Genes and Behavior
Prof. Gary Marcus
Associate Professor of Psychology at New York University
Author, The Algebraic Mind
Author, The Birth of the Mind: How a Tiny Number of Genes Creates the Complexities of Human Thought
As we walk about our daily lives, we live under the perception that our behavior is guided by our own volition. We think, therefore we do. While most of us would concede that the operations of our brain gives rise to our behavior, escaping from view is the role that our genes play in shaping the structure of our brain, and thus our behavior. This debate of nature versus nurture in understanding complex human behaviors is ongoing, but yielding to constant scientific inquiry.
Well, joining us today to discuss these issues is Dr. Gary Marcus. Dr. Marcus is Associate Professor of Psychology at New York University, and was a recipient of the 1996 Robert L. Fance Award for early contributions to cognitive development. He completed his Ph.D. from MIT at the age of 23 under the direction of Prof. Stephen Pinker, and was a fellow at the prestigious Center for Advanced Study in the Behavioral Sciences in 2002-2003. He has written more than 30 articles for journals such as Science and Nature, and is the author of two books: The Algebraic Mind, and The Birth of the Mind: How a Tiny Number of Genes Creates the Complexities of Human Thought. He joins us to discuss these issues of genes, brain, and behavior.
Charles Lee (CL) talks with Gary Marcus (GM) about genes and the brain.
CL: Prof. Marcus, thank you very much for joining us today.
GM: It’s a pleasure to be here.
CL: You’ve written a fascinating book, The Birth of the Mind. So, at last count, the Human Genome Project said we had between 30,000-50,000 genes.
GM: About 30,000.
CL: So, as you put in the subtitle of your book, how can such a tiny number of genes create the complexities of human thought?
GM: Well, it seems almost astounding, 30,000 genes, yet we have billions of neurons. And the question is: how do you get from point A to point B with so few genes and so many neurons. It seems almost impossible, because most of think of the genes as kind of a blueprint. You imagine one gene per one cell in the brain, one gene per one neuron, and if there were 30,000 genes that would explain the first 30,000 neurons, so how would you understand all the rest? But in fact it’s better to think of the genome as a recipe. It’s a kind of process that you can use just as well for a small cake or a large cake, for a tiny brain or a large brain. So the genome describes a process that each individual cell follows on its own independently. Every cell is kind of like a free agent and it’s got this master library, the genome, that gives a bunch of different recipes, and the cell figures out where it is, and it picks out the right set of recipes to follow. And, it’s all coordinated by the individual actions of all those cells.
CL: So, genes can have multiple uses, not just in a one-to-one ratio.
GM: Exactly. So each cell in your brain makes use of maybe half of those genes. Each cell might actually depend on 15,000 genes. And, each one of those genes may participate in the construction of billions of neurons.
CL: So there must be a multiplicative interaction of all these genes.
GM: Exactly. The combinatorial interaction is a key point.
CL: Are there any particular types of genes that have been implicated as being important for the development of our brain and behavior?
GM: Well, there are lots of different genes. One of the surprises is how much many of the genes that participate in building the brain also help to build the rest of the body. So, there are lots of basic structural genes that guide the metabolism of any cell, and they do that in the brain as well. In fact, if you look at a neuron, the basic cell in the brain, it looks different from other kinds of cells because it has these long feelers that we call axons that helps them connect to other neurons. But, they’re just a variation on a theme, a basic cellular theme for how to build any kind of cell. In fact, many of the genes that participate in building the brain are directly evolved for the body itself and then just changed a little bit for the brain. There are only some genes that are really unique to the brain. A lot of them are just variations on things that you would find somewhere else. But, the special part of any particular brain cell is how it communicates with other cells and how it evaluates the information that’s there. And a lot of the other genes are designed for that.
CL: What fraction of the genome is unique to specifying the brain then?
GM: Nobody really knows the answer to that. They’re still working on annotating the genome. So, we have this long list of DNA letters, which we translate into particular genes. Then we have to figure out the functions of those genes. I think what we’re going to find is that there are many genes that are expressed only in the brain, but they’re closely related to genes expressed somewhere else in the body. So, for example, there’s a gene called FOXP2 that you may have heard that’s very important for language. But, it also has some kind of function in the lungs. It may have two different functions in two different places. I think we’ll find that is quite common.
CL: So, in evolution, did this gene already exist and was then co-opted for another purpose?
GM: Exactly. Darwin had this wonderful phrase about ‘descent with modification’. We usually think of that in terms of species. So, one species derived from another ancestral species with some kind of change. So, humans are like chimpanzees, but with some variation. Well, you can think of that idea at the level of individual genes. Each individual gene is descended with modification from pre-existing genes. For, example the genes that help us to see color that build in our retina have actually varied over time. So, there used to be just one version, now there are two versions, one that descended with modification from the other. Ultimately, there are now three versions that allow us to see three fundamental aspects of color by modifying a plan that was already there.
CL: This multiple use of many genes could explain why humans and chimpanzees are quite similar genetically yet have different cognitive capabilities.
GM: That’s right. Our genomes if you just take genetic letter by genetic letter, nucleotide by nucleotide, are about 98.5% the same as chimpanzees. Most of the difference is actually what genes get turned on and when. You can think of every gene as having two parts. There’s a recipe part that says ‘okay, build this particular protein.’ There’s another part that says when you should build that protein and where you should build that protein. The difference between humans and chimpanzees isn’t in the recipe part. We’re basically built of the same stuff as chimpanzees, but it’s put together in a different order. It’s the part of each gene that says when and where you should build this protein that differs between chimps and us. It’s the organization of the material.
CL: How has your work looking at these genetic issues informed an understanding of behavior?
GM: I actually came to these genetic issues from the perspective of someone studying the acquisition of language. How do children acquire their first language? And what I found is that the younger we look at. I started by looking at three year-old children, and eventually started by looking at seven month-old children. And, as we look at younger and younger children, I’ve always been impressed by how much was built into the mind. It’s not that we know a particular language at birth, but we’re built with the mechanisms to learn language. And I got into genetics, trying to understand how genes can actually construct the brain. So, what we’re trying to understand now in my own lab is how do these small numbers of genes give these complicated brain structures that give rise to the complexities of human thought.
CL: So, there does seem to be some innate structure for language?
GM: Absolutely. One question is how that descended with modification from other brain structures that help us to deal with other parts of our mental life. So, for example, the ability to use language depends on a memory system that was already in place in our ancestors. But maybe that memory system is reconfigured in a special way to allow us to represent language.
CL: Has language actually been a relatively recent adaptation in evolution?
GM: Well, we diverged from chimpanzees six million years ago. And some people think that language only arose 50,000-100,000 years ago. It’s quite recent from the perspective of evolution. And so what that tells you is that somehow, in small ways, a pre-existing system was changed to give a really profound impact.
CL: So, what other types of behaviors are linked strongly with genetic causes?
GM: Well, there are lots of behaviors that are tied to genes, but they’re not tied as closely as some people might think. So, the popular way of thinking about genes is that you have a gene for a particular trait. There’s a wonderful cartoon that talks about a gene for delusions of stock market grandeur. But, we don’t really have a gene for delusions of stock market grandeur, but we do have genes that affect all kinds of aspects of our personality. They do that by working together with other genes and by working with the environment. So genes don’t really dictate things. They give us options. They give us opportunities. And they can influence us. Push us a little bit in one direction or another.
CL: Rather than dictating our particular types of behavior, they would guide more our temperament or inclinations.
GM: Yes. They’re more likely to influence something like temperament rather than a particular type of behavior. Genes build the brain. Then, the brain takes us on the day-to-day operations. Genes are too slow to make decisions on a moment-by-moment basis.
CL: Where is this work of understanding the relationship between genes and the brain actually heading?
GM: There are both scientific and practical consequences. As a scientific consequence, I hope that we will eventually understand how the language acquisition device works, and how it gets wired into the brain. On a practical side, we should be better able to diagnose and how to treat things like language disorders, schizophrenia, and so forth. Yet another possibility that I discuss at the end of the book is that we might be able to use genes to identify people who are at risk, and then apply social interventions to help those who are most at risk.
CL: So, there’s a good deal of interaction with the environment that can sway even your genetic predisposition to certain types of disorders.
GM: There’s a great example, which I actually talked about in an op-ed in the L.A. Times last week. There’s a particular gene that causes a predisposition for violence, but only for people that are raised in abusive families. So, if you have the particular version of the gene, you can think of it as giving you two different strategies, one of which isn’t particularly violent, the other of which is. And, it’s actually the environment that brings out the aggression.
CL: Well, we are running out of time, but I am curious how you became interested in these issues of genes and the brain.
GM: I actually started out as a kid trying to program computers to understand language. I found that it was a very difficult task, far beyond what I was able to do, but it pointed me to trying to understand how humans understand language. I got interested in the work of Noam Chomsky, and went to graduate school in the lab of Stephen Pinker. And so, I’ve been studying in my laboratory research how children acquire language, and that’s what led me to want to understand the genetic contribution, because there seemed to be so much built in.
CL: Well, it is certainly very fascinating work, and I want to thank you again for joining us to discuss this fascinating issue.
GM: My pleasure. Thank you very much.