So one important point is that genes are designed to produce human behavior through nurture. But there's another phenomenon going on too, which is equally important and which again people in these kinds of debates over human nature have missed. They couldn't have failed to miss it until recent molecular biology made a difference. That is, behavior affects genes. It doesn't change the code of the gene, and it doesn't change the encoded genome. Sure, you can change your encoded genome by having a mutational accident, by flying in an airplane and having cosmic rays damage your DNA. But what I'm talking about is changing the expression of genes through things you do in your life. The encoded genome is a set of DNA. The expressed genome is the RNA that's translated from it and then made into proteins. That process of expressing the encoded genome is controlled by transcription factors and all these other things that interact with the promoters, which turn the genes on and off and turn the volume of the genes up and down like thermostat switches, or whatever analogy you want to use. That process is itself at the mercy of the way we behave because you can do things in your life that literally lead you to alter the expression of genes.

Two quick examples of that to get the point across: One is stress. When you're under stress, the physiological result is that cortisol increases in your body and has a lot of effects. Cortisol is a transcription factor; it actually alters the expression of certain genes. It does so largely in the immune system, which results in the suppression of immune activity. It lowers your immunity, which is why when you're under stress you're more likely to catch a cold. So here's an example in which something outside you—whatever it is that causes stress, an argument or an exam—affects the expression of your genes in your immune system. But a much more everyday example is simply the process of learning and memory. Associative memory, conditioning the association of one stimulus with another, is an immensely important part of learning. That process involves the changing of the strength of synapses between neurons in real time. As you form a long-term memory, you change the shape and the size of the synapse, so that when two neurons that are connected keep firing together—meaning that you're associating a particular smell with a particular sight or something like that—then the connection gets strengthened. In the future only one of them need fire and it will provoke a memory of the other.

That process of changing the strength of synapses between nerve cells is mediated by genes. It actually requires the switching on and off of genes in order to change the synapses. These genes we now know, because of work on fruit flies, are called the CREB genes. There are about 17 of them in that particular system, and they're also in mammals and humans as well. They prove that memory and learning is a genetic process. That doesn't mean that it's a hereditary process—of course not. What we're talking about here is changing the expression of the genes in real life in response to what is literally the formation of a new memory—a new experience, in other words.

So the simple linear model of going from genes to behavior, which both the nature people and the nurture people have subscribed to—the nurture people saying, that model doesn't work, the nature people saying, that model does work—is wrong. It's nothing as linear, as deterministic, or as frightening as nurture people would have you believe, nor is it anything nearly as linear as the nature people would have you believe. It's much more interesting than that. With these feedback loops, from behavior back into the expression of genes, it means that the process of the creation of human nature and the alteration of human nature throughout your life intimately involves genes and experience at the same time. The more genes you have, the more genetic programs you have, the more experience you can get into the organism.


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