[Paleopsych] Foreign Affairs: Robert M. Sapolsky: A Natural History of Peace

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Robert M. Sapolsky: A Natural History of Peace
http://www.foreignaffairs.org/20060101faessay85110/robert-m-sapolsky/a-natural-history-of-peace.html

    From Foreign Affairs, January/February 2006
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    Summary: Humans like to think that they are unique, but the study of
    other primates has called into question the exceptionalism of our
    species. So what does primatology have to say about war and peace?
    Contrary to what was believed just a few decades ago, humans are not
    "killer apes" destined for violent conflict, but can make their own
    history.

    Robert M. Sapolsky is John A. and Cynthia Fry Gunn Professor of
    Biological Sciences and Professor of Neurology and Neurological
    Sciences at Stanford University. His most recent book is "Monkeyluv:
    And Other Essays on Our Lives as Animals."

    THE NAKED APE

    The evolutionary biologist Theodosius Dobzhansky once said, "All
    species are unique, but humans are uniquest." Humans have long taken
    pride in their specialness. But the study of other primates is
    rendering the concept of such human exceptionalism increasingly
    suspect.

    Some of the retrenchment has been relatively palatable, such as with
    the workings of our bodies. Thus we now know that a baboon heart can
    be transplanted into a human body and work for a few weeks, and human
    blood types are coded in Rh factors named after the rhesus monkeys
    that possess similar blood variability.

    More discomfiting is the continuum that has been demonstrated in the
    realm of cognition. We now know, for example, that other species
    invent tools and use them with dexterity and local cultural variation.
    Other primates display "semanticity" (the use of symbols to refer to
    objects and actions) in their communication in ways that would impress
    any linguist. And experiments have shown other primates to possess a
    "theory of mind," that is, the ability to recognize that different
    individuals can have different thoughts and knowledge.

    Our purported uniqueness has been challenged most, however, with
    regard to our social life. Like the occasional human hermit, there are
    a few primates that are typically asocial (such as the orangutan).
    Apart from those, however, it turns out that one cannot understand a
    primate in isolation from its social group. Across the 150 or so
    species of primates, the larger the average social group, the larger
    the cortex relative to the rest of the brain. The fanciest part of the
    primate brain, in other words, seems to have been sculpted by
    evolution to enable us to gossip and groom, cooperate and cheat, and
    obsess about who is mating with whom. Humans, in short, are yet
    another primate with an intense and rich social life -- a fact that
    raises the question of whether primatology can teach us something
    about a rather important part of human sociality, war and peace.

    It used to be thought that humans were the only savagely violent
    primate. "We are the only species that kills its own," one might have
    heard intoned portentously at the end of nature films several decades
    ago. That view fell by the wayside in the 1960s as it became clear
    that some other primates kill their fellows aplenty. Males kill;
    females kill. Some kill one another's infants with cold-blooded
    stratagems worthy of Richard III. Some use their toolmaking skills to
    fashion bigger and better cudgels. Some other primates even engage in
    what can only be called warfare -- organized, proactive group violence
    directed at other populations.

    As field studies of primates expanded, what became most striking was
    the variation in social practices across species. Yes, some primate
    species have lives filled with violence, frequent and varied. But life
    among others is filled with communitarianism, egalitarianism, and
    cooperative child rearing.

    Patterns emerged. In less aggressive species, such as gibbons or
    marmosets, groups tend to live in lush rain forests where food is
    plentiful and life is easy. Females and males tend to be the same
    size, and the males lack secondary sexual markers such as long, sharp
    canines or garish coloring. Couples mate for life, and males help
    substantially with child care. In violent species, on the other hand,
    such as baboons and rhesus monkeys, the opposite conditions prevail.

    The most disquieting fact about the violent species was the apparent
    inevitability of their behavior. Certain species seemed simply to be
    the way they were, fixed products of the interplay of evolution and
    ecology, and that was that. And although human males might not be
    inflexibly polygamous or come with bright red butts and six-inch
    canines designed for tooth-to-tooth combat, it was clear that our
    species had at least as much in common with the violent primates as
    with the gentle ones. "In their nature" thus became "in our nature."
    This was the humans-as-killer-apes theory popularized by the writer
    Robert Ardrey, according to which humans have as much chance of
    becoming intrinsically peaceful as they have of growing prehensile
    tails.

    That view always had little more scientific rigor than a Planet of the
    Apes movie, but it took a great deal of field research to figure out
    just what should supplant it. After decades' more work, the picture
    has become quite interesting. Some primate species, it turns out, are
    indeed simply violent or peaceful, with their behavior driven by their
    social structures and ecological settings. More important, however,
    some primate species can make peace despite violent traits that seem
    built into their natures. The challenge now is to figure out under
    what conditions that can happen, and whether humans can manage the
    trick themselves.

    PAX BONOBO

    Primatology has long been dominated by studies of the chimpanzee, due
    in large part to the phenomenally influential research of Jane
    Goodall, whose findings from her decades of observations in the wild
    have been widely disseminated. National Geographic specials based on
    Goodall's work would always include the reminder that chimps are our
    closest relatives, a notion underlined by the fact that we share an
    astonishing 98 percent of our DNA with them. And Goodall and other
    chimp researchers have carefully documented an endless stream of
    murders, cannibalism, and organized group violence among their
    subjects. Humans' evolutionary fate thus seemed sealed, smeared by the
    excesses of these first cousins.

    But all along there has been another chimp species, one traditionally
    ignored because of its small numbers; its habitat in remote,
    impenetrable rain forests; and the fact that its early chroniclers
    published in Japanese. These skinny little creatures were originally
    called "pygmy chimps" and were thought of as uninteresting, some sort
    of regressed subspecies of the real thing. Now known as bonobos, they
    are today recognized as a separate and distinct species that
    taxonomically and genetically is just as closely related to humans as
    the standard chimp. And boy, is this ever a different ape.

    Male bonobos are not particularly aggressive and lack the massive
    musculature typical of species that engage in a lot of fighting (such
    as the standard chimp). Moreover, the bonobo social system is female
    dominated, food is often shared, and there are well-developed means
    for reconciling social tensions. And then there is the sex.

    Bonobo sex is the prurient highlight of primatology conferences, and
    leads parents to shield their children's eyes when watching nature
    films. Bonobos have sex in every conceivable position and some
    seemingly inconceivable ones, in pairs and groups, between genders and
    within genders, to greet each other and to resolve conflicts, to work
    off steam after a predator scare, to celebrate finding food or to
    cajole its sharing, or just because. As the sound bite has it, chimps
    are from Mars and bonobos are from Venus.

    All is not perfect in the bonobo commune, and they still have
    hierarchies and conflict (why else invent conflict resolution?).
    Nonetheless, they are currently among the trendiest of species to
    analyze, a wonderful antidote to their hard-boiled relatives. The
    trouble is, while we have a pretty good sense of what bonobos are
    like, we have little insight into how they got that way. Furthermore,
    this is basically what all bonobos seem to be like -- a classic case
    of in-their-nature-ness. There is even recent evidence for a genetic
    component to the phenomenon, in that bonobos (but not chimps) possess
    a version of a gene that makes affiliative behavior (behavior that
    promotes group cohesion) more pleasurable to males. So -- a wondrous
    species (and one, predictably, teetering on the edge of extinction).
    But besides being useful for taking the wind out of we-be-chimps
    fatalists, the bonobo has little to say to us. We are not bonobos, and
    never can be.

    WARRIORS, COME OUT TO PLAY

    In contrast to the social life of bonobos, the social life of chimps
    is not pretty. Nor is that of rhesus monkeys, nor savanna baboons -- a
    species found in groups of 50 to 100 in the African grasslands and one
    I have studied for close to 30 years. Hierarchies among baboons are
    strict, as are their consequences. Among males, high rank is typically
    achieved by a series of successful violent challenges. Spoils, such as
    meat, are unevenly divided. Most males die of the consequences of
    violence, and roughly half of their aggression is directed at third
    parties (some high-ranking male in a bad mood takes it out on an
    innocent bystander, such as a female or a subordinate male).

    Male baboons, moreover, can fight amazingly dirty. I saw this happen a
    few years ago in one of the troops I study: Two males had fought, and
    one, having been badly trounced, assumed a crouching stance, with his
    rear end up in the air. This is universally recognized among savanna
    baboons as an abject gesture of subordination, signaling an end to the
    conflict, and the conventional response on the part of the victorious
    male is to subject the other to a ritualized gesture of dominance
    (such as mounting him). In this instance, however, the winner,
    approaching the loser as if to mount him, instead abruptly gave him a
    deep slash with his canines.

    A baboon group, in short, is an unlikely breeding ground for
    pacifists. Nevertheless, there are some interesting exceptions. In
    recent years, for example, it has been recognized that a certain
    traditional style of chest-thumping evolutionary thinking is wrong.
    According to the standard logic, males compete with one another
    aggressively in order to achieve and maintain a high rank, which will
    in turn enable them to dominate reproduction and thus maximize the
    number of copies of their genes that are passed on to the next
    generation. But although aggression among baboons does indeed have
    something to do with attaining a high rank, it turns out to have
    virtually nothing to do with maintaining it. Dominant males rarely are
    particularly aggressive, and those that are typically are on their way
    out: the ones that need to use it are often about to lose it. Instead,
    maintaining dominance requires social intelligence and impulse control
    -- the ability to form prudent coalitions, show some tolerance of
    subordinates, and ignore most provocations.

    Recent work, moreover, has demonstrated that females have something to
    say about which males get to pass on their genes. The traditional view
    was based on a "linear access" model of reproduction: if one female is
    in heat, the alpha male gets to mate with her; if two are in heat, the
    alpha male and the second-ranking male get their opportunity; and so
    on. Yet we now know that female baboons are pretty good at getting
    away from even champions of male-male competition if they want to and
    can sneak off instead with another male they actually desire. And who
    would that be? Typically, it is a male that has followed a different
    strategy of building affiliative relations with the female -- grooming
    her a lot, helping to take care of her kids, not beating her up. These
    nice-guy males seem to pass on at least as many copies of their genes
    as their more aggressive peers, not least because they can go like
    this for years, without the life-shortening burnout and injuries of
    the gladiators.

    And so the crude picture of combat as the sole path to evolutionary
    success is wrong. The average male baboon does opt for the combative
    route, but there are important phases of his life when aggression is
    less important than social intelligence and restraint, and there are
    evolutionarily fruitful alternative courses of action.

    Even within the bare-knuckle world of male-male aggression, we are now
    recognizing some surprising outposts of primate civility. For one
    thing, primates can make up after a fight. Such reconciliation was
    first described by Frans de Waal, of Emory University, in the early
    1980s; it has now been observed in some 27 different species of
    primates, including male chimps, and it works as it is supposed to,
    reducing the odds of further aggression between the two ex-combatants.
    And various primates, including male baboons, will sometimes
    cooperate, for example by supporting one another in a fight.
    Coalitions can involve reciprocity and even induce what appears to be
    a sense of justice or fairness. In a remarkable study by de Waal and
    one of his students, capuchin monkeys were housed in adjacent cages. A
    monkey could obtain food on its own (by pulling a tray of food toward
    its cage) or with help from a neighbor (by pulling a heavier tray
    together); in the latter case, only one of the monkeys was given
    access to the food in question. The monkeys that collaborated proved
    more likely to share it with their neighbor.

    Even more striking are lifelong patterns of cooperation among some
    male chimps, such as those that form bands of brothers. Among certain
    primate species, all the members of one gender will leave their home
    troop around puberty, thus avoiding the possibility of genetically
    deleterious inbreeding. Among chimps, the females leave home, and as a
    result, male chimps typically spend their lives in the company of
    close male relatives. Animal behaviorists steeped in game theory spend
    careers trying to figure out how reciprocal cooperation gets started
    among nonrelatives, but it is clear that stable reciprocity among
    relatives emerges readily.

    Thus, even the violent primates engage in reconciliation and
    cooperation -- but only up to a point. For starters, as noted in
    regard to the bonobo, there would be nothing to reconcile without
    violence and conflict in the first place. Furthermore, reconciliation
    is not universal: female savanna baboons are good at it, for example,
    but males are not. Most important, even among species and genders that
    do reconcile, it is not an indiscriminate phenomenon: individuals are
    more likely to reconcile with those who can be useful to them. This
    was demonstrated in a brilliant study by Marina Cords, of Columbia
    University, in which the value of some relationships among a type of
    macaque monkey was artificially raised. Animals were again caged next
    to each other under conditions in which they could obtain food by
    themselves or through cooperation, and those pairs that developed the
    capacity for cooperation were three times as likely to reconcile after
    induced aggression as noncooperators. Tension-reducing reconciliation,
    in other words, is most likely to occur among animals who already are
    in the habit of cooperating and have an incentive to keep doing so.

    Some deflating points emerge from the studies of cooperation as well,
    such as the fact that coalitions are notoriously unstable. In one
    troop of baboons I studied in the early 1980s, male-male coalitions
    lasted less than two days on average before collapsing, and most cases
    of such collapse involved one partner failing to reciprocate or, even
    more dramatically, defecting to the other side during a fight.
    finally, and most discouraging, is the use to which most coalitions
    are put. In theory, cooperation could trump individualism in order to,
    say, improve food gathering or defend against predators. In practice,
    two baboons that cooperate typically do so in order to make a third
    miserable.

    Goodall was the first to report the profoundly disquieting fact that
    bands of related male chimps carry out cooperative "border patrols" --
    searching along the geographic boundary separating their group from
    another and attacking neighboring males they encounter, even to the
    point of killing other groups off entirely. In-group cooperation can
    thus usher in not peace and tranquility, but rather more efficient
    extermination.

    So primate species with some of the most aggressive and stratified
    social systems have been seen to cooperate and resolve conflicts --
    but not consistently, not necessarily for benign purposes, and not in
    a cumulative way that could lead to some fundamentally non-Hobbesian
    social outcomes. The lesson appears to be not that violent primates
    can transcend their natures, but merely that the natures of these
    species are subtler and more multifaceted than previously thought. At
    least that was the lesson until quite recently.

    OLD PRIMATES AND NEW TRICKS

    To some extent, the age-old "nature versus nurture" debate is silly.
    The action of genes is completely intertwined with the environment in
    which they function; in a sense, it is pointless to even discuss what
    gene X does, and we should consider instead only what gene X does in
    environment Y. Nonetheless, if one had to predict the behavior of some
    organism on the basis of only one fact, one might still want to know
    whether the most useful fact would be about genetics or about the
    environment.

    The first two studies to show that primates were somewhat independent
    from their "natures" involved a classic technique in behavioral
    genetics called cross-fostering. Suppose some animal has engaged in a
    particular behavior for generations -- call it behavior A. We want to
    know if that behavior is due to shared genes or to a
    multigenerationally shared environment. Researchers try to answer the
    question by cross-fostering the animal, that is, switching the
    animal's mother at birth so that she is raised by one with behavior B,
    and then watching to see which behavior the animal displays when she
    grows up. One problem with this approach is that an animal's
    environment does not begin at birth -- a fetus shares a very intimate
    environment with its mother, namely the body's circulation, chock-full
    of hormones and nutrients that can cause lifelong changes in brain
    function and behavior. Therefore, the approach can be applied only
    asymmetrically: if a behavior persists in a new environment, one
    cannot conclude that genes are the cause, but if a behavior changes in
    a new environment, then one can conclude that genes are not the cause.
    This is where the two studies come in.

    In the early 1970s, a highly respected primatologist named Hans Kummer
    was working in Ethiopia, in a region containing two species of baboons
    with markedly different social systems. Savanna baboons live in large
    troops, with plenty of adult females and males. Hamadryas baboons, in
    contrast, have a more complex, multilevel society. Because they live
    in a much harsher, drier region, hamadryas have a distinctive
    ecological problem. Some resources are singular and scarce -- like a
    rare watering hole or a good cliff face to sleep on at night in order
    to evade predators -- and large numbers of animals are likely to want
    to share them. Other resources, such as the vegetation they eat, are
    sparse and widely dispersed, requiring animals to function in small,
    separate groups. As a result, hamadryas have evolved a "harem"
    structure -- a single adult male surrounded by a handful of adult
    females and their children -- with large numbers of discrete harems
    converging, peacefully, for short periods at the occasional desirable
    watering hole or cliff face.

    Kummer conducted a simple experiment, trapping an adult female savanna
    baboon and releasing her into a hamadryas troop and trapping an adult
    female hamadryas and releasing her into a savanna troop. Among
    hamadryas, if a male threatens a female, it is almost certainly this
    brute who dominates the harem, and the only way for the female to
    avoid injury is to approach him -- i.e., return to the fold. But among
    savanna baboons, if a male threatens a female, the way for her to
    avoid injury is to run away. In Kummer's experiment, the females who
    were dropped in among a different species initially carried out their
    species-typical behavior, a major faux pas in the new neighborhood.
    But gradually, they assimilated the new rules. How long did this
    learning take? About an hour. In other words, millennia of genetic
    differences separating the two species, a lifetime of experience with
    a crucial social rule for each female, and a miniscule amount of time
    to reverse course completely.

    The second experiment was set up by de Waal and his student Denise
    Johanowicz in the early 1990s, working with two macaque monkey
    species. By any human standards, male rhesus macaques are unappealing
    animals. Their hierarchies are rigid, those at the top seize a
    disproportionate share of the spoils, they enforce this inequity with
    ferocious aggression, and they rarely reconcile after fights. Male
    stump tail macaques, in contrast, which share almost all of their
    genes with their rhesus macaque cousins, display much less aggression,
    more affiliative behaviors, looser hierarchies, and more
    egalitarianism.

    Working with captive primates, de Waal and Johanowicz created a
    mixed-sex social group of juvenile macaques, combining rhesus and
    stump tails together. Remarkably, instead of the rhesus macaques
    bullying the stump tails, over the course of a few months, the rhesus
    males adopted the stump tails' social style, eventually even matching
    the stump tails' high rates of reconciliatory behavior. It so happens,
    moreover, that stump tails and rhesus macaques use different gestures
    when reconciling. The rhesus macaques in the study did not start using
    the stump tails' reconciliatory gestures, but rather increased the
    incidence of their own species-typical gestures. In other words, they
    were not merely imitating the stump tails' behavior; they were
    incorporating the concept of frequent reconciliation into their own
    social practices. When the newly warm-and-fuzzy rhesus macaques were
    returned to a larger, all-rhesus group, finally, their new behavioral
    style persisted.

    This is nothing short of extraordinary. But it brings up one last
    question: When those rhesus macaques were transferred back into the
    all-rhesus world, did they spread their insights and behaviors to the
    others? Alas, they did not. For that, we need to move on to our final
    case.

    LEFT BEHIND

    In the early 1980s, "Forest Troop," a group of savanna baboons I had
    been studying -- virtually living with -- for years, was going about
    its business in a national park in Kenya when a neighboring baboon
    group had a stroke of luck: its territory encompassed a tourist lodge
    that expanded its operations and consequently the amount of food
    tossed into its garbage dump. Baboons are omnivorous, and "Garbage
    Dump Troop" was delighted to feast on leftover drumsticks, half-eaten
    hamburgers, remnants of chocolate cake, and anything else that wound
    up there. Soon they had shifted to sleeping in the trees immediately
    above the pit, descending each morning just in time for the day's
    dumping of garbage. (They soon got quite obese from the rich diet and
    lack of exercise, but that is another story.)

    The development produced nearly as dramatic a shift in the social
    behavior of Forest Troop. Each morning, approximately half of its
    adult males would infiltrate Garbage Dump Troop's territory,
    descending on the pit in time for the day's dumping and battling the
    resident males for access to the garbage. The Forest Troop males that
    did this shared two traits: they were particularly combative (which
    was necessary to get the food away from the other baboons), and they
    were not very interested in socializing (the raids took place early in
    the morning, during the hours when the bulk of a savanna baboon's
    daily communal grooming occurs).

    Soon afterward, tuberculosis, a disease that moves with devastating
    speed and severity in nonhuman primates, broke out in Garbage Dump
    Troop. Over the next year, most of its members died, as did all of the
    males from Forest Troop who had foraged at the dump.[See Footnote #1]
    The results were that Forest Troop was left with males who were less
    aggressive and more social than average and the troop now had double
    its previous female-to-male ratio.

    The social consequences of these changes were dramatic. There remained
    a hierarchy among the Forest Troop males, but it was far looser than
    before: compared with other, more typical savanna baboon groups,
    high-ranking males rarely harassed subordinates and occasionally even
    relinquished contested resources to them. Aggression was less
    frequent, particularly against third parties. And rates of affiliative
    behaviors, such as males and females grooming each other or sitting
    together, soared. There were even instances, now and then, of adult
    males grooming each other -- a behavior nearly as unprecedented as
    baboons sprouting wings.

    This unique social milieu did not arise merely as a function of the
    skewed sex ratio; other primatologists have occasionally reported on
    troops with similar ratios but without a comparable social atmosphere.
    What was key was not just the predominance of females, but the type of
    male that remained. The demographic disaster -- what evolutionary
    biologists term a "selective bottleneck" -- had produced a savanna
    baboon troop quite different from what most experts would have
    anticipated.

    But the largest surprise did not come until some years later. Female
    savanna baboons spend their lives in the troop into which they are
    born, whereas males leave their birth troop around puberty; a troop's
    adult males have thus all grown up elsewhere and immigrated as
    adolescents. By the early 1990s, none of the original low
    aggression/high affiliation males of Forest Troop's tuberculosis
    period was still alive; all of the group's adult males had joined
    after the epidemic. Despite this, the troop's unique social milieu
    persisted -- as it does to this day, some 20 years after the selective
    bottleneck.In other words, adolescent males that enter Forest Troop
    after having grown up elsewhere wind up adopting the unique behavioral
    style of the resident males. As defined by both anthropologists and
    animal behaviorists, "culture" consists of local behavioral
    variations, occurring for nongenetic and nonecological reasons, that
    last beyond the time of their originators. Forest Troop's low
    aggression/high affiliation society constitutes nothing less than a
    multigenerational benign culture.

    Continuous study of the troop has yielded some insights into how its
    culture is transmitted to newcomers. Genetics obviously plays no role,
    nor apparently does self-selection: adolescent males that transfer
    into the troop are no different from those that transfer into other
    troops, displaying on arrival similarly high rates of aggression and
    low rates of affiliation. Nor is there evidence that new males are
    taught to act in benign ways by the residents. One cannot rule out the
    possibility that some observational learning is occurring, but it is
    difficult to detect given that the distinctive feature of this culture
    is not the performance of a unique behavior but the performance of
    typical behaviors at atypically extreme rates.

    To date, the most interesting hint about the mechanism of transmission
    is the way recently transferred males are treated by Forest Troop's
    resident females. In a typical savanna baboon troop, newly transferred
    adolescent males spend years slowly working their way into the social
    fabric; they are extremely low ranking -- ignored by females and noted
    by adult males only as convenient targets for aggression. In Forest
    Troop, by contrast, new male transfers are inundated with female
    attention soon after their arrival. Resident females first present
    themselves sexually to new males an average of 18 days after the males
    arrive, and they first groom the new males an average of 20 days after
    they arrive (normal savanna baboons introduce such behaviors after 63
    and 78 days, respectively). Furthermore, these welcoming gestures
    occur more frequently in Forest Troop during the early post-transfer
    period, and there is four times as much grooming of males by females
    in Forest Troop as elsewhere. From almost the moment they arrive, in
    other words, new males find out that in Forest Troop, things are done
    differently.

    At present, I think the most plausible explanation is that this
    troop's special culture is not passed on actively but simply emerges,
    facilitated by the actions of the resident members. Living in a group
    with half the typical number of males, and with the males being nice
    guys to boot, Forest Troop's females become more relaxed and less
    wary. As a result, they are more willing to take a chance and reach
    out socially to new arrivals, even if the new guys are typical jerky
    adolescents at first. The new males, in turn, finding themselves
    treated so well, eventually relax and adopt the behaviors of the
    troop's distinctive social milieu.

    NATURAL BORN KILLERS?

    Are there any lessons to be learned here that can be applied to
    human-on-human violence -- apart, that is, from the possible
    desirability of giving fatal cases of tuberculosis to aggressive
    people?

    Any biological anthropologist opining about human behavior is required
    by long-established tradition to note that for 99 percent of human
    history, humans lived in small, stable bands of related
    hunter-gatherers. Game theorists have shown that a small, cohesive
    group is the perfect setting for the emergence of cooperation: the
    identities of the other participants are known, there are
    opportunities for multiple iterations of games (and thus the ability
    to punish cheaters), and there is open-book play (players can acquire
    reputations). And so, those hunter-gatherer bands were highly
    egalitarian. Empirical and experimental data have also shown the
    cooperative advantages of small groups at the opposite human extreme,
    namely in the corporate world.

    But the lack of violence within small groups can come at a heavy
    price. Small homogenous groups with shared values can be a nightmare
    of conformity. They can also be dangerous for outsiders. Unconsciously
    emulating the murderous border patrols of closely related male chimps,
    militaries throughout history have sought to form small, stable units;
    inculcate them with rituals of pseudokinship; and thereby produce
    efficient, cooperative killing machines.

    Is it possible to achieve the cooperative advantages of a small group
    without having the group reflexively view outsiders as the Other? One
    way is through trade. Voluntary economic exchanges not only produce
    profits; they can also reduce social friction -- as the macaques
    demonstrated by being more likely to reconcile with a valued partner
    in food acquisition.

    Another way is through a fission-fusion social structure, in which the
    boundaries between groups are not absolute and impermeable. The model
    here is not the multilevel society of the hamadryas baboons, both
    because their basic social unit of the harem is despotic and because
    their fusion consists of nothing more than lots of animals
    occasionally coming together to utilize a resource peacefully. Human
    hunter-gatherers are a better example to follow, in that their small
    bands often merge, split, or exchange members for a while, with such
    fluidity helping to solve not only environmental resource problems but
    social problems as well. The result is that instead of the
    all-or-nothing world of male chimps, in which there is only one's own
    group and the enemy, hunter-gatherers can enjoy gradations of
    familiarity and cooperation stretching over large areas.

    The interactions among hunter-gatherers resemble those of other
    networks, where there are individual nodes (in this case, small
    groups) and where the majority of interactions between the nodes are
    local ones, with the frequency of interactions dropping off as a
    function of distance. Mathematicians have shown that when the ratios
    among short-, middle-, and long-distance interactions are optimal,
    networks are robust: they are dominated by highly cooperative clusters
    of local interactions, but they also retain the potential for less
    frequent, long-distance communication and coordination.

    Optimizing the fission-fusion interactions of hunter-gatherer networks
    is easy: cooperate within the band; schedule frequent joint hunts with
    the next band over; have occasional hunts with bands somewhat farther
    out; have a legend of a single shared hunt with a mythic band at the
    end of the earth. Optimizing the fission-fusion interactions in
    contemporary human networks is vastly harder, but the principles are
    the same.

    In exploring these subjects, one often encounters a pessimism built
    around the notion that humans, as primates, are hard-wired for
    xenophobia. Some brain-imaging studies have appeared to support this
    view in a particularly discouraging way. There is a structure deep
    inside the brain called the amygdala, which plays a key role in fear
    and aggression, and experiments have shown that when subjects are
    presented with a face of someone from a different race, the amygdala
    gets metabolically active -- aroused, alert, ready for action. This
    happens even when the face is presented "subliminally," which is to
    say, so rapidly that the subject does not consciously see it.

    More recent studies, however, should mitigate this pessimism. Test a
    person who has a lot of experience with people of different races, and
    the amygdala does not activate. Or, as in a wonderful experiment by
    Susan Fiske, of Princeton University, subtly bias the subject
    beforehand to think of people as individuals rather than as members of
    a group, and the amygdala does not budge. Humans may be hard-wired to
    get edgy around the Other, but our views on who falls into that
    category are decidedly malleable.

    In the early 1960s, a rising star of primatology, Irven DeVore, of
    Harvard University, published the first general overview of the
    subject. Discussing his own specialty, savanna baboons, he wrote that
    they "have acquired an aggressive temperament as a defense against
    predators, and aggressiveness cannot be turned on and off like a
    faucet. It is an integral part of the monkeys' personalities, so
    deeply rooted that it makes them potential aggressors in every
    situation." Thus the savanna baboon became, literally, a textbook
    example of life in an aggressive, highly stratified, male-dominated
    society. Yet within a few years, members of the species demonstrated
    enough behavioral plasticity to transform a society of theirs into a
    baboon utopia.

    The first half of the twentieth century was drenched in the blood
    spilled by German and Japanese aggression, yet only a few decades
    later it is hard to think of two countries more pacific. Sweden spent
    the seventeenth century rampaging through Europe, yet it is now an
    icon of nurturing tranquility. Humans have invented the small nomadic
    band and the continental megastate, and have demonstrated a
    flexibility whereby uprooted descendants of the former can function
    effectively in the latter. We lack the type of physiology or anatomy
    that in other mammals determine their mating system, and have come up
    with societies based on monogamy, polygyny, and polyandry. And we have
    fashioned some religions in which violent acts are the entrée to
    paradise and other religions in which the same acts consign one to
    hell. Is a world of peacefully coexisting human Forest Troops
    possible? Anyone who says, "No, it is beyond our nature," knows too
    little about primates, including ourselves.

    [Footnote #1] Considerable sleuthing ultimately revealed that the
    disease had come from tainted meat in the garbage dump, which had been
    sold to the tourist lodge thanks to a corrupt meat inspector. The
    studies were the first of this kind of outbreak in a wild primate
    population and showed that, in contrast to what happens with humans
    and captive primates, there was little animal-to-animal transmission
    of the tuberculosis, and so the disease did not spread in Forest Troop
    beyond the garbage eaters.