The Birth of the Mind

How a Tiny Number of Genes Creates The Complexities of Human Thought


By Gary Marcus

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In The Birth of the Mind , award-winning cognitive scientist Gary Marcus irrevocably alters the nature vs. nurture debate by linking the findings of the Human Genome project to the development of the brain. Startling findings have recently revealed that the genome is much smaller than we once thought, containing no more than 30,000-40,000 genes. Since this discovery, scientists have struggled to understand how such a tiny number of genes could contain the instructions for building the human brain, arguably the most complex device in the known universe. Synthesizing up-to-the-minute biology with his own original findings on child development, Marcus is the first to resolve this apparent contradiction by chronicling exactly how genes create the infinite complexities of the human mind. Along the way, he dispels the common misconceptions people harbor about genes, and explores the stunning implications of this research for the future of genetic engineering. Vibrantly written and completely accessible to the lay reader, The Birth of the Mind will forever change the way we think about our origins and ourselves.


"Gary Marcus has written the real deal. This book is smart, tough, and insightful."
—Michael S. Gazzaniga, author of Nature's Mind
"If you've ever wondered about the space between genes and the mind, here's the book that connects the dots. Marcus's book is clear, fascinating, and up-to-date."
—Howard Gardner
"This is without a doubt the clearest account of the relationship between genes and environment that I have ever read. Marcus has digested an enormous amount of information from very diverse fields, and integrated it into a smooth-flowing, user-friendly whole."
—Derek Bickerton, author of Language and Human Behavior
"Engaging and clear . . . covers a truly amazing range."
—Richard C. Atkinson, past President of the University of California
"The Birth of the Mind is fast, accurate, and informative—an excellent overview of new ideas and research. Gary Marcus will light up all eight of your brain's lobes!"
—Greg Bear, author of Darwin's Children
"A lucid, lively text."
Library Journal
"A wonderful book which I heartily recommend to any interested readers who want to explore either genomics or the workings of the mind/brain."
"Interesting and accessible . . . progress in genomics means that we are now ready to learn about the mind by understanding how genes build the brain."
Genome News Network
"An engaging and often witty look at how our genetic code can be simple enough to make basic proteins and complicated enough to help us learn languages."
— Editorial Review
"Psychologist Marcus provides an accessible discussion of genetic history and current research. . . . A lucid, pleasing chronicle of how genes construct the human mind."

The Algebraic Mind: Integrating Connectionism and Cognitive Science

The genetic code is not a blueprint for assembling a body from a set of bits; it is more like a recipe for baking one from a set of ingredients. If we follow a particular recipe, word for word, in a cookery book, what finally emerges from the oven is a cake. We cannot now break the cake into its component crumbs and say: this crumb corresponds to the first word in the recipe; this crumb corresponds to the second word in the recipe, etc.
—Richard Dawkins
FRANCIS CRICK, codiscoverer of the structure of DNA, recently argued in his book The Astonishing Hypothesis that the activity in our minds has its basis in our brains: "To understand ourselves, we must understand how nerve cells behave and how they interact."1
Crick is surely right that the mind arises from the activity of the brain. But, having grown up in the late twentieth century, the son of a software engineer who once studied the biophysics of neurons, I can't say that I am astonished. To many people of my generation, it has become obvious (maybe even banal) that our thoughts are the product of our brains. In the words of MIT cognitive scientist Steven Pinker, "The mind is what the brain does."2
In contemporary society, we are surrounded by evidence of the influence of the brain on the mind. Science has shown that Prozac affects our mood by targeting the brain, that strokes can cause brain lesions that alter our behavior, and that distinct parts of the brain are active in different aspects of cognitive functioning—the right brain when we listen to music, the left when we listen to speech,3 the amygdala when we are frightened,4 the right prefrontal cortex during orgasm.5
But although most people have by now accepted the fact that the mind has its origins in the brain, far fewer have become comfortable with a second fact: that the origins of the brain are in the genes. The molecule that Crick helped to decipher just over fifty years ago has had an enormous impact on science, medicine, even law. Yet it has had almost no impact on theories of the mind.
If genes can predispose us to cancer or diabetes, it stands to reason that they might significantly shape our minds. It is easy to admit that genes have something to do with why one breed of dogs is friendlier (or meaner) than another, but even scientists can be reluctant to accept the notion that they might affect our own thoughts and behavior. In a recent issue of Current Anthropologist, two Stanford biologists, Paul Ehrlich and Marcus Feldman, wrote that "the concept of overall heritability should be restricted in its employment to plant and animal breeding. . . . [When it comes to humans] genes can control some general patterns . . . but they cannot be controlling our individual behavioral choices."6
Ehrlich has gone so far as to argue that the effect of genes must be limited because of what he has dubbed a "gene shortage." Our species has perhaps 30,000 genes, yet our brains have on the order of 20 billion neurons. "Given that ratio," Ehrlich concluded, "it would be quite a trick for genes typically to control more than the most general aspects of human behavior."7 This view was recently echoed in the writings of cultural critic Louis Menand, who, in the pages of The New Yorker, wrote that "every aspect of life has a biological foundation in exactly the same sense, which is that unless it was biologically possible it wouldn't exist. After that, it's up for grabs"8—echoing an old boast by John B. Watson (no relation to Crick's collaborator James) that he could raise any child to do anything, so long as he had his "own specified world to bring them up in."9 People don't want to accept that genes play an important role in our mental life because this notion challenges our sense of being able to shape our own destinies.
But it is patently clear that genes do shape our mental lives. Although Ehrlich and Feldman are, strictly speaking, correct—genes certainly don't control our destinies—genes do contribute to our personalities, our temperaments, and the qualities that make each individual unique, as well as to the qualities that make the human species unique. Modern science has revealed dozens of ways in which genes have a demonstrable effect on mental life. Animal studies have shown that aspects of behavior and personality can be genetically transmitted (as in the example of the dog breeds that I mentioned earlier, and in studies in which mouse geneticists have bred rodents to be as anxious as Woody Allen).10 Studies of twins have shown time and again that people who share more genes (such as identical twins) are more similar than those who share fewer genes (such as fraternal twins), not only in physical attributes, but in personality and intelligence, indeed just about anything mental that can be measured.
Of course, not every similarity between twins depends on genes. When two seventy-one-year-old Finnish twins died within hours of each other on March 6, 2002, each of a bicycle accident, each while riding on the same road, it really was just a coincidence.11 Genes might have predisposed them both to enjoy physical activity or to enjoy taking risks, but it was sheer chance that caused them to die on the same day and in the same way. Nevertheless, the influence of genes on our mental structure is undeniable.
This influence extends to the structure of the brain itself. For example, a team of brain imagers from the University of California at Los Angeles combined with a team of geneticists from Helsinki to take three-dimensional magnetic resonance images of the brains of twenty sets of twins—ten identical, ten fraternal—carefully matched in terms of their social class, when they were born, and how much time they had spent together.12 The density of the gray matter of the brain—the part of the brain that is most likely to stay constant regardless of experience—was much more similar in the brains of identical twins than in the brains of fraternal twins.
Another team found that the volume of white matter—the part of the brain that consists of modifiable neural connections and that might be expected to be most influenced by experience—was also more similar in identical twins.13 The brains of identical twins are more similar than those of fraternal twins in the patterns of convolutions14 and in the size of particular structures, such as the corpus callosum (which connects the left and right hemispheres).15 Studies with cats suggest that those similarities may extend even to finer-grained details, such as the spacing and layout of microscopic cortical columns, sets of densely connected brain cells that share functional properties.16 Genes thus appear to shape even the finest details of the brain.
Yet another hint that genes must play an important role in the development of the mind comes from newborn babies. Within hours of their birth, newborns can imitate facial gestures,17 connect what they hear with what they see,18 distinguish the rhythms of Dutch from the rhythms of Japanese,19 and tell the difference between someone who is looking at them and someone who isn't,20 suggesting that even with relatively little experience, newborns are ready to start observing the world. Building on the ideas of the pioneering linguist Noam Chomsky, "nativists" such as Steven Pinker and the French cognitive neuroscientist Stanislas Dehaene have argued that babies are born with a "language instinct"21 and a built-in "number sense."22 The tradition of a newborn as a "blank slate" shaped solely by experience (uninfluenced by genes) is, as Pinker has forcefully argued, no longer tenable.23
By now, these results shouldn't come as news. But whether we read newspapers and magazines or the professional literature in psychology, we find very few theories of the mind that make genuine contact with genes; in psychology, it's almost as if Watson and Crick never met DNA.
My goal in this book is not to try to prove that genes make a difference—a matter that is no longer in serious doubt—but to describe how they work and to explain, for the first time, what that means for the mind. I won't argue that genes dictate our destinies (they most certainly do not, and I'll explain why not), nor that they outweigh the contributions of culture or experience (which are difficult to measure). The thesis of this book is that the only way to understand what nature brings to the table is to take a look at what genes actually do.
Almost everything that is written about genes in the popular press is misleading in one way or another. We read that genes are blueprints or maps. We are told that they are like books, libraries, recipes, computer programs, codes, or factories.24 But we're never let in on the secret of what genes really do. Thus, readers have no basis with which to evaluate competing claims. Could evolution have built a language instinct? Is there truly a gene shortage? Without a clear explanation of how genes work, there is no way to tell. What does it mean when newspapers report that a gene for alcoholism or obesity has been discovered? There is no way to interpret the daily onslaught of exciting biological discoveries without understanding what genes actually do.
In order to understand how genes influence human traits and capacities, we must first abandon the familiar idea of a genome (the set of genes within a particular organism) as a blueprint. The genome is not an exact wiring diagram for the mind or a picture of a finished product, even if newspaper headlines so often seem to suggest otherwise. Athena was said to have sprung fully formed from the head of Zeus, and the seventeenth-century scientists known as "preformationists" thought that babies were tiny, fully formed creatures within the sperm or egg cells in which they originated. But nowadays, biologists realize that in early development, such little creatures are not to be found. There are at least five good reasons to think that genomes do not provide detailed blueprints that specify a final product in intricate detail:
• In blueprints, there is a direct correspondence between the elements of the drawing and the elements of the building it describes. There is no such one-to-one correspondence between genes and the cells and structures that make up an organism. As British zoologist Patrick Bateson put it, "The idea that genes might be likened to the blueprint of a building . . . is hopelessly misleading because the correspondences between plan and product are not to be found. In a blueprint, the mapping works both ways. Starting from a finished house, the room can be found on the blueprint, just as the room's position is determined by the blueprint. This straightforward mapping is not true for genes and behaviour, in either direction."25
• A blueprint that is 1 percent different from the next yields a building that is 1 percent different. But a genome that is 1 percent different can lead to a radically different mind. A single change in our genetic makeup can lead to disorders ranging from sickle-cell anemia to certain kinds of specific language impairment. Our genomes are only about 1 percent different from those of chimpanzees, yet our minds are radically different.
• Genomes are too small to contain the kind of detail one would expect if genes were truly an exact blueprint for the wiring of the mind. The human genome contains far fewer than 100,000 genes—perhaps as few as 30,000,26 paltry in comparison to the 20 billion or so neurons found in the human brain.27 Ehrlich's gene shortage militates against any idea of the genome as a literal blueprint.
• Identical genomes do not yield identical nervous systems. In the mid-1970s, neurobiologist Corey Goodman showed that the nervous systems of grasshopper clones with identical genotypes were similar, but not identical.28 More recent studies using newly developed brain imaging technologies have shown that the same is true of human twins: The brains of identical twins are similar, but decidedly not identical.29
• Just as identical twins do not have identical brains, they also do not have identical minds. One twin may be more ambitious, the other more nurturing. These differences, presumably, correlate with differences in brain structure. Identical twins can differ in weight, religion, and even sexual orientation. Even with identical genomes, identical twins are separate people with separate minds.
Clearly, the blueprint metaphor is flawed. Yet, as we will see, many discussions of nature and nurture founder precisely because they wrongly assume genes to be simplistic blueprints.
The second biggest misconception people harbor about genetics: that it will be possible one day to determine, once and for all, whether nurture or nature is "more important." Genes are useless without an environment, and no organism could make any use of the environment at all if it were not for its genes. Asking which one is more important is like asking which gender, male or female, is more important, as the deliberately obtuse British comedian Ali G did in an interview with a feminist scholar. (I quote the dialogue verbatim, in Ali G's own unique dialect. Fans of his will recognize that Ali G is a fictional character and that the whole bit is shtick. True fans will know that the talented young man who plays Ali G, Sascha Baron-Cohen, is a cousin of Simon Baron-Cohen, one of the world's leading scholars of cognitive development.)
Ali G: Now one in two people in the country is a woman. We's got to know about them. Women: They is important aren't they?
Professor: They indeed are. Very important. As important as men.
Ali G: Which is better? Man or woman?
Professor: Well, equality is not about being better.
Ali G: But which one is better?
Professor: Either is better.
Ali G: But one must be a little bit better . . .
Professor: [pauses] In what respect?
Ali G: Like in the way, you know, that something is worse and something is better.30
In the interaction between nature and nurture, neither is better. The better question is not "which" but "how": How do genes work together with the environment to build a human mind?
Before we begin to tackle this question, let me note that there actually is a statistic known as "heritability" that at first blush seems to measure which one, genes or environment, is "better." More precisely, heritability is an estimate of the contribution of genes and the environment to individual differences in any given trait. Are differences in intelligence between people more a matter of genes or the environment? How about assertiveness? Neuroticism? Self-discipline?
To answer these questions, researchers can assess and calculate how individual differences in attributes such as IQ or personality vary as a function of genetic relatedness.31 Heritability is determined not by poring over DNA sequences (no microscope required) but by comparing the total amount of variation in one trait with the extent to which that variation is shared between related people. If, controlling for environment, closely related people are significantly more similar on a particular trait than less closely related people, that trait is said to be highly heritable.32 As you might expect, fingerprints come as close as anything to being set in stone (nature), whereas the extent to which hands are callused is largely a function of one's line of work (nurture). Some physical traits, such as biceps size, are a mix of an individual's inherent constitution and experiential factors such as diet and work-out regime. Similarly, most mental traits fall somewhere in the middle. For example, "identical" twins (who share all their genes) are never actually identical, but on almost anything one can measure, they are more similar than fraternal twins (who share only half their genes): IQ, temperament, even the extent to which they are religiously devout.33 Likewise, siblings are more similar than half-sibs or cousins.34
Heritability scores, in principle, can range from 0 percent, which would mean that none of the differences between individuals can be attributed to differences in genes, to 100 percent, which would mean that all the differences between individuals can be attributed to differences in genes. The heritability of getting hit by lightning would come in at close to zero—in other words, getting hit by lightning is not at all determined by genes. Fingerprints, in contrast, come in at nearly 100 percent—individual differences in fingerprints are almost completely genetically determined.35 In nearly every measure of the mind, scores are well above 30 percent, and often as high as 60 to 70 percent. That's high enough that we can be confident that genes are in some way involved, but low enough to make it clear that there is something beyond genes (it could be environment, or it could be random chance) that is important.36
Heritability scores have an air of authority, but they are easily misunderstood. For example, it is tempting to interpret a heritability score of 60 percent on IQ tests as showing that "60 percent of intelligence comes from heredity."37 Although twin studies do suggest that IQ has a heritability that is not far from 60 percent, that does not mean that 60 percent of your intelligence comes from your genes. In fact, the heritability measure doesn't say what percentage of any trait comes from the genes. Here's why.38
First, heritability doesn't reveal what percentage of a trait comes from the genes, it only measures what percentage of the variation in that trait can be attributed to those genes. What do I mean by "percentage of variation in that trait"? (I'll get to the equally tricky phrase "attributed to those genes" in the next paragraph.) Heritability measures can't see the forest for the trees; all they can see is the differences between trees. What enters into the statistic is not the average height of the trees, but the differences in height between them. As a consequence, heritability can only speak to what makes some trees bigger than others (is it light and moisture, or just rapid-growth genes?), not what makes a tree have a trunk or roots. In humans, heritability only looks at differences that in the grand perspective of life on earth are tiny: whether Jimmy has a bigger vocabulary than Johnny or whether Janey is better with a wrench than Susan, not that which makes all humans intelligent creatures. It is entirely possible that 5,000 different genes contribute to human intelligence and that only a few hundred of those vary in ways that contribute to the differences between one person and the next. In the words of psychoanalyst Harry Stack Sullivan, oft-repeated by my mother, a social worker and lifelong student of human nature, "We are all more human than otherwise." Heritability scores tell us only how differences in those few genes correlate with differences in scores like IQ, not about the contribution of the genes that we all share, or of how genes contribute to making humans different from chimpanzees.
Second, saying that a trait can be "attributed" to genes is not the same thing as saying it is caused by genes; heritabilities are just measures of correlation, and correlation never guarantees causation. Almost all Jedi Knights are male and hence bear Y chromosomes, so statistically speaking, the chance of being a Jedi Knight is tied to the presence or absence of a Y chromosome. But Princess Leia may have the Force, too; perhaps the real problem is not a lack of talent, but a lack of opportunity—maybe the Jedi powers-that-be in her era tended not to give females equal consideration for Yoda's Jedi boot camp (though I hear that equal opportunity could reach the Force in Episode VII).39 Y chromosomes would still then be correlated with who gets to be a Jedi, but they would not be a cause of being one. Likewise, differences on verbal IQ tests can be statistically related to the genes for gender, but that doesn't mean those differences are caused by those genes—they might instead result from how society treats people of different genders. By treating all relations, causal or otherwise, the same, heritability scores can mislead us about the contribution of genes to the finished product.
Third, as any behavioral geneticist would explain, heritability scores inevitably reflect the range of environments from which the data are collected.40 IQ tests taken from a homogeneous society in which all children receive compulsory education—say, the contemporary United States—tend to minimize the possible impact of environmental variation and thereby yield relatively high heritability scores. Heritability measurements taken from a society with more radical variation—such as an earlier period in the United States when only the wealthy could afford education—would likely yield lower estimates of heritability. There simply is no fact of the matter, no absolute number. Heritability scores are a little like an auto manufacturer's gas mileage estimates. They may be a good relative guide (body weight is more sensitive to environment than fingerprints, just as your subcompact will get better mileage than my four-wheel-drive SUV), but the absolute numbers mean relatively little. Whether you get 35 or 26 miles to the gallon will depend on the road you travel, how recently your car has been tuned, how aggressively you drive, and so forth. In a similar way, any heritability score is a complex reflection of the measure used and the population studied.
Until recently, there wasn't much more to say. Scientists knew that nature and nurture both mattered, but they didn't know why or how. As the late Nobel laureate Peter Medawar put it in 1981, the only tool biologists had for investigating the development of the mind was to study differences between individuals. If we wanted to know whether "any common characteristics human beings possess" (for example, the ability to learn language) were in some way "genetically encoded and part, therefore, of our inheritance," we were stuck.41 Such theories might be true but we had no way to verify them. The debate between nature and nurture was widely seen as a "wearisome,"42 unanswerable question.
But much has changed since 1981, and we are finally in a position to move past the long-standing impasse, not by trying to decide which one is better, but by trying to better understand how the two—genes and the environment—work together. Newly invented biological techniques allow scientists to assess the contributions of individual genes, and even to deliberately alter those genes, launching a whole new scientific enterprise that studies the molecules that help shape the mind. The goal of this book is to unite the results of groundbreaking scientific research with studies of the psychology of humans and other animals—in other words, to take insights from the genome and use them to revamp our understanding of nature, of nurture, and of how they work together to create a human mind.
To do that, I must immerse you in a world of cells and proteins, the domain in which genes actually do their work. That may seem peculiar in a book about the mind—most books about the mind are about psychology, not cells—but my argument is that the workings of the cellular world cast enormous light on the mental world, and that the mental world cannot be properly understood without a firm grip on the cellular world. Anything else would just be business as usual, "nature and nurture" without the nature.
Any theory that puts the role of genes front and center must deal with two of the most difficult challenges in the science of the mind, which I will call the Two Paradoxes. First, any adequate theory must face the challenge of neural flexibility. For every study that tells us that a newborn can understand something about the world, there is another that shows that the brain can continue to function even when its structure is altered. How can the mind be at once so richly structured and so flexible? The second challenge is Ehrlich's "gene shortage": How can the complexity of the brain emerge from a relatively small genome, 20 billion neurons versus just 30,000 genes?43


On Sale
Aug 5, 2008
Page Count
288 pages
Basic Books

Gary Marcus

About the Author

Gary Marcus is Associate Professor of Psychology at New York University. Author of The Algebraic Mind, Marcus received his Ph.D. from MIT at the age of twenty-three. In 2002-2003, he is a Fellow of the Stanford Center for Advanced Study in Behavioral Sciences. He lives in New York City. To learn more about Marcus’ work, please visit

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