Indeed, until recently the innateness of brain structure was an embarrassment for neuroscience. The brain could not possibly be wired by the genes down to the last synapse, because there isn’t nearly enough information in the genome to do so. And we know that people learn throughout their lives, and the products of that learning have to be stored in the brain somehow. Unless you believe in a ghost in the machine, everything a person learns has to affect some part of the brain; more accurately, learning is a change in some part of the brain. But it was difficult to find the features of the brain that reflected those changes amid all that innate structure. Becoming stronger in math or motor coordination or visual discrimination does not bulk up the brain the way becoming stronger at weightlifting bulks up the muscles.
Now, at last, neuroscience is beginning to catch up with psychology by discovering changes in the brain that underlie learning. As we shall see, the boundaries between swatches of cortex devoted to different body parts, talents, and even physical senses can be adjusted by learning and practice. Some neuroscientists are so excited by these discoveries that they are trying to push the pendulum in the other direction, emphasizing the plasticity of the cerebral cortex. But for reasons that I will review in Chapter 5, most neuroscientists believe that these changes take place within a matrix of genetically organized structure. There is much we don’t understand about how the brain is laid out in development, but we know that it is not indefinitely malleable by experience.
THE THIRD BRIDGE between the biological and the mental is behavioral genetics, the study of how genes affect behavior.41 All the potential for thinking, learning, and feeling that distinguishes humans from other animals lies in the information contained in the DNA of the fertilized ovum. This is most obvious when we compare species. Chimpanzees brought up in a human home do not speak, think, or act like people, and that is because of the information in the ten megabytes of DNA that differ between us. Even the two species of chimpanzees, common chimps and bonobos, which differ in just a few tenths of one percent of their genomes, part company in their behavior, as zookeepers first discovered when they inadvertently mixed the two. Common chimps are among the most aggressive mammals known to zoology, bonobos among the most peaceable; in common chimps the males dominate the females, in bonobos the females have the upper hand; common chimps have sex for procreation, bonobos for recreation. Small differences in the genes can lead to large differences in behavior. They can affect the size and shape of the different parts of the brain, their wiring, and the nanotechnology that releases, binds, and recycles hormones and neurotransmitters.
The importance of genes in organizing the normal brain is underscored by the many ways in which nonstandard genes can give rise to nonstandard minds. When I was an undergraduate an exam question in Abnormal Psychology asked, “What is the best predictor that a person will become schizophrenic?” The answer was, “Having an identical twin who is schizophrenic.” At the time it was a trick question, because the reigning theories of schizophrenia pointed to societal stress, “schizophrenogenic mothers,” double binds, and other life experiences (none of which turned out to have much, if any, importance); hardly anyone thought about genes as a possible cause. But even then the evidence was there: schizophrenia is highly concordant within pairs of identical twins, who share all their DNA and most of their environment, but far less concordant within pairs of fraternal twins, who share only half their DNA (of the DNA that varies in the population) and most of their environment. The trick question could be asked—and would have the same answer—for virtually every cognitive and emotional disorder or difference ever observed. Autism, dyslexia, language delay, language impairment, learning disability, left-handedness, major depressions, bipolar illness, obsessive-compulsive disorder, sexual orientation, and many other conditions run in families, are more concordant in identical than in fraternal twins, are better predicted by people’s biological relatives than by their adoptive relatives, and are poorly predicted by any measurable feature of the environment.42
Genes not only push us toward exceptional conditions of mental functioning but scatter us within the normal range, producing much of the variation in ability and temperament that we notice in the people around us. The famous Chas Addams cartoon from The New Yorker is only a slight exaggeration:
© The New Yorker Collection 1981. Charles Addams from cartoonbank.com. All rights reserved.
Identical twins think and feel in such similar ways that they sometimes suspect they are linked by telepathy. When separated at birth and reunited as adults, they say they feel they have known each other all their lives. Testing confirms that identical twins, whether separated at birth or not, are eerily alike (though far from identical) in just about any trait one can measure. They are similar in verbal, mathematical, and general intelligence, in their degree of life satisfaction, and in personality traits such as introversion, agreeableness, neuroticism, conscientiousness, and openness to experience. They have similar attitudes toward controversial issues such as the death penalty, religion, and modern music. They resemble each other not just in paper-and-pencil tests but in consequential behavior such as gambling, divorcing, committing crimes, getting into accidents, and watching television. And they boast dozens of shared idiosyncrasies such as giggling incessantly, giving interminable answers to simple questions, dipping buttered toast in coffee, and—in the case of Abigail van Buren and Ann Landers—writing indistinguishable syndicated advice columns. The crags and valleys of their electroencephalograms (brainwaves) are as alike as those of a single person recorded on two occasions, and the wrinkles of their brains and distribution of gray matter across cortical areas are also similar.43
The effects of differences in genes on differences in minds can be measured, and the same rough estimate—substantially greater than zero, but substantially less than 100 percent—pops out of the data no matter what measuring stick is used. Identical twins are far more similar than fraternal twins, whether they are raised apart or together; identical twins raised apart are highly similar; biological siblings, whether raised together or apart, are far more similar than adoptive siblings. Many of these conclusions come from massive studies in Scandinavian countries where governments keep huge databases on their citizens, and they employ the best-validated measuring instruments known to psychology. Skeptics have offered alternative explanations that try to push the effects of the genes to zero—they suggest that identical twins separated at birth may have been placed in similar adoptive homes, that they may have contacted each other before being tested, that they look alike and hence may have been treated alike, and that they shared a womb in addition to their genes. But as we shall see in the chapter on children, these explanations have all been tested and rejected. Recently a new kind of evidence may be piled on the heap. “Virtual twins” are the mirror image of identical twins raised apart: they are unrelated siblings, one or both adopted, who are raised together from infancy. Though they are the same age and are growing up in the same family, the psychologist Nancy Segal found that their IQ scores are barely correlated.44 One father in the study said that despite efforts to treat them alike, the virtual twins are “like night and day.”
Twinning and adoption are natural experiments that offer strong indirect evidence that differences in minds can come from differences in genes. Recently geneticists have pinpointed some of the genes that can cause the differences. A single wayward nucleotide in a gene called FOXP2 causes a hereditary disorder in speech and language.45 A gene on the same chromosome, LIM-kinasel, produces a protein found in growing neurons that helps install the faculty of spatial cognition: when the gene is deleted, the person has normal intelligence but cannot assemble objects, arrange blocks, or copy shapes.46 One version of the gene IGF2R is associated with high general intelligence, accounting for as many as four IQ points and two percent of the variation in intelligence among normal individuals.47 If you have a longer than average version of the D4DR dopamine receptor gene, you are more likely to be
a thrill seeker, the kind of person who jumps out of airplanes, clambers up frozen waterfalls, or has sex with strangers.48 If you have a shorter version of a stretch of DNA that inhibits the serotonin transporter gene on chromosome 17, you are more likely to be neurotic and anxious, the kind of person who can barely function at social gatherings for fear of offending someone or acting like a fool.49
Single genes with large consequences are the most dramatic examples of the effects of genes on the mind, but they are not the most representative examples. Most psychological traits are the product of many genes with small effects that are modulated by the presence of other genes, rather than the product of a single gene with a large effect that shows up come what may. That is why studies of identical twins (two people who share all their genes) consistently show powerful genetic effects on a trait even when the search for a single gene for that trait is unsuccessful.
In 2001 the complete sequence of the human genome was published, and with it came a powerful new ability to identify genes and their products, including those that are active in the brain. In the coming decade, geneticists will identify genes that differentiate us from chimpanzees, infer which of them were subject to natural selection during the millions of years our ancestors evolved into humans, identify which combinations are associated with normal, abnormal, and exceptional mental abilities, and begin to trace the chain of causation in fetal development by which genes shape the brain systems that let us learn, feel, and act.
People sometimes fear that if the genes affect the mind at all they must determine it in every detail. That is wrong, for two reasons. The first is that most effects of genes are probabilistic. If one identical twin has a trait, there is usually no more than an even chance that the other will have it, despite their having a complete genome in common. Behavioral geneticists estimate that only about half of the variation in most psychological traits within a given environment correlates with the genes. In the chapter on children, we will explore what this means and where the other half of the variation comes from.
The second reason that genes aren’t everything is that their effects can vary depending on the environment. A simple example may be found in any genetics textbook. While different strains of corn grown in a single field will vary in height because of their genes, a single strain of corn grown in different fields—one arid, the other irrigated—will vary in height because of the environment. A human example comes from Woody Allen. Though his fame, fortune, and ability to attract beautiful women may depend on having genes that enhance a sense of humor, in Stardust Memories he explains to an envious childhood friend that there is a crucial environmental factor as well: “We live in a society that puts a big value on jokes…. If I had been an Apache Indian, those guys didn’t need comedians, so I’d be out of work.”
The meaning of findings in behavioral genetics for our understanding of human nature has to be worked out for each case. An aberrant gene that causes a disorder shows that the standard version of the gene is necessary to have a normal human mind. But what the standard version does is not immediately obvious. If a gear with a broken tooth goes clunk on every turn, we do not conclude that the tooth in its intact form was a clunk-suppressor. And so a gene that disrupts a mental ability need not be a defective version of a gene that is “for” that ability. It may produce a toxin that interferes with normal brain development, or it may leave a chink in the immune system that allows a pathogen to infect the brain, or it may make the person look stupid or sinister and thereby affect how other people react to him. In the past, geneticists couldn’t rule out the boring possibilities (the ones that don’t involve brain function directly), and skeptics intimated that all genetic effects might be boring, merely warping or defacing a blank slate rather than being an ineffective version of a gene that helps to give structure to a complex brain. But increasingly researchers are able to tie genes to the brain.
A promising example is the FOXP2 gene, associated with a speech and language disorder in a large family.50 The aberrant nucleotide has been found in every impaired member of the family (and in one unrelated person with the same syndrome), but it was not found in any of the unimpaired members, nor was it found in 364 chromosomes from unrelated normal people. The gene belongs to a family of genes for transcription factors—proteins that turn on other genes—that are known to play important roles in embryogenesis. The mutation disrupts the part of the protein that latches onto a particular region of DNA, the key step in turning on the right gene at the right time. The gene appears to be strongly active in fetal brain tissue, and a closely related version found in mice is active in the developing cerebral cortex. These are signs, according to the authors of the study, that the normal version of the gene triggers a cascade of events that help organize a part of the developing brain.
The meaning of genetic variation among normal individuals (as opposed to genetic defects that cause a disorder) also has to be thought through with care. An innate difference among people is not the same thing as an innate human nature that is universal across the species. Documenting the ways that people vary will not directly reveal the workings of human nature, any more than documenting the ways that automobiles vary will directly reveal how car engines work. Nonetheless, genetic variation certainly has implications for human nature. If there are many ways for a mind to vary genetically, the mind must have many genetically influenced parts and attributes that make the variation possible. Also, any modern conception of human nature that is rooted in biology (as opposed to traditional conceptions of human nature that are rooted in philosophy, religion, or common sense) must predict that the faculties making up human nature show quantitative variation, even if their fundamental design (how they work) is universal. Natural selection depends on genetic variation, and though it reduces that variation as it shapes organisms over the generations, it never uses it up completely.51
Whatever their exact interpretation turns out to be, the findings of behavioral genetics are highly damaging to the Blank Slate and its companion doctrines. The slate cannot be blank if different genes can make it more or less smart, articulate, adventurous, shy, happy, conscientious, neurotic, open, introverted, giggly, spatially challenged, or likely to dip buttered toast in coffee. For genes to affect the mind in all these ways, the mind must have many parts and features for the genes to affect. Similarly, if the mutation or deletion of a gene can target a cognitive ability as specific as spatial construction or a personality trait as specific as sensation-seeking, that trait may be a distinct component of a complex psyche.
Moreover, many of the traits affected by genes are far from noble. Psychologists have discovered that our personalities differ in five major ways: we are to varying degrees introverted or extroverted, neurotic or stable, incurious or open to experience, agreeable or antagonistic, and conscientious or undirected. Most of the 18,000 adjectives for personality traits in an unabridged dictionary can be tied to one of these five dimensions, including such sins and flaws as being aimless, careless, conforming, impatient, narrow, rude, self-pitying, selfish, suspicious, uncooperative, and undependable. All five of the major personality dimensions are heritable, with perhaps 40 to 50 percent of the variation in a typical population tied to differences in their genes. The unfortunate wretch who is introverted, neurotic, narrow, selfish, and undependable is probably that way in part because of his genes, and so, most likely, are the rest of us who have tendencies in any of those directions as compared with our fellows.
It’s not just unpleasant temperaments that are partly heritable, but actual behavior with real consequences. Study after study has shown that a willingness to commit antisocial acts, including lying, stealing, starting fights, and destroying property, is partly heritable (though like all heritable traits it is exercised more in some environments than in others).52 People who commit truly heinous acts, such as bilking elderly people out of their life savings, raping a succession of women, or shooting convenience store clerks lying on the floor during a robbery, a
re often diagnosed with “psychopathy” or “antisocial personality disorder”53 Most psychopaths showed signs of malice from the time they were children. They bullied smaller children, tortured animals, lied habitually, and were incapable of empathy or remorse, often despite normal family backgrounds and the best efforts of their distraught parents. Most experts on psychopathy believe that it comes from a genetic predisposition, though in some cases it may come from early brain damage.54 In either case genetics and neuroscience are showing that a heart of darkness cannot always be blamed on parents or society.
And the genes, even if they by no means seal our fate, don’t sit easily with the intuition that we are ghosts in machines either. Imagine that you are agonizing over a choice—which career to pursue, whether to get married, how to vote, what to wear that day. You have finally staggered to a decision when the phone rings. It is the identical twin you never knew you had. During the joyous conversation it comes out that she has just chosen a similar career, has decided to get married at around the same time, plans to cast her vote for the same presidential candidate, and is wearing a shirt of the same color—just as the behavioral geneticists who tracked you down would have bet. How much discretion did the “you” making the choices actually have if the outcome could have been predicted in advance, at least probabilistically, based on events that took place in your mother’s Fallopian tubes decades ago?
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