Okay, hi. Welcome back. This module is the first module in the seventh week, unit seven. And to kind of frame or give you the context for this unit, the seventh unit I want to go back to something I talked with you about, in act, the very beginning of the course. And I told you that the way I see behavioral genetics, and what behavioral geneticists do, is they're using genetic methodologies and concepts to try to understand individual differences in behavior. Certainly a big portion of that is to understand the heritability of behavior, but also, behavioral geneticists want to use those methodologies to try to understand the environment or development. And really this seventh unit, what I'm going to try to do is highlight for you some of what I think are the most significant contributions of behavioral genetics. To understanding development, to understanding the nature of environmental influences and in fact, from my perspective, some of the more interesting and provocative findings coming out of behavior genetics actually concern the nature of environmental and not genetic influence. So, in this first module what we'll do is, I'm going to go back and, and talk a little bit more in depth than we have at this point about a distinction that I've made multiple times so far in the course. And that's the distinction between shared and non-shared environmental influences. And just by way of review. What behavioral geneticists mean by shared environmental influence, which they usually denote with the letter C, is the shared environmental influence corresponds to the effects of factors that reared together relatives share. Things like the socioeconomic status of the home you grow up in, the neighborhood you live in, the school district you reside in, how your parents approach child rearing. Two things I want to emphasize about this at this point, first is that strictly speaking, the shared environmental effect that behavior geneticists are estimating in their biometric models corresponds to the effects of these environmental factors. So for example, whether or not you grew up in poverty. Poverty is potentially a shared environmental factor, but the effect of that factor might differ for different individuals in the home. The shared environmental effect is the common effect of that factor. To the extent different siblings in a home react differently to poverty, that would be non-shared. So the first thing it it sounds a little bit nuanced, and I suppose it is, but the first thing I should really point out is we're talking about the effects, not necessarily the objective reality of these types of factors. The second thing I want to point out is that the signal, or hallmark, of a shared environmental effect is that it makes relatives who grew up together phenotypically, behaviorally, more similar than they would have been had they not grown up together. Non-shared environmental factors are denoted by the biometricians or viable geneticists, by the letter E. And these correspond to the effects of environmental factors you don't share with relatives you grew up in, with. This might be things like peer-groups or accidents that you might experience, or if your parents treat you differently than they treated your brother or sister. Non-shared environmental factors are the basis for phenotypic. dissimilarity or differences among individuals who are growing up in the same home. Now in this course, I focus, primarily, not exclusively, but primarily on two prototypical traits. Schizophrenia, a psychiatric phenotype and general cognitive ability, a psychological phenotype. And if we think about, the environmental sources there. For Schizophrenia, the predominant environmental source, was of the non-shared environmental, variety. For general cognitive ability, the predominant environmental source, was the shared environment. It accounted for about 35% of the variance in general cognitive ability in the biometrical models. In general, however, what most behavioral geneticists would complete is that general cognitive ability is actually exceptional in this regard. That for most psychological and psychiatric traits, overwhelmingly the most important source of environmental effects is the non-shared environment. And this is actually rather non-intuitive and provocative conclusion, and it dates back, originally, to this man, John Loehlin, who's a professor in the Department of Psychology at the University of Texas in Austin. And many years ago, back in the mid-1970s, he was doing a, a twin study with Paul Nichols, and at that time what they were interested in doing is actually determining whether or not, using a standard twin study, like the twin studies we've talked about here, they're interested in just determining whether or not personality was heritable. But along the way, they made a very interesting and unexpected observation about the nature of environmental influence. They certainly found that the environment was important to personality, but what Loehlin and Nichols concluded is that the important source of environment appeared to operate almost randomly with respect to the types of environmental influences developmental psychologists had traditionally emphasized in their theoretical models. In short, it was the non-shared rather the shared environment that they found to be important. To understand why they made this conclusion, we have to go back to something we talked about in week three. You may recall that to estimate the shared environmental effect from a twin study denoted here by C squared, sometimes just be C, but usually by C squared. That's the proportion of variance or individual differences in a trait that is associated with shared environmental effects. The Falconer estimate is take the DZ correlation and double it and subtract the MZ correlation. The corr, the, the formula's I, I'm sure not very intuitive, and that's really not important at the moment. What I want to do is note here, is that when C squared is going to be small or trivial. It's going to be small or trivial whenever the MZ correlation is at least twice the DZ correlation. And in fact, when it's at least twice the DZ correlation, C squared to be estimated at zero or some hopefully negative, small negative value, that is that there would be no evidence of a shared environmental effect. What Loehlin and Nichols observed back in 1976 when they first started studying personality was that the MZ correlation was systematically at least twice as great as the DZ correlation. C squared was not important. Now, here's a a more recent set of data from from John Loehlin. These are large twin studies; there are actually five twin studies of extraversion here. Extraversion is a major personality factor that corresponds to your tendency to be positively and actively engaged in your environment. And what they're reporting here for both women and men is that monozygotic twin correlation, the dizygotic twin correlation. And, again, you can see the samples are quite large in general, and the, the, it's kind of a complicated table. What I really want to highlight is the very general pattern here, that in every one of the ten possibilities here the MZ correlation is at least twice the DZ correlation. The estimate of C squared is trivial or is zero. It's not as if the environment is not important; MZ are never perfectly correlated. But the lack of perfect correlation in the MZ twins reflects non-shared environmental factors. That's important. The shared environment does not appear to be important. This is for one broad and very important dimension of personality, extraversion. In the same publication Loehlin al, also summarizes research on neuroticism, the tendency to, to experience psychological distress. And again, it large samples MZ DZ correlation, and all ten possibilities the MZ correlation is at least twice the DZ correlation. The shared environmental effect appears to be trivial or nonexistent. The non-shared is very important because the MZ correlations are about .5. So about 50% of the variants appears to be environmental, but non-shared environment. Not the shared environment. This is something that you'll see consistently if the twin study of personality is large enough, consistently you see this pattern, what Loehlin is reporting here. Now, if we reflect back to the way this course has gone, hopefully one thing you might wonder about is, well, maybe that's really that consistent pattern reflects some underlying flaw in the twin methodology. Can we show the same type of effect using a somewhat different research study design? And in fact, you can. One way you can find the same conclusion or come to the same conclusion, that the shared environment isn't, doesn't seem to be very important to personality is by looking at adopted siblings. We've talked about adopted siblings before in this course. Adopted siblings are actually individuals who grow up in the same home. They're reared together, but they're not genetically related. If growing up in the same home helps to shape our personalities, then people growing up in the same home should have similar personalities to, to some degree, even if they're not genetically similar. This is actually based on a study we recently did here of about 400 pairs of adopted siblings. It's much harder to get adopted siblings than twins, but so, 400 is actually a reasonably sized sample. Here's the correlation for extraversion and neuroticism in this study cited here. They're zero, they're effective, they're in both cases, not statistically, significantly different from zero. Both adoption studies and twin studies suggest for personality, the shared environment is not important. Even though the environment is very important. This pattern appears to be actually much more general than for personality and Schizophrenia. As I indicated before. And I'm going to try to talk about more in this module, and more in the subsequent modules, behavioral geneticists feel or believe based on the data, that this is a very general pattern. The environment is important, but it's the non-shared part of it. And probably the person who's done most to establish this imperical cor, conclusion, is this man here Robert Plomin. Robert Plomin is an American, but he now works at the Institute of Psychiatry in London. He's one of the most prominent behavioral geneticists in the world. And early in his career, he really began as a developmental psychologist, and like most developmental psychologists of his era, he thought that it was reasonable to assume that what shapes us environmentally, what shapes our psychologies in terms of our environment is the homes we grew up in. But yet, if you look at the behavior genetic literature, it doesn't seem that the shared environment is important. It's the non-shared. It's the things, those environmental factors we don't share with the people we are growing up with that appear to be important. That's really unexpected. That's provocative. It's non-intuitive. Now, again hopefully some of you are saying, hold on here. There are certainly some behavioral traits that do share, show a shared environmental effect. So why do behavioral geneticists draw this conclusion, right? Last week we talked about general cognitive ability, and 35% of the variance in general cognitive ability is associated with the shared environment. So, right there, there's a very important trait showing shared environmental effects. In fact, there's three domains to my mind that show important sizable shared environmental effects. One we've already talked about. The general cognitive ability. The other two we really haven't. The second is rule-breaking or antisocial behavior. The third is social attitudes including religiousness. All of these in the behavioral genetic literature show a shared environmental effect. And if you think about that, that maybe makes some sense. If you think about what parents are trying to achieve when they're rearing their children, they're probably not worrying too much about whether or not they're sociable or they, they'd rather stay home. But per, they're probably more interested in how they achieve in school, whether or not they follow the rules, and whether or not they're moral beings. And maybe our, the behavior genetic finding that you get shared environmental effects here really reflect where parents place their emphasis in socialize, excuse me, socializing their children. To understand why it is that even though there's evidence of shared environmental effects for these three major domains of behavior. Behavior geneticist generally conclude, that shared environment is not particularly important. We're going to need to go next time, and we'll take this up next time, in the next module, to bring up the concept of development. Thank you.
