Welcome back to an introduction to human behavioral genetics. This is lecture unit two and this is the third module. In lecture unit two, we're talking about twin studies, and actually a little bit later, we'll be talking about alternatives to twin studies. But today we're going to actually look, I'm going to try to give you an overview of what twin studies in psychology have found. Recall that a twin study is a natural experiment. What psychologists are trying to find out is whether monozygotic twins are behaviorally more similar than dizygotic twins, in which case we're going to infer that genetic factors play some role in contributing to individual differences on whatever trait we'll measure that phenotype. Alternatively, if they're approximately equally similar, then we're going to say that it's their rearing environment that's the most important in shaping individual differences on that phenotype. Before getting involved in twin studies themselves, we should say something, a little something about the feasibility of twin studies. We called it in the first module in this lecture, we talked about the frequency of twins. Twins are actually extraordinarily frequent. In the U.S. approximately two percent of the population is a twin. So twins are there, but how do twin researchers go about finding twins? I know this is kind of a busy slide, but this was this last year, in 2013 this journal, Twin Research in Human Genetics, which is one of the leading twin research journals in the world, published a special issue on twin registries throughout the world. And in this special issue, every star here on the world map is actually a twin registry. And I think every continent is represented, except may be South America. And although they're not represented in this special issue, I suspect there are also twin registries there that just for some reason they, they didn't get represented in this special issue of this journal on twin registries. And, this special issue talks about over 1,500,000 twins participating in research and all these different sites throughout the world. Twin research is a very active area of research. In many of these countries, certainly the European countries and some of the Asian countries they have public health systems that allow researchers to identify twins through the public health systems. For example, I work in Denmark and in Denmark, we have a twin registry. And, every twin in the country is registered in that twin registry. And, so we, when we want to ascertain or identify a sample of twins, we go to the registry and identify where they're living now, and we contact them. Of course, there are ethical guidelines for doing that, but it's relatively easy to identify twins. In other parts of the world where we don't have public health systems, it might be a little more difficult to identify twins. But researchers, twin researchers, have come up with a variety of ways of doing this. So, for example, in the state of Minnesota, we identified twins through birth records, which happened to be public records, in the state of Minnesota in the US. People identify twins through the school system or through special clubs run for mothers of twins. So, there's a lot of ways of systematically ascertaining twins. And, researchers throughout the world have accumulated quite large samples of twins in order to undertake twin studies. The other issue about feasibility that we should say something about before looking at the studies is, well, how do you tell twins apart? How do you, how can you surely how can you be sure that this is a monozygotic pair versus a dizygotic pair? In fact, Galton recognize the utility of twins. Well, over a hundred years ago, but Galton could never quite figure out how to reliably tell monozygotic twins from dizygotic twins. How do you determine their zygosity? Well, there's several ways of doing this. The gold standard is to do genetic testing. Taking a biological specimen that might be blood or saliva and actually genotyping genetic markers. Monozygotic twins, because for all intents and purposes, they should have the same genetic makeup, should be, have the same ABO blood type or the same rH factor, and so on and so forth. So if you genotype them on a multiple markers, you can fairly quickly determine that these are monozygotic twins versus dizygotic twins. Methods of genetic testing are over 99.9% accurate in identifying monozygotic twins from dizygotic twins. The drawback of using this gold standard is it can be expensive and you don't always acc, have access to the twins themselves. Sometimes, surveys are done through the mail, for example, where you wouldn't have ready access to gaining a biological specimen. So alternative is to use questionnaires and these questionnaires actually seem very simple-minded. You ask questions like, this would be questions asked of a parent, do t, teachers often mistake the twins for each other or are the twins as alike as two peas in the pod? But there's an overwhelming amount of information now on twins that show that just answers to two simple questions like this can lead to 90 pher, 95% accuracy or sometimes a little bit more in differentiating monozygotic from dizygotic twins. And, for most purposes, that's adequate. Finally, a last way of differentiating twins, and you'd actually have to see them, is by having an expert look at the twins very carefully. This is a picture, actually, it's a composite of two twins; they're monozygotic twins. But the thing I want to point out here, is their irises. Their iris, our irises are very distinct. You look at people's irises and monozygotic twins have very, very similar irises. You can see they actually have diff, different faces, but they have very similar irises here. Twins also have very we, we, each of us have very distinctive earlobes and attachments. And if, if an expert looks at your irises and looks at your earlobes, they pretty well can tell if you're monozygotic or di, dizygotic, usually with 90% or even greater accuracy. So twins are common and we've developed ways of reliably differentiating monozygotic from dizygotic twins. What if twin studies told us about human behavior? Well, there have been hundreds of twin studies in psychology and I'm not going to go through all of those studies here. Again, that would be very, very tedious to do that. In fact, I think the data are extraordinarily consistent from what we're finding from twin studies. Three things we see, one is that very consistently on behavioral phenotypes monozygotic twins are more similar then dizygotic twins. Secondly, monozygotic twins are never perfectly similar for a psychological trait. Finally, the similarity we see in twins for psychological trait doesn't look all that much different from the similarity we see for twins for physical or medical traits. So, let me give you just a couple illustrative review findings on this rather than go through a lot of individual studies that show the same pattern pretty much over and over again. So this is a review actually of a paper published 20 years ago now, but not much has really changed in terms of what we see in twin similarity than what Robert Plomin published in Science in 1994. What he's plotting here are twin concordances for various psychological or psychiatric conditions. So recall concordance is a measure that can theoretically vary from 0% to 100%. In this case the dark bars, the solid bars are the monozygotic twin concordances. The hatch bars are the dizygotic twin concordances and these are then the categorical phenotypes. Two things are evident here. One is the dark, the solid bars are consistently larger than the hatch bars. MZ twins, for psychiatric traits, are consistently more concordant than dizygotic twins. Traits that aren't on here that also would, this would be true for things like depression or bipolar disorder. It's a very consistent pattern. There's some variability in the magnitude of the concordance. But what really doesn't vary much is that monozygotic twins are more similar than dizygotic twins. Genetics seems to be important here. Secondly, the other salient and consistent finding is that the monozygotic are never 100% concordant for a psychiatric or psychological condition. Sometimes they're discordant. That's an important observation, because if, that means that monozygotic twins are never behaviorally identical. Because they're genetically the same, but not behaviorally identical. That lack of behavioral identity has to be due to their environment. They have the same genotype. So if they end up different, it has to because, be, because their environments have shaped them somewhat differently. So this observation of less than perfect concordance in monozygotic twins is a very important one and that it underscores the importance of the environment. In the same article that Paulman published, these concordance figures for psychiatric phenotypes, he also published concordance figures for physical phenotypes. Physical disorders here. And again, with the dark bar as being the monozygotic twins and the hatch bars being the dizygotic twins. And again, the point of including it here is that, the pattern, the qualitative pattern again, the concordance rates will vary depending upon the condition. But the qualitative pattern doesn't look all that markedly different for the physical disorders versus the mental disorders. If anything, some of the physical disorders, some of the cancers, like breast cancer or Parkinson's Disease, look less genetically influenced than some of the psychiatric disorders. That's concordance. But what about correlation? What about quantitative psychological traits? Traits that vary within the normal population. Do we see the same pattern of results? Well, we do. Here's again from that Plomin article. Correlation coefficients for monozygotic twins in the solid bars, dizygotic twins in the hatch bars, for various, in this case cognitive and personality factors, again, you see the two very general patterns we see over and over again. Monozygotic bars are higher than the dizygotic bars. Monozygotic twins are more similar than dizygotic twins on these quantitative phenotypes of twins. Monozygotic twins are never perfectly similar. The correlations are not one. The environment is also important. This pattern I'll confess something to you here, this pattern is so consistent that I rarely read a twin study anymore, because I already know if it's a twin study that they're going to report the correlations for phenotype x, I pretty much know what the results are going to be. The monozygotic twins are going to look more like the dizygotic twins. We've seen this in hundreds and hundreds of studies. It's a very consistent pattern. It's so consistent that you could look at traits that you would think the patten would be very improbable for, yet we still see it. Here are three traits where you might not expect monozygotic twins to be more similar than dizygotic twins. The first is divorce, the second is religiousness, and the third is political attitudes, whether or not you're conservative versus liberal, essentially. These first two studies, the divorce study and the religious study were studies that I was involved in and actually I got a lot of grief for this divorce study. People thought I was crazy to publish a finding where monozygotic twins were more concordant here for whether or not their marriage broke up than dizygotic twins. They weren't perfectly concordant, again. The environment's certainly very important. But come on. Could genetics influence a risk of divorce or whether or not we're religious, or who we vote for in an election? That seems pretty preposterous. Am I trying to tell you there's a God gene, a divorce gene, or a conservatism gene? Well no, I'm definitely not trying to tell you that. There's, there are no such genes. But, at the same time, I am going to try to tell you that things like whether or not our marriages end in divorce, whether or not we vote for this political candidate or another political candidate, or whether or not we believe in God, might be genetically influenced to some degree. It certainly cannot be genetically determined, but it might be to some certain degree genetically influenced. Why might it be genetically influenced? Why might we be seeing this very consistent finding? Well there are two possibilities. Only one of which I subscribe to, but there are two possibilities think we really should entertain in this course. The first is that genetic factors do influence these traits. That the twin studies, the observation that monozygotic twins are more similar than dizygotic twins, is really telling us something about the importance of genetics. But we know, from basic biology, that there aren't genes for behavior. There aren't genes for divorce, but there aren't genes for extroversion of IQ or schizophrenia as well. That isn't what genes do. If there are genes that influence our behaviors, they do it indirectly. It would be a pleiotropic effect of the gene. In the case of divorce, where again, I, I took a lot of grief for publishing that paper, but I, I, I actually kind of like the paper. In the, in the case of divorce, there aren't, there aren't any genes that code for your likelihood of divorce. But one of the things we went on to show. Is that whether or not you are divorced is related to the personality you have. Certain personality characteristics increase the likelihood that your marriage ends in divorce. And you can entertain that the types of personality characteristics those might be, but that's not a preposterous notion to think that our personalities might influence our likelihood of divorce. One of the reasons, maybe the major reason, divorce is heritable, is because personalities, our personality characteristics are heritable. And why is personality heritable? Well, neuroscientists would say our personality characteristics are heritable because our brains are in part heritable, the neurotransmission systems in our brains. So when we're looking at something like divorce or even personality or schizophrenia we're seeing something that might indirectly influenced by genes. So that's one possibility to explain the consistent observation in twin studies that monozygotic twins are more similar than dizygotic twins. What's the second possibility? Well, the second possibly is that twin studies just might be wrong. They might be selling us something consistently, but they might just be consistently wrong. They might be invalid. And some very learned people have argued that twin studies might be wrong. So this is a subject that we need to take, consider very seriously here. And that's something that we'll look at in the next module. [BLANK_AUDIO]
