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Twin studies and adoption studies

Twin and adoption studies help scientists understand the roles of nature (genes) and nurture (environment) in our behaviors and health. Identical twins share 100% of their genes, while fraternal twins share 50%, making them ideal for such research. Adoption studies compare adopted individuals to their biological and adoptive families, offering insights into genetic and environmental effects.

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Video transcript

- [Voiceover] Two types of studies that are very important in the behavioral and social sciences, but are also important in the health sciences as well, are twin studies and adoption studies. And these studies are important because they can help researchers tease apart nature, our genes, our genetic code, and nurture, our environment. So they can help us figure out what things we inherit from our parents and what comes about through our interaction with our environment. Where we live, our surroundings, our parents, our peers. So let's start off by talking about twin studies. And there are two types of twins. Monozygotic twins, who are also known as identical twins, and dizygotic, or fraternal twins. Monozygotic twins develop from a single fertilized egg. And I'll underline mono here, because that means one. So we have one egg and we have one sperm, and they come together, the sperm fertilizes the egg, and the egg splits into two. And because the egg splits after fertilization has occurred, monozygotic twins are genetically identical to each other. They share 100 percent of their genes. So identical twins don't just look the same, they have the same genetic code in every cell. Fraternal, or dizygotic twins, develop from two separate fertilized eggs. And here I'll underline di, which indicates two. So instead of one egg being released during a woman's cycle, two are released instead. And these two different eggs are fertilized by two different sperm. And as a result of this, fraternal twins share 50 percent of their genes. So fraternal twins have the same genetic relatedness as regular siblings. And just like you and your siblings, they might look alike, but they aren't going to be identical. But unlike regular siblings, both identical and fraternal twins are unique in that they share the same environment. First off, they share the same prenatal environment. They're exposed to the same things while they're in the womb. But twins are also raised by the same parents at the same time. They eat the same food and share the same toys and the same germs. And so we say that both identical and fraternal twins can be said to share 100 percent of the same environment. Or at least as close as two people could possibly get. But what about regular siblings, don't they share an environment? Sure they do. And it can be kind of hard to assess how similar their environments are, but it certainly isn't 100 percent. And you might think no, that doesn't really sound right. Siblings are raised by the same parents, how different could their environments really be? And on the one hand, you're totally right. Siblings definitely have a more similar environment than they would with some random person on the street. But the environments of siblings can actually vary quite a bit. So for example, I have a younger sister, and I think my parents were way less strict with her than they were with me. I mean, I was their first kid, they didn't really know what they were doing. But they were much more prepared when she came around, and so they were much more relaxed. Also, my sister and I grew up in different places. I grew up in an apartment, but by the time my sister was born, we had already moved into a house. And while we may have gone to the same schools, we had different teachers, and different friends. So while we certainly shared a similar environment, it is not as similar as we would see with twins. So let's say that you're a scientist and you're very interested in learning about what causes schizophrenia. Is it genes, or is it something in the environment? Maybe a toxin that people with this disorder were exposed to in childhood. And one thing that you might have noticed in your past research is that children are more likely to develop schizophrenia if one of their parents has the disorder. And you might take that as an indication that there's a genetic component. And certainly, there might be. But one problem with drawing this conclusion is that we're not sure about the effects of the environment. It's entirely possible that both parent and child develop schizophrenia not because of any genetic component, but because there's some kind of chemical in their environment, maybe something in the water, that's actually triggering this disorder. And so what researchers want to do is to try to isolate genes and the environment, to try to look at one without the other to see whether or not that will give us a clearer picture of what causes this disorder. And one of the ways that we can do this is with twin studies. So we could look at the rates of schizophrenia in both identical twins and fraternal twins. And what this allows us to do is it allows us to examine the effect of genes while the environment is held constant. So as we said, identical twins have 100 percent of the same genes and fraternal twins have 50 percent of the same genes, and they both share 100 percent of the same environment. So these two identical twins grew up in the same time and the same place, and these two fraternal twins grew up in the same time and the same place as well. So in this study, if schizophrenia was primarily caused by genes, you would expect to see different rates of the disorder in identical and fraternal twins. You would see that if one of these identical twins had schizophrenia, there would be a higher chance the second twin would have it as well. At least higher than you would see in fraternal twins, or regular siblings. However, if schizophrenia was more of a factor of what environment someone had been exposed to, we would expect to see similar rates of the disorder in both sets of twins, because it is likely that each set of twins was exposed to the same environmental effects. So if something within that environment caused the disorder, both sets of twins would've interacted with it. It wouldn't make a difference whether or not they were identical or fraternal. So to review, if identical twins, who share 100 percent of the same genetic material, resemble each other more than fraternal twins, who share 50 percent of their genetic material, then we would conclude that that trait or disorder or behavior had a strong genetic component. However, just like with every study methodology, there are some problems with twin studies. For example, it's possible that identical twins are treated more similarly than fraternal twins are. And this could have some kind of unexpected effect. And so even though both identical and fraternal twins share more of their environment than regular siblings, it is possible that identical twins share even more of the same environment than fraternal twins do. Another type of study that can help us tease apart nature and nurture are adoption studies. In these studies, individuals who have been adopted are compared to their adopted families and their biological families. So sticking with our schizophrenia example, if rates of schizophrenia in adopted individuals resemble the rates seen in their biological family, but not their adopted one, then we would conclude that there is a strong genetic component. But if we find that there is no relation between an individual and their biological parents, but that there are similar rates for adopted children and their adopted parents, then we would conclude that environment plays an important role. So because adopted individuals are exposed to different environments from their biological relatives, their genetic relatives, it makes it easier for scientists to tease apart genetic and environmental effects. But there are problems with this methodology as well. Because a child has been adopted, we might have incomplete information about their biological families. Also, placement for adoption is not random. Children are not just randomly handed to any member of the population. And some agencies might even try to match individuals with an adopted family that is similar to their biological family. In terms of community and culture. And this can make it more difficult for us to determine whether or not something is genetically driven or environmentally driven. So we have twin studies and we have adoption studies, but it's also possible for these two types of studies to be combined. In some very rare cases, identical twins are put up for adoption and are each adopted by different families. So we have two people who are genetically identical, but they are being raised in different environments. And you can see why this would really be an ideal situation for scientists in terms of studying the underlying effects of nature and nurture. Using our schizophrenia example, if it was primarily caused by genes, if it had a strong genetic component, you would expect to see similar rates of the disorder in both identical twins raised apart and identical twins raised together. But if it was mainly driven by the environment, you would not expect to see the same rates within these two twin populations. But once again, we have the problem that adoption isn't random. In fact, families that adopt tend to be very similar to one another. They tend to be wealthy, because adoption is very expensive, but they also clearly want a child, and they're really going out of their way to get one. And this might result in these families having similar parenting styles. And so even though these two identical twins might be raised by different families, their environments might not be as different as researchers would like. So to review, if something has a strong genetic component, whether it's a behavior or a trait or disorder, we would expect to see more similarity between identical twins than fraternal twins, we would expect to see no difference between identical twins raised together and identical twins raised apart, and adopted children would have more similarities to their biological families than their adopted families. But if something is environmentally driven, then we would expect to see no differences between identical twins and fraternal twins. We would also expect to see closer rates in identical twins raised together, closer than we would see in identical twins raised apart, and we would expect adopted children to have rates closer to their adopted families as compared to their biological families.