A Troublesome Inheritance: Genes, Race and Human History (7 page)

Human sociability is two-edged. The trust extended to members of one’s own group is mirrored by the suspicion and potential mistrust shown toward strangers. Willingness to defend one’s own people is the counterpart of readiness to kill the enemy. Human morality is not universal, as philosophers have argued: it is strictly local, at least in its instinctual form. Reflections of this ambivalence are now apparent from the level of the genes.

If human social nature is innate and has evolved, as seems highly likely, there will be evidence of its evolution in the genome. Very little about the genes that govern the human brain is yet understood, so it need be no surprise that not much is yet known about the genetic basis of human social behavior. A prominent exception concerns the neural hormone called oxytocin, sometimes known as the hormone of trust. It is synthesized in a region in the base of the brain known as the hypothalamus and from there is distributed to both brain and body, with separate roles in each. In the body, oxytocin is released when a woman gives birth and when she gives milk to her child.

In the brain, oxytocin has a range of subtle effects that are only beginning to be explored. In general, it seems that oxytocin has been co-opted in the course of evolution to play a central role in social cohesion. It’s a hormone of affiliation. It dampens down the distrust usually felt toward strangers and promotes feelings of solidarity. “It increases men’s trust, generosity and willingness to cooperate,” say the authors of a recent review.
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(The same is doubtless true of women too but most such experiments are performed only in men because of the risk that oxytocin might make a woman miscarry if she were unknowingly pregnant.)

The trust promoted by oxytocin is not of the brotherhood of man variety—it’s strictly local. Oxytocin engenders trust toward members of the in-group, together with feelings of defensiveness toward outsiders. This limitation in oxytocin’s radius of trust was discovered only recently by Carsten De Dreu, a Dutch psychologist who doubted the conventional wisdom that oxytocin simply promoted general feelings of trust. Any individual who blindly trusts everyone is not going to prosper in the struggle for survival, De Dreu supposed, and his genes would be rapidly eliminated; hence it seemed much more likely that oxytocin promoted trust only in certain contexts.

De Dreu showed in several ingenious experiments that this is indeed the case. In one, the young Dutch men who were his subjects
were presented with standard moral dilemmas, such as whether to save five people in the path of a train for the loss of one life, that of a bystander who could be thrown onto the tracks to stop the train. The people to be saved were all Dutch but the person to be killed was sometimes given a Dutch first name, like Pieter, and sometimes a German or Muslim name, like Helmut or Muhammad. (Opinion polls show that neither is a favorite nationality among the Dutch.)

When the subjects had taken a sniff of oxytocin, they were much more inclined to sacrifice the Helmuts and the Muhammads, De Dreu found, showing the dark side of oxytocin in making people more willing to punish outsiders. Oxytocin does not seem to promote positive aggression toward outsiders, he finds, but rather it heightens the willingness to defend the in-group.
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The two-edged nature of oxytocin is just what might be expected to suit the needs of ancestral humans living in small tribal groups where every stranger was a possible enemy. In larger societies, for instance in cities, where people must often do business with strangers, the general level of trust needs to be considerably higher than in tribal societies, where most interactions are with close kin.

So deep are oxytocin’s roots that it is involved in the most basic aspect of human sociality, that of recognizing people’s faces. Doses of oxytocin improve a subject’s recognition of human faces. Genetic variations in the gene that specifies the oxytocin receptor protein are associated with impaired face recognition.
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When oxytocin reaches a target neuron, it interacts with a receptor protein that juts out from the neuron’s surface and is crafted to recognize oxytocin specifically. The strength with which these receptors bind to oxytocin can be varied by making small changes in the receptor’s gene. An experiment to test this of course cannot be done in people, but relevant evidence comes from comparing two species of vole. Male prairie voles are monogamous and make caring, trustworthy fathers, whereas male meadow voles are roaming
polygamists who leave much to be desired in the fatherhood realm. If meadow voles are genetically engineered so as to stud their neurons with extra receptors for vasopressin, a hormone very similar to oxytocin, these Lotharios suddenly become monogamous.
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It’s easy to see how natural selection could increase the general degree of trust in human societies, whether by raising the brain’s production of oxytocin, by inserting more oxytocin receptors into people’s neurons, or by enhancing the tenacity with which the receptors hold on to oxytocin. The opposite processes would lower the degree of social trust. It is not yet known by what specific mechanism the oxytocin levels in people are controlled. But the oxytocin mechanism can evidently be modulated by natural selection so as to achieve either more or less of the same effect. If an inclination to distrust others should favor survival, people with lower oxytocin levels will flourish and have more children, and in several generations, a society will become less trusting. Conversely, if stronger bonds of trust help a society flourish, genes that increase oxytocin levels will become more common.

This is not to imply that trust in human societies is set exclusively by the genes. Culture is far more important in most short-term interactions. As with most human behaviors, the genes provide just a nudge in a certain direction. But these small nudges, acting on every individual, can alter the nature of a society. Small changes in social behavior can, in the long term, deeply modify the social fabric and make one society differ significantly from another.

Besides trust, another important social behavior that is clearly under genetic influence is that of aggression, or rather the whole spectrum of behaviors that runs from aggression to shyness. The fact that
animals can be domesticated is proof that the trait can be modulated by the selective pressures of evolution.

One of the most dramatic experiments on the genetic control of aggression was performed by the Soviet scientist Dmitriy Belyaev. From the same population of Siberian gray rats he developed two strains, one highly sociable and the other brimming with aggression. For the tame rats, the parents of each generation were chosen simply by the criterion of how well they tolerated human presence. For the ferocious rats, the criterion was how adversely they reacted to people. After many generations of breeding, the first strain was now so tame that when visitors entered the room where the rats were caged, the animals would press their snouts through the bars to be petted. The other strain could not have been more different. The rats would hurl themselves screaming toward the intruder, thudding ferociously against the bars of their cage.
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Rodents and humans use many of the same genes and brain regions to control aggression. Experiments with mice have shown that a large number of genes are involved in the trait, and the same is certainly true of people. Comparisons of identical twins raised together and separately show that aggression is heritable. Genes account for between 37% and 72% of the heritability, the variation of the trait in a population, according to various studies. But very few of the genes that underlie aggression have yet been identified, in part because when many genes control a behavior, each has so small an effect that it is hard to detect. Most research has focused on genes that promote aggression rather than those at the other end of the behavioral spectrum.

One of the genes associated with aggression is called MAO-A, meaning that it makes one of two forms of an enzyme called monoamine oxidase. The enzyme has a central role in maintaining normal mental states through its cleanup function—it breaks down three of the small neurotransmitter chemicals used to convey signals from one neuron to another. The three neurotransmitters, serotonin,
norepinephrine, and dopamine, need to be disposed of after accomplishing their signaling task. If allowed to accumulate in the brain, they will keep neurons activated that should have returned to rest.

The role of MAO-A in the control of aggression came to light in 1993 through the study of a Dutch family in which the men were inclined to violently deviant behavior, such as impulsive aggression, arson, attempted rape and exhibitionism. The eight affected men had inherited an unusual form of the MAO-A gene. A single mutation in the gene causes the cell’s assembly of the MAO-A enzyme to be halted halfway through, rendering it ineffective. In the absence of functioning MAO-A enzymes, neurotransmitters build up in excess, causing the men to be overaggressive in social situations.
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Mutations that totally disrupt a gene like MAO-A have serious consequences for the individual. There are more subtle ways in which a gene like MAO-A can be modulated by natural selection so as to make people either more or less aggressive. Genes are controlled by elements called promoters, which are short stretches of DNA that lie near the genes they control. And being made of DNA, the promoters can incur mutations just like the DNA of the genes.

As it happens, the promoter for MAO-A is quite variable in the human population. People may have two, three, four or five copies of it, and the more copies they have, the more of the MAO-A enzyme their cells produce. What difference does this make to a person’s behavior? Quite a lot, it turns out. People with three, four or five copies of the MAO-A promoter are normal but those with only two copies have a much higher level of delinquency. From a questionnaire given to 2,524 youths in the United States, Jean Shih and colleagues found that men with just two promoters were significantly more likely to report that they had committed both serious delinquency within the previous 12 months, such as theft, selling drugs or damaging property, and violent assaults, such as hurting someone badly enough to need medical care or threatening someone with a knife or gun.
Women with two promoters also reported much higher levels of serious and violent delinquency than those with more promoters.
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If individuals can differ in the genetic structure of their MAO-A gene and its controls, is the same also true of races and ethnicities? The answer is yes. A team led by Karl Skorecki of the Rambam Health Care Center in Haifa looked at variations in the MAO-A gene in people from seven ethnicities—Ashkenazi Jews, Bedouins, African pygmies, aboriginal Taiwanese, East Asians (Chinese and Japanese), Mexicans and Russians. They found 41 variations in the portions of the gene they decoded, and the pattern of variation differed from one ethnicity to the next, revealing a “substantial differentiation between populations.”

The pattern of variation could have arisen from random mutations in the DNA that had no effect on the MAO-A enzyme or on people’s behavior. But after applying various tests, the researchers concluded there was possible evidence for “positive selection, potentially acting on MAO-A-related phenotypes.”
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This means they think that natural selection could have favored particular behavioral traits in the various ethnicities, whether more or less aggressive, and that this could have caused the particular patterns of variation in the MAO-A gene. But the researchers did not examine the behaviors of the various ethnicities so could not establish causal links between each pattern of variations in the MAO-A enzyme and specific behavioral traits.

Such a link has been asserted by a research team led by Michael Vaughn of Saint Louis University. He and his colleagues looked at the MAO-A promoters in African Americans. The subjects were the same 2,524 American youths in the study by Shih mentioned above. Of the African American men in the sample, 5% carried two MAO-A promoters, the condition that Shih had found to be associated with higher levels of delinquency. Members of the two-promoter group were significantly more likely to have been arrested and imprisoned than African Americans who carried three or four promoters. The same
comparison could not be made in white, or Caucasian, males, the researchers report, because only 0.1% carry the two-promoter allele.
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A finding like this has to be interpreted with care. First, like any scientific report, it needs to be repeated by an independent laboratory to be sure it is valid. Second, a large number of genes are evidently involved in controlling aggression, so even if African Americans are more likely to carry the violence-linked allele of MAO-A promoters than are Caucasians, Caucasians may carry the aggressive allele of other genes yet to be identified. Indeed a variant of a gene called HTR2B, an allele that predisposes carriers to impulsive and violent crimes when under the influence of alcohol, has been found in Finns.
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It is therefore impossible, by looking at single genes, to say on genetic grounds that one race is genetically more prone to violence than any other. Third, genes don’t determine human behavior; they merely create a propensity to behave in a certain way. Whether a propensity to violence is exercised depends on circumstances as well as genetic endowment, so that people who live in conditions of poverty and unemployment may have more inducements to violence than those who are better off.

The wider point illustrated by the case of the MAO-A gene is that important aspects of human social behavior are shaped by the genes and that these behavior traits are likely to vary from one race to another, sometimes significantly so.

Trust and aggression are two significant components of human social behavior whose underlying genetics have already been to some extent explored. There are many other aspects of social behavior, such as conformity to rules, the willingness to punish violators of social norms
or the expectation of fairness and reciprocity, that most probably have a genetic basis, although one that remains to be discovered.

The fact that human social behavior is to some extent shaped by the genes means that it can evolve and that different kinds of society can emerge as the underlying social behaviors shift. Conversely, major changes in human society, such as the transition from hunter-gathering to settled life, were almost certainly accompanied by evolutionary changes in social behavior as people adapted to their new way of life. (The words adapt and adaptation are always used here in the biological sense of a genetically based evolutionary response to circumstances.)

There are two important factors to consider in the emergence of social change. One is that a society develops through changes in its institutions, which are blends of culture and genetically shaped social behavior. The other is that the genes and culture interact. This may seem paradoxical to anyone who considers genes and culture to be entirely separate realms. But it is scarcely surprising from an evolutionary perspective, given that the genome is designed to respond to the environment, and a major component of the human environment is society and its cultural practices.

The working components of a society are its institutions. Any socially agreed-upon form of behavior, from a tribal dance to a parliament, may be considered an institution. Institutions reflect both culture and history, but their basic building blocks are human behaviors. Follow an institution all the way down, and beneath thick layers of culture, it is built on instinctual human behaviors. The rule of law would not exist if people didn’t have innate tendencies to follow norms and to punish violators. Soldiers could not be made to follow orders were not army discipline able to invoke innate behaviors of conformity, obedience and willingness to kill for one’s own group.

So consider the intricate dynamics of the natural system in which the members of a human society are embedded. Their basic
motivation is their own survival and that of their families. Unlike species that can only interact directly with their environment, people often do so through their society and its institutions. In responding to an environmental change, a society adjusts its institutions, and its members adjust to the new institutions by changing their culture in the short term and their social behavior in the long term.

The idea that human behavior has a genetic basis has long been resisted by those who see the mind as a blank slate on which only culture can write. The blank slate notion has been particularly attractive to Marxists, who wish government to mold socialist man in its desired image and who see genetics as an impediment to the power of the state. Marxist academics led the attack on Edward O. Wilson when he proposed in his 1975 book
Sociobiology
that social behaviors such as conformity and morality had a genetic basis. Wilson even suggested that genes might have some influence “in the behavioral qualities that underlie variations between cultures.”
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Although his term sociobiology is not now widely used—evolutionary psychology is a less controversial term for much the same thing—the tide has turned in favor of Wilson’s ideas now that many human faculties seem to be innate. From the social repertoire of babies to the moral instincts discernible from psychological tests, it is clear that the human mind is hereditarily predisposed to act in certain ways.

Social behavior changes because, over a period of generations, genes and culture interact. “The genes hold culture on a leash,” Wilson writes. “The leash is very long but inevitably values will be constrained in accordance with their effects on the human gene pool.”
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Harmful cultural practices may lead to extinction, but advantageous ones create selective pressures that can promote specific genetic variants. If a cultural practice provides a significant survival advantage, genes that enable a person to engage in that practice will become more common.

This interaction between the genome and society, known as gene-culture evolution, has probably been a powerful force in shaping human societies. At present it has been documented for only minor dietary changes, but these establish the principle. The leading example is that of lactose tolerance, the ability to digest milk in adulthood by means of the enzyme lactase, which breaks down lactose, the principal sugar in milk.

Figure 3.1. Distribution of lactose tolerance in present-day Europe (dark gray = 100%). Dotted area shows homeland of Funnel Beaker Culture, which flourished 6,000 to 5,000 years ago.

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In most human populations, the lactase gene is permanently switched off after weaning so as to save the energy required to make the lactase enzyme. Lactose, the sugar metabolized by the lactase
enzyme, occurs only in milk, so that when a person has finished breast-feeding, lactase will never be needed again. But in populations that learned to herd cattle and drink raw cow’s milk, notably the Funnel Beaker Culture that flourished in north central Europe from 6,000 to 5,000 years ago, there was a great selective advantage in keeping the lactase gene switched on. Almost all Dutch and Swedish people today are lactose tolerant, meaning they carry the mutation that keeps the lactase gene permanently switched on. The mutation is progressively less common in Europe with increasing distance from the core region of the ancient Funnel Beaker Culture.

Three different mutations that have the same result have been detected in pastoral peoples of eastern Africa. Natural selection has to work on whatever mutations are available in a population, and evidently different mutations were available in the European and various African peoples who took up cattle raising and drinking raw milk. The lactase-prolonging mutations conferred an enormous advantage on their owners, letting them leave ten times more surviving children than those without the mutation.
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Lactose tolerance is a fascinating example of how a human cultural practice, in this case cattle raising and drinking raw milk, can feed back into the human genome. The genes that underlie social behavior have for the most part not yet been identified, but it’s a reasonable assumption that they too would have changed in response to new social institutions. In larger societies requiring a higher degree of trust, people who trusted only their close kin would have been at a disadvantage. People who were more trusting would have had more surviving children, and any genetic variation that promoted this behavior would become more common in each successive generation.