A Troublesome Inheritance

by Nicholas Wade

  Human sociality is often assumed to be entirely a matter of culture, originating from the age of life when children are taught to be nice to one another. A cascade of discoveries, many in the past decade, has made clear that this is not the case. Human sociality has been shaped by natural selection, just as might be expected for any feature so crucial to survival. Sociality is written into our physical form, as with the whites of the eyes and the self-mortifying phenomenon of blushing as a signal of embarrassment. It is engraved in our neural circuitry too, most obviously in the faculty of language—there is no point in talking to oneself—and in many other behaviors. These include an inclination to follow rules and an urge to punish others when they fail to do so. Shame and guilt are the penalties for our own failings. To achieve status and avert retribution, we are always seeking to burnish our reputation. We trust the members of our in-group and are prepared to distrust the out-group. We often know instinctively what is right and wrong.

  The genes that set up the circuitry of these social instincts have not yet been identified, but their presence can be inferred from several lines of evidence that are described below. The salient fact is that all types of human society, from the hunter-gatherer band to the modern nation, are rooted in a suite of social behaviors. These behaviors, which most probably have a genetic basis, interact with culture to produce the institutions that are characteristic of each society and help it survive in its particular environment.

  Any trait that has a genetic basis can be changed by natural selection. The existence of genes that have some bearing on human social behavior means that social behavior can be reworked by evolution and therefore can vary in time and place. But natural selection’s remodeling of human societies is far harder to identify than changes in skin color, say, because skin color depends primarily on the genes whereas social behavior, harder to measure in any case, is strongly influenced by culture.

  Nonetheless, it is reasonable to assume that if traits like skin color have evolved in a population, the same may be true of its social behavior, and hence the very different kinds of society seen in the various races and in the world’s great civilizations differ not just because of their received culture—in other words, in what is learned from birth—but also because of variations in the social behavior of their members, carried down in their genes.

  Given the vast power of culture to shape human social behavior, it’s necessary to look far back in evolutionary history to glimpse the signs of social behavior genes at work.

  From Chimpanzee Society to Human Society

  The nature of human society can most clearly be understood by tracing how it evolved. Humans and chimpanzees, our closest evolutionary cousins, split apart some 5 to 6 million years ago. There is reason to think that the joint ancestor of humans and chimps was far more chimplike than human. Chimpanzees seem to live in much the same habitat as they did 5 million years ago, and their basic way of life hasn’t changed. The apes at the head of the human lineage, on the other hand, abandoned the forest and ventured out into the open savannahs of Africa, obliging them to go through many evolutionary transitions in both body and behavior as they grew more and more unlike the joint ancestor they shared with chimps.

  If the joint ancestor of chimps and humans was chimplike, so too was its social behavior. The society of living chimps can thus with reasonable accuracy stand in as a surrogate for the society of the joint ancestor and hence describe the baseline from which human social behavior evolved.

  Chimp bands are hierarchical. An alpha male and one or two allies dominate the male hierarchy, and below that is a less visible female hierarchy. The males are fiercely territorial, probably to protect the fruit trees that are the community’s chief source of food. Females usually stay and feed in one region of the territory. The larger each female’s region is and the more fruit trees it contains, the more children she can bear.

  To maintain and increase the size of their territory, male chimps conduct regular patrols around its perimeter, with occasional forays into their neighbors’ territory. Male chimps are unremittingly hostile to strange males and if possible will kill them on sight. Their favorite tactic on invading enemy territory is to surprise and kill any male whom they find alone. If the raiding party senses that it is outnumbered, it will retreat. A neighboring territory will be captured after its resident males have been killed off one by one in a campaign that may last several years.

  Chimp reproductive behavior requires a female to mate with all the males in her band, or at least as many as possible. She is estimated to copulate between 400 and 3,000 times per conception. This labor provides an insurance policy for her children, since each male who thinks he might be the father of her child is more likely to refrain from killing it.

  Despite the flamboyant promiscuity of female chimps, the alpha male somehow manages to fulfill his droit du seigneur of fathering many of the community’s offspring—about 36% in one study based on DNA paternity tests, or 45% excluding the close female relatives with whom he would avoid mating. The high-ranking males who were his allies together scored 50% of paternities.

  An important feature of chimp communities is that the females mostly disperse to neighboring groups when they reach adolescence, while the males stay in the community where they were born, an arrangement called patrilocality. Dispersal at puberty, which serves to avoid inbreeding, is common in primate communities, except that most are matrilocal, meaning that it’s the males who disperse and the females who stay in their home community. Chimps, many hunter-gatherer societies, and to some extent gorillas, are patrilocal. This arrangement probably has much to do with the chimp and human propensity for warfare: a group of males who have grown up together will be more cohesive in defending their own territory against rival groups. Since the males need to stay together, this obliges the females to move so as to avoid inbreeding.

  A strange feature of chimp society, at least from the human perspective, is that kinship is almost invisible. If you are born into a chimp society, you will know your mother and the siblings born a few years before or after you, because these are the chimps who hang out around your mother. But you will have no idea who your father is, though he must be one of the males in the community, nor any notion of who his relatives are, even though you see them every day. You are equally ignorant of your mother’s relatives, whom she left behind in her home community when she migrated to yours as a teenager. When a chimp raiding party enters neighboring territory, the males it kills may often be relatives or in-laws of the invaders’ daughters and sisters who dispersed there. But this kinship is unknown to the raiders.

  How then was the profound transition made from the chimplike society of the joint ancestor to the hunter-gatherer societies in which all humans lived until 15,000 years ago and in which kinship was a central institution? The likely steps in this process have been persuasively worked out by the primatologist Bernard Chapais. The critical behavioral step, in his view, was formation of the pair bond, or at least a stable breeding relationship between male and female.

  Consider a population of chimplike creatures living in a forest in Africa more than 5 million years ago. A fierce drought gripped Africa from 6.5 to 5 million years ago, and the forests shrank, giving way to open woodland or savannah. This was perhaps the event that forced the population apart into two groups, one of which led to chimps and the other to humans. In response to the drought, some of the population clung to the traditional habitat and became the ancestors of chimps. Others left the trees and sought new sources of food on the ground, despite the risk of being caught in the open by large cats and other predators. This group became the ancestors of the human lineage.

  The group trying life on the ground eventually started to walk upright, probably because walking on two feet is more efficient than knuckle-walking, the great ape method of making the knuckles of the hands serve as a pair of forefeet. Freeing the hands, though an accidental by-product of wa
lking upright, was an adaptation of far-reaching significance because the hands could now be used for gripping tools and for gesturing.

  Another adaptation, equally accidental and far-reaching, led to a transformation of social structure. This was the practice of mate guarding, which developed into the formation of stable breeding relationships and eventually of the pair bond between one male and one female.

  Males of almost all primate species, even chimps, guard females to some extent, so as to deter other males and improve their own chances of fathering the females’ children. Among the population of chimplike ancestors that had left the trees, mate guarding would have become more common than usual because of the more dangerous environment on the ground.

  With the male often around for defense, he could also help in feeding and taking care of the children. Having at least two people involved in child rearing made an enormous difference, Chapais argues. The period of juvenile dependence could last for several years longer. Children could be born at an earlier stage in their development since they would be more protected, and earlier birth enabled the brain to do more of its growing outside the womb. The human brain eventually reached three times the size of that of chimps.

  At first each male guarded as many females as he could, but another development drove them unwillingly toward monogamy. This was the emergence of weapons. At first, physical strength was decisive in fending off other males. But weapons are great equalizers because they tend to negate the advantages of size. The cost of maintaining a large harem became too high for most males. Weapons forced most to settle for one wife. The pair bond between male and female became established.

  Having a dad around makes all the difference to social networks. In highly promiscuous societies like those of chimps, an individual knows only its mother and the siblings it grows up with. With pair bonding, people know not only their father as well as their mother, but all their father’s relatives too. The males in a community now recognized both their daughters and, when their daughters dispersed to a neighboring group, a daughter’s husband and his parents.

  The neighbors, who used to be treated as hostile, began now to be seen in an entirely different light. Those males, who once had to be killed on sight, were not the enemy—they were the in-laws, with an equal interest in promoting the welfare of one’s daughter’s or sister’s children. Thus in the incipient human line, a new and more complex social structure came into being, that of the tribe, a group of bands bound to one another by exchange of women.

  Warfare between neighboring bands, the chimp practice, was now pushed upward to the tribal level. Tribes would fight as savagely as before, but among the bands within each tribe cooperation was now the rule.

  This profound transition in social structure started some time after the split of the ancestral populations leading to chimps and humans. Pair bonding, an essential element of the new social structure, probably did not become significant until the emergence of Homo ergaster some 1.7 million years ago. This is the first human ancestor in which the males were not very much larger than the females. A large size difference between the sexes, as in gorillas, indicates competition between males and a harem structure. The size difference diminishes as pair bonding becomes more common.

  Given the distinctiveness of chimp social behavior, there is no reason to doubt that it has a genetic basis. Both the chimp and human lineages would have inherited a suite of genes governing social behavior and in each species the genes for social behavior would have evolved as social structure changed in response to the society’s requirements for survival.

  Chimp social structure, in fact, may not differ much from that of the joint chimp-human ancestor. But human social structure has changed profoundly over the past 5 million years. Just as physical form was changing from ape to human, social behavior was undergoing a radical transformation from the chimplike behavior of multimale bands to the human pair-bond system. There is every reason to suppose that the development of distinctive social behavior in humans had a genetic basis, just as surely as the physical changes did. And if social behavior was under genetic control during the evolution of human society from that of a chimplike ancestor, it is hard to see why it should not have continued to be molded by evolutionary forces up until the present day.

  Social behavior changes in response to changes in the environment. As the hominid groups abandoned the trees, for eons the primates’ safe refuge, their societies had to adapt to the richer opportunities and more serious perils of life on the ground. This highly risky endeavor required a thorough makeover of standard ape social behavior, most pertinently in the degree of cooperation between individuals.

  The Distinctive Human Virtue: Cooperation

  Chimps will cooperate in certain ways, like assembling in war parties to patrol the borders of their territory. But beyond the minimum requirements for being a social species, they have little instinct to help one another. Chimps in the wild forage for themselves. Even chimp mothers regularly decline to share food with their children, who are able from a young age to gather their own food. When the mothers do share food, it’s always the rind or husk or less desirable part that is given to the child.3

  In the laboratory, chimps don’t naturally share food either. With some exceptions, most experiments show chimps to be severely lacking in altruistic sentiments for other chimps. If a chimp is put in a cage where he can pull in one tray of food for himself or, with no greater effort, a tray that also provides food for a neighbor in the next cage, he will pull indiscriminately—he just doesn’t care whether his neighbor gets fed or not. Yet he’s perfectly aware that one of the trays carries a portion of food made available to the neighboring cage. If the cage next door is empty and the chimp is allowed access to it, he will usually pull the tray with the double portion. Chimps are truly selfish.4

  Human children, on the other hand, are inherently cooperative. From the earliest ages, they desire to help others, to share information and to participate in pursuing common goals. The developmental psychologist Michael Tomasello has studied this cooperativeness in a series of experiments with very young children. He finds that if infants aged 18 months see an unrelated adult with hands full trying to open a door, almost all will immediately try to help. If the adult pretends to have lost an object, children from as young as 12 months will helpfully point out where it is.

  There are several reasons to believe that the urges to help, inform and share are “naturally emerging” in young children, Tomasello writes, meaning that they are innate, not taught.5 One is that these instincts appear at a very young age before most parents have started to train their children to behave socially. Another is that the helping behaviors are not enhanced if the children are rewarded.

  A third reason is that social intelligence develops in children before their general cognitive skills, at least when compared with apes. Tomasello gave human and chimp children a battery of tests related to understanding the physical and social worlds. The human children, aged 2.5 years, did no better than the chimps on the physical world tests but were considerably better at understanding the social world.6

  The essence of what children’s minds have and chimps’ don’t is what Tomasello calls shared intentionality. Part of this ability is that they can infer what others know or are thinking, a skill called theory of mind. But beyond that, even very young children want to be part of a shared purpose. They actively seek to be part of a “we,” a group that has pooled its talents and intends to work toward a shared goal.

  Children of course have the selfish motivations necessary for survival, like any other animal, but a vigorous social instinct is overlaid on their behavior from a very young age. The social instinct gets modulated in later life as the children learn to make distinctions about whom they can trust and who does not reciprocate.

  Besides shared intentionality, another striking social behavior is that of following norms, or rules generally agreed on withi
n the “we” group. Allied with the rule following are two other basic principles of human social behavior. One is a tendency to criticize, and if necessary punish, those who do not follow the agreed-upon norms. Another is to bolster one’s own reputation, presenting oneself as an unselfish and valuable follower of the group’s norms, an exercise that may involve finding fault with others.

  The first two behaviors are already evident in very young children. Tomasello showed a group of two- and three-year-olds a new game. A puppet then appeared and performed the game incorrectly. Almost all the children protested the puppet’s actions and many explicitly objected, telling the puppet how the game should be played. “Social norms—even of this relatively trivial type—can only be created by creatures who engage in shared intentionality and collective beliefs,” Tomasello writes, “and they play an enormously important role in maintaining the shared values of human cultural groups.” 7

  The urge to punish deviations from social norms is a distinctive feature of human societies. In principle it carries great risks for the punisher. In tribal or hunter-gatherer societies, anyone who punishes a miscreant is likely to have vengeance wreaked upon him by the miscreant’s family. So punishment in practice is meted out fairly deliberately. First, through social gossip, a consensus is arrived at that an individual’s behavior merits correction. Punishment may then be carried out collectively, by shunning or even ostracizing the deviant member. A different problem arises when the offender refuses to reform and must be killed. Hunter-gatherers will usually persuade his family to do the job because anyone else will bring down a blood feud on his head.