Three. How Genes Make Us Different

In considering the cast of characters I described in the last chapter, you may have wondered how they got to be so different. If you’re like most people, you probably assumed that their personality patterns were mainly caused by social circumstances and upbringing. But it’s likely that you also toyed with another explanation that is becoming increasingly popular: genes.

The growing interest in the genetics of personality is reflected in its extensive media coverage. Consider, for example, this excerpt from a New York Times column about the genetics of excitement-seeking:

Jason Dallas used to think of his daredevil streak—a love of backcountry skiing, mountain bikes and fast vehicles—as “a personality thing.” Then he heard that scientists at the Fred Hutchison Cancer Research Center had linked risk-taking in mice to a gene. Those without it pranced unprotected along a steel beam instead of huddling in safety like the other mice. Now Mr. Dallas, a chef in Seattle, is convinced he has a genetic predisposition for risk-taking, a conclusion that researchers say is not unwarranted, since the similar variations in human genes can explain why people perceive danger differently. “It’s in your blood,” Mr. Dallas said. “You hear people say that kind of thing, but now you know it really is.”¹

What I find remarkable about this report is that Jason Dallas so readily accepts the idea that a gene that affects the personality of a mouse may also affect his own. Although there is no evidence that the gene in this study,² neuroD2, has anything to do with his love of excitement, Dallas has been primed to make this connection by the widely publicized findings that there is, in fact, a close relationship between mouse genes and human genes, and between mouse brains and human brains. Needless to say, there are also important differences. But as I show in this chapter, Dallas does have good reason to believe that his daredevil streak has some genetic basis, even though his neuroD2 may have nothing to do with it.

The belief that some personality traits are innate is hardly new. What is new is our growing understanding of the degree and nature of this genetic influence. In this chapter, I take you beyond the vague idea that genes affect personality, to a deeper conception of the role they play in making us who we are.

A New Foundation for Psychology

Charles Darwin, who revolutionized our understanding of the origins of personality differences, didn’t begin with a particular interest in this subject. He was after something much bigger: the origin of all the differences among all living things. Of the clues that led him to that answer, the most revealing came from domestic animal breeding.

Dogs were especially informative. People already knew in Darwin’s time that breeds as different as greyhounds and spaniels descended from the same wild ancestors. Darwin also understood that their selective breeding depended on the transmission of inherited characteristics from parents to pups, and that new breeds arose “by the careful selection of the individuals which present the desired character.” Furthermore, the creation of strikingly different breeds was a gradual affair, accomplished through a succession of little steps. As Darwin explained this in 1859 in Origin of Species:

[W]hen we compare the many breeds of dogs, each good for man in different ways ... we cannot suppose that all the breeds were suddenly produced as perfect and useful as we now see them; indeed, in many cases, we know this has not been their history. The key is man’s power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful for him. In this sense he may be said to have made for himself useful breeds.³

Once Darwin recognized that the creation of dog breeds depends on the breeder’s selection of heritable variations, it occurred to him that nature does the same thing: It selects those heritable variations—spontaneous modifications of genes, now called mutations—that are advantageous in the wild. This process of natural selection ensures that desirable mutations are passed on from generation to generation and may eventually become stable features of the species.

A good example is a mutation in a gene, SLC24A5, that controls the deposit of melanin, a black pigment. What makes this mutation so interesting is that it caused a dramatic change in the color of human skin, from black to white. In sunny Africa black skin is favored to block harmful ultraviolet rays while still allowing enough through to stimulate the skin’s production of vitamin D. This explains why the native African population has SLC24A5 genes that provide lots of black pigment. But in regions far from the equator, where sunlight is scarce, a mutation that inactivates this gene⁴ took over because the pale skin that results lets through more of the limited light to make vitamin D.⁵ As with the evolution of many other human differences, this one became prevalent through accidental DNA mutations and natural selection based on adaptation to specific environmental conditions.

Darwin wasn’t in a position to provide such a persuasive illustration. But this didn’t stop him from extending his idea from biology to psychology. It was clear to him that selective breeding affected not only physical characteristics, but also behavioral ones. For example, breeders have selected dogs not only for their shape and size, but also for their skills, such as herding or pointing, and for personality traits such as agreeableness or aggressiveness. So why wouldn’t natural selection of behavioral traits also increase fitness in the wild? By the end of Origin of Species, Darwin was sufficiently convinced of this to predict that “In the distant future...psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation.” To put this in modern terms, Darwin predicted that our understanding of psychology would one day rely on knowledge of the genetic variations that affect behavior.

But Darwin was initially reluctant to extend this prediction from animals to people. The mere hint that physical features of humans had animal origins would cause him trouble enough. He would, for some time, leave human psychology to others.

Experiments of Nature

The man who first took up the challenge was Darwin’s cousin, Francis Galton, whom you met in Chapter 1, and he was willing to take it even further. It seemed to Galton that if the characteristic behaviors of a species are inherited, the behavioral differences between individual people—our distinctive intellectual abilities and personality traits—might also be inherited.

Such variations in human talents and traits were already of great personal interest to Galton. A precocious child who was proud of his intelligence and achievements, he had long believed that both he and Charles Darwin had inherited their special gifts from their common grandfather, the distinguished physician and scientist Erasmus Darwin. But Galton was aware that his family also provided him with a privileged upbringing that fostered whatever gifts he inherited by placing him “in a more favourable position for advancement than if he had been the son of an ordinary person.”⁶ So was he gifted because of favorable heredity or favorable upbringing?

To address this question, Galton turned to an experiment of nature: twins. Galton knew that some twins looked so much alike that they were probably genetically identical, whereas others were no more similar than siblings born at different times. Because both identical and fraternal twins were usually raised together by their parents, the members of each pair would have a comparable upbringing. If he found that the behavior of identical twins was more similar than that of same-sex fraternal twins, this would support his hunch that greater genetic similarity leads to greater behavioral similarity.

In 1875, Galton reported that 35 sets of identical twins showed much greater behavioral similarities than 20 sets of fraternal twins, which he took as support for the importance of heredity. In “The History of Twins As a Criterion of the Relative Powers of Nature and Nurture,” he announced, “[T]here is no escape from the conclusion that nature prevails enormously over nurture.”⁷ His observations that adopted children of gifted adoptive parents are no more gifted than ordinary children, even though they are provided with a privileged environment, also supported this conclusion.⁸ This was the first use of another natural experimental approach—adoption—in assessing the role of inheritance and upbringing.

Although Galton’s ways of studying behavior were crude, his results were sufficiently persuasive to convince his most eminent critic. As Darwin wrote to him after studying some of Galton’s publications, “I do not think I ever in all my life read anything more interesting and original—and how well and clearly you put every point! ... You have made a convert of an opponent in one sense, for I have always maintained that, excepting fools, men did not differ much in intellect, only in zeal and hard work.”⁹ While Darwin’s praise was not wholly merited in its time, it was subsequently justified by more persuasive research using Galton’s approach.

How Much of Our Personality Differences Is Heritable?

The biggest impediment to Galton’s research is that he didn’t know how to measure personality differences. He had tried to make objective assessments in his work with twins, but he was painfully aware that his methods weren’t very good. Frustrated by these difficulties, Galton turned his attention to the inheritance of height, which he could measure accurately. His studies of the relationship between the heights of parents and their children led him to develop the formula for calculating correlations that I mentioned earlier, and that was later adapted to create the Big Five personality tests.

The Big Five tests are just what Galton had hoped for, and they are now routinely used to investigate the influence of genes on the personalities of identical and fraternal twins. In a typical study, each twin is given a Big Five test, and the scores are compared with those of the other twin. If genes influence these personality traits, both twins should have scores that are somewhat similar. But the similarities of pairs of identical twins, who share 100% of their genes (because they are derived from a single fertilized egg that split after conception), should be twice as great as the similarities of same-sex fraternal twins (derived from different eggs), who share only 50% of their genes.

This is just what researchers have found. For example, in a study using hundreds of subjects, the Extraversion scores of the two members of a pair of fraternal twins had an average correlation of 0.23 (on a scale of 0 to 1). In contrast, the two members of a pair of identical twins had an average correlation about twice as large, 0.48. The difference in correlations (0.48 − 0.23 = 0.25) is assumed to reflect the difference between having all the same genes (identical twins) and having half the same genes (fraternal twins). Therefore, this difference measures only half the effect of having all the same genes. To get the full effect, which geneticists call heritability, 0.25 is doubled to get 0.5, or 50%.¹⁰ Studies of Agreeableness, Conscientiousness, Neuroticism, and Openness also found heritability to be around 50%.¹¹

When the evidence for such substantial heritability of personality traits was first published, critics pointed out another possible explanation for the greater psychological resemblance of the identical twin pairs. Instead of resulting from genes alone, it might also result from the identical twins being treated more alike than the fraternal twins. Fortunately, the contribution of shared family environment can be evaluated through another experiment of nature: studying identical twins separated after birth and raised in different families.

Thomas Bouchard and his colleagues at the University of Minnesota did just that.¹² They tracked down more than 100 pairs of identical twins who had been raised apart and persuaded them to volunteer for a week of psychological testing. Many twins had been raised in very different environments, some in different countries and cultures, and their reunions attracted a great deal of media attention.

A certain pair of British identical twin girls would have been especially interesting to Galton because they addressed the issue of social privilege he had wondered about. One twin had been raised by an upper-class family, had attended exclusive schools, and spoke with a refined accent to prove it; the other had been raised by a lower-class family, had quit school at 16, and spoke like Liza Doolittle did before she met Henry Higgins. Yet their test scores were very similar. The same was true of the other sets of twins. As Bouchard summed it up, “[O]n multiple measures of personality and temperament, occupational and leisure-time interests, and social attitudes, monozygotic twins reared apart are about as similar as monozygotic twins reared together.”¹³ These and other family and adoption studies support the conclusion that personality traits are highly heritable.¹⁴

The studies with identical twins also tell us something else that should not be ignored: They challenge the assumption that the shared family environment of those raised together is responsible for some of their similarities. Were this the case, the scores of identical twins raised together should be more similar than those raised apart. But as Bouchard pointed out, they’re not.¹⁵ Scores of genetically unrelated children who were adopted and raised in the same family also show no effect of this shared environment.¹⁶

The lack of effect of a shared family environment on these measures of personality doesn’t mean that parents are just part of the furniture. Studies indicate that parents do have some influence, but it is transmitted by their unique relationship with each child,¹⁷ including each identical twin. The studies also indicate that most environmental influences on personality cannot be specifically attributed to interactions within the family.¹⁸

How Many Gene Variants Shape a Personality Trait?

So now that we know that genes do, indeed, have a big effect on personality differences, how do they do it? To answer this question, it’s necessary to review a few facts about human genetics.

The total number of human genes is surprisingly small, about 20,000. Each is made from the four chemical building blocks of DNA—adenine (A), cytosine (C), guanine (G), and thymine (T)—strung together in a long chain whose precise sequence (such as AGACTCAAG ...)contains the instructions for manufacturing a particular protein. Each protein interacts with many others to build and maintain us. The major reason so few genes are sufficient for this complex task is that various combinations work together to control our biological and psychological functions. Furthermore, the actions of each gene and each protein can influence the actions of many others.

The main way genes interact is by turning the activities of each other on or off. To make this possible, each gene has a specialized piece of DNA, called a promoter, that serves as a dial to control the amount of the protein that gene makes. The dial can be turned up and down by internal or environmental signals that may work through controls in other regions of DNA. This process, called regulation of gene expression,¹⁹ adjusts the amounts of the proteins that shape our bodies and minds.

Regulating the expression of a variety of genes in different cells helps explain how just 20,000 elements can give rise to such complexity. But it doesn’t explain our heritable differences. These differences are explained by variants of the genes—modifications of the sequence of bases in their DNA or the DNA of their regulatory regions—that have accumulated in the collective human genetic repertoire, called the human genome. These structural modifications of DNA, which arose through random mutations, may cause big changes in the manufacture of a specific protein or the way it functions in the body. Some of the variants, such as those that influence skin color, are carried by billions of people. Others are rare. The combined effects of the assortment of gene variants that were handed down to each of us—our own personal selection from the human genome—defines our genetic uniqueness.

But not all gene variants have such major and obvious effects as the small number that control human skin color. For example, hundreds of different genes²⁰ influence human height, which is about 80% heritable in well-nourished human populations, and each of these genes has a tiny effect. The same is true for highly heritable personality traits.

Persuasive evidence that personality traits reflect the joint action of multiple gene variants comes from selective breeding of mice. A notable example is John DeFries’s classic study²¹ of a seemingly simple mouse trait: the inclination to explore an unfamiliar and potentially dangerous territory. This personality trait is related to both the excitement-seeking and anxiety facets of the Big Five, in that high excitement-seeking would increase exploration while high anxiety would inhibit it. Together these facets would also influence the risk taking that is such a cherished part of Jason Dallas’s personality.

To prepare for the experiment, DeFries randomly chose ten litters of mice and observed their behavior in a brightly lit large box called an open field. Mice prefer dim light and narrow spaces, but there are individual differences. Some mice froze in the open field, like a deer in the headlights, while others sniffed around and explored. Electronic sensors measured each mouse’s behavior, recording the total distance it traveled in a six-minute period. After DeFries had scored each animal from the initial ten litters, he selectively bred the mice to raise two extreme lines. He began by mating the most active male and female from each litter. They became the founders of what I call the fearless (F) line. He also mated the least active pairs, who became the founders of what I call the anxious (A) line. DeFries then took another ten litters and mated a randomly selected male and female from each to be the founders of the control line. He repeated this process in each generation. Because mouse pregnancies take only three weeks, and pups become sexually mature in about three months, he was able to breed and evaluate 30 generations in the course of ten years.

The results were dramatic. After 30 generations, the average member of the F line roamed freely across the open field. In contrast, the average member of the A line huddled in a corner of the box. Members of the control line maintained their original modest level of exploration, which hadn’t changed through 30 generations.

The other notable finding was that separation of the two lines was gradual, with steady increments from one generation to the next. When the open field behavior of the F line was plotted as a graph, it looked like that of a long-term growth stock that kept rising year after year over the ten-year period. In contrast, the pattern of the A line looked like the stock of a weak company in a failing industry, heading progressively downward until it hovered near zero. This pattern of gradual change has two implications: Variants of many genes together affect this personality trait; and the behavioral effects keep adding up as more of the relevant gene variants are selected in each generation. Direct analysis of the DNA of these mouse lines²² confirms these conclusions.²³

Genetic Thinking vs. Genetic Testing

As behavioral scientists were accumulating evidence that many gene variants work together to influence personality differences, geneticists were busily deciphering the complete DNA sequences of the human and mouse genomes, and the structures of common gene variants. This provided the foundation for searching the entire genome for variants that influence personality traits in people and in mice. But progress with this genome-wide approach has been slow.²⁴

Frustrated by this limited success, some researchers have taken a more focused approach. It was based on the knowledge that drugs such as Prozac and Ritalin affect personality and that they do this by influencing the way serotonin or dopamine act in the brain. This raised the possibility that inherited variations in genes that control certain actions of serotonin or dopamine in the brain might be responsible for heritable personality differences. To look into this, researchers examined variants of several dozen of these genes to see if they were correlated with scores on personality tests.

All the genes that they examined influence the emotional circuits of the brain. Of these, the most widely studied, called SERT, was singled out because it makes the serotonin transporter protein, Prozac’s target. The SERT protein vacuums up (transports) serotonin from the fluid around nerve cells that are activated by frightening experiences so that the serotonin can be used again. By controlling the amount of serotonin that bathes these nerve cells, the SERT protein affects the intensity of the emotional response. Therefore, it is easy to imagine how variants of the SERT gene might influence the tuning of brain circuits that control traits such as fearfulness.

To see whether the SERT gene affects personality, researchers focused on two common variants, one with a long promoter and the other with a short one. Several studies have found that groups of people who have two copies of the gene with the long promoter, which makes more SERT protein, have a slightly lower average score on Neuroticism.²⁵ Furthermore, brain imaging studies indicate that if such people are shown frightening pictures, they have less activation of the amygdala, a brain structure involved in fear processing.²⁶ Taken together, the studies suggest that these differences in the amount of SERT protein account for a fraction—but only a tiny fraction—of the variation in the tendency to be frightened.

A similar conclusion emerged in studies of another gene, DRD4, which makes a receptor for dopamine. Scientists studied this gene because Ritalin and amphetamine stimulate behavior by releasing dopamine, which activates dopamine receptors. Researchers found that groups of people with variants of DRD4 had different average scores on novelty-seeking and impulsivity, traits expected to be influenced by dopamine.²⁷ Once again, the gene variants accounted for just a tiny fraction of the variation in these traits.²⁸

So don’t rush out to your nearest DNA testing service to have your SERT or DRD4 genes examined: They are just two examples of the thousands of gene variants that work together to influence personality,²⁹ sometimes in unexpected ways.³⁰ And even though new techniques, such as the complete sequencing of a person’s DNA,³¹ will eventually be used to search for variants that influence specific traits, it will still be very difficult to identify the mixture that shapes a particular personality.

But the fact that such genetic testing hasn’t proved useful, at least for now, doesn’t mean that you can also dismiss genetic thinking. When trying to make sense of someone, it still helps to remember that a person’s specific combination of gene variants has a substantial effect on his or her personality. And we have a good idea where these many variants came from.

The Deep Roots of Our Diversity

Our view of the accumulation of so many variants in the human genome is based on Darwin’s key insight that the environment keeps selecting those that increase fitness. For example, a consistent environmental factor, such as the relatively low amount of sunlight, exerted a relentless selective force on gene variants that eventually made Northern Europeans white. But Darwin also realized that environments keep changing over the course of evolution and that this led to the selection of variants that were suitable for different contingencies. Among them were those that influence personality.

To see what I mean, consider the environments that influenced the selection of the variants that control the open field behavior of mice. In dangerous territories with many predators, variants that favor caution would be selected because those who carried them would be more likely to live long enough to reproduce. But when the cats are away, the mice will play. In such safer environments, the variants that favor exploration would be selected because those who carried them might find more food and more sexual partners. Fluctuations of these alternative environments would lead to retention of both types of variants in the group’s genetic repertoire. Furthermore, many of them would be kept as the species continued to evolve. This explains why some that arose in distant ancestors have been passed down to you and me.³²

Predators are not the most important instruments of selection of heritable human personality traits. People are. They are the ones we depend on and compete with, and there are benefits and costs in the many tactics they and we use to interact. These fluctuating social environments have contributed to the selection of the wide range of gene variants that influence our personalities.³³

In thinking about people in terms of the Big Five, it therefore helps to remember that high or low rankings on each of them have tradeoffs.³⁴ For example, people high in Extraversion enjoy the pleasures of intense engagement with others and the opportunities provided to those who take charge. Studies show that, like Bill Clinton, they tend to have many sexual partners, which, in a precontraceptive world, would have led to more children—Darwin’s gold standard for an adaptive trait. But intense engagement comes with risks; taking charge invites jealousy and insurrection; and high excitement-seeking makes it more likely to get into accidents, engage in criminal activity, get arrested, and even get killed by rivals. So, high Extraversion is a mixed blessing.

High Agreeableness is also a mixed blessing. By promoting cooperation it builds alliances that can pool resources for the common good and for protection against competing groups. But the downside of high Agreeableness is that it increases the chances of being taken advantage of. In contrast, disagreeable people are more likely to fight for themselves and what they believe in. Studies show that people who rank high in Agreeableness tend to earn less money, even though they are valued as team players. In contrast, those who are low in Agreeableness are more likely to rise to the top of their fields.

Great achievement is also favored by high Conscientiousness, which has the benefits of purposeful self-control and long-range planning. But high Conscientiousness has the potential downsides of oppressive perfectionism and the inability to abandon well-practiced routines in the face of changing circumstances. By always taking the long view, people high in Conscientiousness may be less opportunistic, and this can translate into fewer sexual partners, fewer children, and less transmission of their genes. On the other hand, the children they have are more likely to enjoy the benefits of a devoted parent.

Only high Neuroticism might seem to have little to recommend it because it includes an increased likelihood of experiencing painful negative emotions. But the world can be a dangerous place, and emotions such as fear and sadness are adaptive if properly modulated. Studies show that high Neuroticism is correlated with high achievement and creativity in people whose other traits keep them from falling into the deep hole that can be dug by persistent emotional distress. Sigmund Freud, who was very high in Neuroticism, is an example.

In contrast with the assumption that high Neuroticism is always bad, most people who read books like this assume that high Openness is unreservedly good. This is because they value curiosity and are interested in new ideas. But people with low Openness are happy to exchange these pleasures for the comforts of constancy and tradition.

The fact that particular rankings on a trait have advantages and disadvantages does not, of course, mean that we consciously chose the ones we have. The reason I’ve pointed out their relative costs and benefits in various social environments is to help you understand why the many gene variants that influence these traits are retained in the collective human genome. Furthermore, variants that influence one trait may have been selected to balance out others. For example, it is easy to imagine how environments that favored the selection of variants for high Extraversion might have had some of their effects balanced out by the selection of variants that favored high Conscientiousness.

Such a balance of selective forces may also control the proportion of people with high or low expression of a heritable personality trait among the members of a population.³⁵ Consider, for example, the proportion of people with high or low Agreeableness. In a population in which almost everyone ranked high on A, the rare antisocials (with low A) would find it relatively easy to steal from their warm-hearted neighbors. These stolen resources would allow the antisocials to have more children, who would, in turn, inherit gene variants that favor low A. But as the number of antisocials increased, their high-A neighbors might band together, mount defenses to protect their resources, and turn back this growing tide. As these forces came into a stable balance over many generations, the result might be a group with a few crafty antisocials and a majority of members with a range of higher rankings on A.³⁶

The Grandeur in This View of Personality Differences

The realization that human psychological diversity reflects conflicting forces of natural selection has profound implications for making sense of people. But many remain reluctant to embrace this idea because it confronts us with our primitive animal nature. Darwin himself struggled with the seemingly anti-humanistic implications of his discoveries. Nevertheless, being unable to dismiss the evidence in favor of natural selection, he eventually came to see evolution as awe-inspiring. As he explained in Origin’s famous last sentence: “There is grandeur in this view of life ... that ... from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

To me, recognizing the role that natural selection of gene variants played in many of our personality differences is a prime example of the grandeur that comes with such understanding. And as I show in the next chapter, it has opened a way to a deeper analysis of the decades-long process by which each of us gradually develops into a unique person.