The classic and best experiment in this is Susan Mineka's work with a group of monkeys in Madison in the '80s, where she set out to examine the ontogeny of an instinct—in this care fear of snakes. Wild-born monkeys are afraid of snakes. They're so scared of snakes that they will cower in the back of the cage screaming rather than reach across a plastic model snake to get at a peanut when they're very hungry. Captive-born monkeys are not afraid of snakes; they happily reach across the model snake to get at a peanut. So what's going on here? That means that fear of snakes must be learned. But how on earth do you learn fear of snakes? The conventional classical conditioning wouldn't work very well, would it, because either you have a bad experience with a snake to learn from, in which case you're dead, or you don't have a bad experience, in which case you don't learn that snakes are frightening. So how are you going to end up acquiring a fear of snakes? It seems an absurd thing to acquire. She argues that what's happening is that there is a program for fear of snakes, an instinct if you like, but that that instinct needs to be socially triggered—in some sense triggered by a vicarious experience, by observing another monkey having a fear of snakes. So she set up an experiment in which she videotaped the wild-born monkey reacting with fear to a snake, and she then showed this video to a captive-born monkey, which immediately acquired a fear of snakes and was not then prepared to reach across even a model snake to get a peanut. She now doctors the video, so that it has the same monkey reacting in the same way in the background, but the bottom half of the screen now instead of having a snake has a flower. Again, the captive-born monkey has never seen a flower, so after it sees a monkey reacting with extreme fear to this new thing called a flower it should just as easily learn a fear of flowers. But it doesn't. It just learns that some monkeys are crazy. So what's going on here is that there is clearly an instinct for fear of snakes, and that's not surprising. Human beings have snake phobia. It's the commonest of all the phobias, even though most of us hardly even ever see a snake in our lives, but it requires an input from the environment. It requires a nurture input to be triggered. We know this is happening in the amygdala, and we're getting a bit of a handle on which cells are involved. We're not yet down to the gene level, but I'd bet my bottom dollar there's going to be a little pathway of genes in here that's mediating this process.


Judith Harris has made an immensely important contribution in that she has blown the whistle on a huge mistake that's been made, which is to assume from the correlation between parents and children that children are learning things from parents. It turns out that once you control for heredity, through the use of behavior genetics, twin studies and adoption studies, you find that in the development of personality in particular—and that's quite a narrow point—children do very little learning from their parents, but they do quite a lot of learning from their peers. That seems to me a very important breakthrough. Judith Harris is building upon the behavioral genetics studies, where people like Thomas Bouchard and others with their studies of twins have made an immensely valuable contribution. But just because they're proving that genes are important in things like personality doesn't mean they're proving that environment is not important.

What it means is that they're proving that variations in family environment within a particular society don't change personality. It's a bit like vitamin C. If you don't get enough vitamin C it can cause a huge variation in your health, in this case scurvy. But as long as you're getting enough vitamin C, having extra vitamin C doesn't make you any healthier. And that's probably the way families are. You've got to have a sufficient level of love, affection, interest and stimulation from a family, but once you've got that, having extra doesn't change your development, whereas genes do vary all across the spectrum, and can change your development even in a constant environment. It's a bit like saying a kid with one toy in its entire life is obviously massively worse off than a kid with ten toys, but a kid with ten toys is not noticeably worse off than a kid with a hundred toys—or in my son's case, five million toys, as far as I can make out.

There's a lot of people who want the twin studies to go away. They want them to turn out to be methodologically flawed. They want to find that these twins knew each other all along, or that somebody's faking the data—as indeed happened in the case of Cyril Burt, as far as we can make out, although there are some people who don't accept that. People who wish for that are going down the wrong alley. The methodological criticisms have run out of room to be any use. For example, people will say that you can't learn anything from comparing identical twins because identical twins have shared a womb. The point is that a lot of the argument's answered by the fact that you're also comparing non-identical twins reared apart, as well as identical twins reared apart. And once you've got a decent database of these things, you come up with these very strong results saying that variations of personality within American society are caused by variations in genes. Variations in intelligence within American society are caused mostly by genes, partly by family environment. These results are fantastically robust now. They're not just from Bouchard's study in Minnesota; there are also in the Virginia studies, the Australian ones, the Dutch ones, and the Danish ones. There are big studies about twins reared apart all over the world, and they're all coming to the same conclusions, so it's no good wishing them away. But the people who don't like these studies, and who wish them away, are actually allowing them to be more powerful than they are, because they're essentially thinking that they're proving that genes are important at the expense of environmental factors, and they're not. Often, the stronger the environmental factor, the more genetic variation you're going to pick up.

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