The impact of that shared environment decreases, the non-shared environment remains relatively stable. So yes, there are shared environmentally effects in these three domains, but they appear to be developmentally limited. They appear to be primarily limited to the period in which those relatives are residing together. And those effects might not endure much beyond that. So what have we learned about the nature of environmental influence from biometrical studies? First of all, and I know I've said this many times in the course, but I said it many times because I believe it's a very fundamental fact that we have to state over and over again. Environmental influences always exist. Behavior is not genetically determined. But the major source of environmental influence is non-shared environment. MZ twins are not perfectly psychologically similar, and their lack of perfect psychological similarity is owed to non-shared environment. Environmental factors that they don't share that underlie their behavioral differences. So we know that's a major source of environmental influence. Shared environmental influences don't appear to be important in general. There are some traits for which they do appear to be important, but that, the period in which they are important seems to be developmentally limited. That is, once we set out and begin to establish our own lives, the shaping influence of our homes maybe the impact of that actually declines. Certainly that's what the behavioral genetic literature suggests. So this, in this module, I've tried to describe for you this pattern that emerges repeatedly in the behavior genetic literature. When we begin to look at the, at, at developmental data. Next time, I'll try to explain why behavioral geneticists believe the heritability would increase while the shared environmental influence would decrease with age. Thank you. [SOUND] [BLANK_AUDIO]
