So, what have we learned from the methods of behavioral genetics? There are two big findings, and they're both kind of surprising. The first is, that there is high heritability for just about everything. Just about every psychological trait in which people differ, a large proportion of the difference is due to the genes. This is true for intelligence, it's true for personality, it's true for religiosity, is true for happiness, something we'll discuss in the final lecture. The way in which you're different from the person next to, you from your siblings, from your friend is to a large extent determined by your genes. Now, there's a critical qualification that's important to raise here, which is that, this conclusion about the importance of heredity; the importance of genes, applies within human groups. We know that there are differences between human groups, different ethnicities, different cultures, different countries, vary in intelligence. But it's fully consistent with the high importance of heredity for differences within groups, that the differences between groups may be due to environmental forces. To see this, consider an example by the geneticists, Richard Lewontin. Imagine two plots of land, each which have genetically diverse seeds, and you fertilize one and you don't fertilize the other. Well, planting the ones you don't fertilize are going to not grow as well. This has been an entire, and so they'll be a big difference between the plots that has entirely in environmental cause your choice of which plot to fertilize. This could be true even if within each plot the variation in the size of the plants is determined entirely by the genes, you can have a 100 percent heredity within each group but the differences between the groups have an entirely environmental cause. Lewontin makes his analogy on purpose because he's speaking to differences we find in human groups, you often find cases where human groups differ in all sorts of ways, but it turns out that they've had different environments. Imagine one group gets worse access to healthcare, education, to social opportunities, and so on, this group might fare poorly relative to a group that has a much better access to all of these things. This can be true even if the differences within each of the groups is due to environmental factors. Furthermore, we know that environmental factors can have a huge influence on intelligence, and here's how we know it, we know it through what's been called the Flynn effect. It goes like this, whenever we do IQ tests we always standardize them to 100, so if you want to know what the average IQ was in 1960 was 100, and if you want to know what the average IQ will be in 2050 it will be 100. But what Flynn did was, he asked the question, what would happen if you didn't standardized tests? His results were astonishing. Basically, if you look at people say from 1950 to people in 1980, or compare from the 1930s to 1990s, it turns out our intelligence has been rising. That is, if you take somebody from roughly 1985 and tested them using the same test as you did in the 50s, they would have an average IQ of about 120. Put simply, our IQs have been going up rapidly over the last 70 or so years, maybe 70 to 100 years. Nobody exactly knows why, but this is proof positive that groups IQs can change, can move around due to factors that have nothing to do with genes, because clearly there wasn't a genetic difference over this time. So, the first finding is the strong heritability, though, within group not between group. The second finding is almost as surprising. The second finding is that, all of the remaining variance, almost all the remaining variance, is due to non-shared environment. So remember, everything adds up to heredity, plus shared environment, plus non-shared environment and it turns out that not that shared environment, sort of environments you get from your parents, from within a family, statistically count for little or nothing. So, when it comes to personality or intelligence, an adopted child is no more similar to his siblings than to a stranger. To put it clear for example, IQ correlates with genetically unrelated adults who were raised in the same family is about zero. If I have a family and I'm high intelligence, introvert, with highly conscientious, whatever, my children are likely to share some of my traits, but my adopted children are not, suggesting that whatever I do to my kid, whatever effect I have, it's not shaping their basic psychological processes. This is such a surprising and weird conclusion, I'll devote the next part of the lecture to talking about it.
