So we reviewed the couple of ways in which people are different. We're different in our personality and we're different in our intelligence. But now let's turn to the question of what makes people different, why are we different? And very generally, there's two different possibilities, there is genes and environment, or sometimes known as heredity and environment, and often knowns as nature and nurture. The way we define these considerations, they're exhaustive, there is nothing else. So first look at genes, and in particular look heredity. So psychologists behavior or geneticists talk about the proportion of a trait that's heritable. And it's important to know what that means. It doesn't mean how much of the trait is due to your genes. I mean, asking how much of your height or intelligence is due to your genes is actually a very confusing question. Rather, when we talk about heredity, we're talking about differences. And in particular, we're talking proportion of variance that's due to genetic differences. And the way people think about differences in this field is you go from zero to one. And zero to one means 0% and one is a 100%. So a way to think about it is, height is fairly heritable because the different heights that people have is due to a large extent in their genes. But think about the number of legs people have. The number of legs people have, most people have two legs, but some have one, and some have zero. Is the number of legs that people have heritable? Well, no, it's actually not highly heritable. For the most part, the reason why you have only one leg or no legs is due to some accident, some sort of experience. So that's genes. And then what remains is environment. And we could parcel environment up into two parts. There's shared environment, this is the proportion of the variance in people that's due to environment that different family members share. So people living in the same household might share, your siblings might share. And then there's non-shared environment, which is everything else. So if you're different from your brother because you got hit by a snowball, or you got the flu one day, or somebody bullied you, or you won the lottery, that's non-shared environment. And together, this is it, everything is either heredity, shared environment, or non-shared environment. And now we can ask the question of how to resolve the puzzle of the extent to which these different factors explain variation. In the real world, it's kind of hard to pull them apart. So I'm a lot like my sister. And we could say, why? Well, in part, well, we share a lot of the same genes because we have the same biological parents, but we were also raised the same way in the same family. And so to pull them apart, you look at some interesting cases. So for instance, you could look at monozygotic twins. Monozygotic twins are essentially clones, they share all the same genes. And one could compare them to dizygotic twins. Dizygotic twins, although they're born at the same time, are just siblings, they're regular old siblings. They have a 50% overlap in genes. And you could compare these to adopted siblings, which have no special genetic overlap. If my wife and I adopt a kid into our family, that kid's relationship to my sons, they'll have to share an environment, we'll raise them the same way, for the most part. But he won't share the same genes. And finally, two people raised in the same house, by the same parents, by definition have a 100% the same shared environment. So that's all background, but now this gives you a way of asking some questions. So you might ask, for instance, are monozygotic twins identical twins, much more similar to one another than dizygotic twins? And if so, this might suggest there's a high role of genes, high heritability. On the other hand, a critic could say, well, maybe monozygotic twins will be treated more similarly than dizygotic twins, both within the family and outside the family, because they look so much alike. So we have to bear that qualification in mind. You might ask, what if monozygotic twins turn out to be just the same as dizygotic twins? Well, that shows the genes don't matter that much. What about adopted children? Well, similarly, if they turn out to be highly similar to their brothers and sisters, that suggests there's a high role of shared environment. Suppose all of the children in the Bloom family have an IQ of 105. And we adopt children, and they grow up and they have an IQ of 105 too. That would suggest that the Bloom's family environment is shaping their IQs. On the other hand, if they entered a family and they end up with widely different IQs depending on where they're from, depending on their genes, this would suggest that IQ is shaped more by the genes than by the environment. The most exotic case, and of course, this is is very rare, is twins raised apart. If twins raised apart, identical twins are raised apart, then they would be, and they're entirely dissimilar, dissimilar as any other people, any two strangers. This suggests that with regard to what you're looking at, there's no effect of genes. On the other hand, if they're highly similar, there's a high effect of genes. And in fact, to get to the next lecture, which is about what we know about these differences, it actually turns out very surprising that identical twins raised apart turn out to be surprisingly alike. They have similar attitudes to the death penalty, religion, to music, similar behaviors with regard to crime, gambling, and divorce. In one study, they took two identical twins raised apart and then scanned their brains. And they were so similar that you couldn't tell them apart. But now what we'll do, more generally, in the next lecture, is talk about what we know, about where these differences come from, using the tools we've just discussed.
