The Better Angels of Our Nature: Why Violence Has Declined (139 page)
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Authors: Steven Pinker
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BOOK: The Better Angels of Our Nature: Why Violence Has Declined
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Oxytocin
, the so-called cuddle hormone which encourages sympathy and trust, acts on receptors in several parts of the brain, and the number and distribution of those receptors can have dramatic effects on behavior. In a famous experiment, biologists inserted a gene for the receptor for vasopressin (a hormone similar to oxytocin that operates in the brains of males) into meadow voles, an aggressive and promiscuous species that lacks it. Higamous, hogamous, the voles were monogamous, just like their evolutionary cousins the prairie voles, which come with the receptors preinstalled.
149
The experiment suggests that simple genetic changes in the oxytocin-vasopressin system can have profound effects on sympathy, bonding, and by extension the inhibition of aggression.
, the so-called cuddle hormone which encourages sympathy and trust, acts on receptors in several parts of the brain, and the number and distribution of those receptors can have dramatic effects on behavior. In a famous experiment, biologists inserted a gene for the receptor for vasopressin (a hormone similar to oxytocin that operates in the brains of males) into meadow voles, an aggressive and promiscuous species that lacks it. Higamous, hogamous, the voles were monogamous, just like their evolutionary cousins the prairie voles, which come with the receptors preinstalled.
149
The experiment suggests that simple genetic changes in the oxytocin-vasopressin system can have profound effects on sympathy, bonding, and by extension the inhibition of aggression.
Testosterone
. A person’s response to a challenge of dominance depends in part on the amount of testosterone released into the bloodstream and on the distribution of receptors for the hormone in his or her brain.
150
The gene for the testosterone receptor varies across individuals, so a given concentration of testosterone can have a stronger effect on the brains of some people than others. Men with genes that code for more sensitive versions of the receptor have a greater surge of testosterone when conversing with an attractive woman (which can lead to reduced fear and greater risk-taking), and in one study were overrepresented in a sample of convicted rapists and murderers.
151
The genetic pathways that regulate testosterone are complicated, but they offer a target by which natural selection could alter people’s willingness to take up aggressive challenges.
. A person’s response to a challenge of dominance depends in part on the amount of testosterone released into the bloodstream and on the distribution of receptors for the hormone in his or her brain.
150
The gene for the testosterone receptor varies across individuals, so a given concentration of testosterone can have a stronger effect on the brains of some people than others. Men with genes that code for more sensitive versions of the receptor have a greater surge of testosterone when conversing with an attractive woman (which can lead to reduced fear and greater risk-taking), and in one study were overrepresented in a sample of convicted rapists and murderers.
151
The genetic pathways that regulate testosterone are complicated, but they offer a target by which natural selection could alter people’s willingness to take up aggressive challenges.
Neurotransmitters
are the molecules that are released from a neuron, seep across a microscopic gap, and lock onto a receptor in the surface of another neuron, changing its activity and thereby allowing patterns of neural firing to propagate through the brain. One major class of neurotransmitters are the catecholamines, which include dopamine, serotonin, and norepinephrine (also called noradrenaline, and related to the adrenaline that triggers the fight-or-flight response). The catecholamines are used in several motivational and emotional systems of the brain, and their concentration is regulated by proteins that break them down or recycle them. One of those enzymes is monoamine oxidase-A, MAO-A for short, which helps to break down these neurotransmitters, preventing them from building up in the brain. When they do build up, the organism can become hyperreactive to threats and more likely to engage in aggression.
are the molecules that are released from a neuron, seep across a microscopic gap, and lock onto a receptor in the surface of another neuron, changing its activity and thereby allowing patterns of neural firing to propagate through the brain. One major class of neurotransmitters are the catecholamines, which include dopamine, serotonin, and norepinephrine (also called noradrenaline, and related to the adrenaline that triggers the fight-or-flight response). The catecholamines are used in several motivational and emotional systems of the brain, and their concentration is regulated by proteins that break them down or recycle them. One of those enzymes is monoamine oxidase-A, MAO-A for short, which helps to break down these neurotransmitters, preventing them from building up in the brain. When they do build up, the organism can become hyperreactive to threats and more likely to engage in aggression.
The first sign that MAO-A can affect violence in humans was the discovery of a Dutch family that carried a rare mutation that left half the men without a working version of the gene.
152
(The gene is found on the X chromosome, which men have only one of, so if a man’s MAO-A gene is defective, he has no backup copy to compensate.) Over at least five generations, the affected men in the family were prone to aggressive outbursts. One, for example, forced his sisters to disrobe at knifepoint; another tried to run over his boss with his car.
152
(The gene is found on the X chromosome, which men have only one of, so if a man’s MAO-A gene is defective, he has no backup copy to compensate.) Over at least five generations, the affected men in the family were prone to aggressive outbursts. One, for example, forced his sisters to disrobe at knifepoint; another tried to run over his boss with his car.
A more common kind of variation is found in the part of the gene that determines how much MAO-A is produced. People with a low-activity version of the gene build up higher levels of dopamine, serotonin, and norepinephrine in the brain. They also are more likely to have symptoms of antisocial personality disorder, to report that they have committed acts of violence, to be convicted of a violent crime, to have amygdalas that react more strongly and an orbital cortex that reacts less strongly to angry and fearful faces, and, in the psychology lab, to force a fellow participant to drink hot sauce if they think he has exploited them.
153
Unlike many other genes that affect behavior, the low-activity version of the MAO-A gene seems to be fairly specific to aggression; it does not correlate well with any other personality trait.
154
153
Unlike many other genes that affect behavior, the low-activity version of the MAO-A gene seems to be fairly specific to aggression; it does not correlate well with any other personality trait.
154
The low-activity version of the MAO-A gene makes people more prone to aggression primarily when they have grown up with stressful experiences, such as having been abused or neglected by their parents or having been held back in school.
155
It’s hard to pinpoint the exact stressors that have this effect, because stressful lives are often stressful in many ways at once. In fact, the modulating factor may consist of other genes which are shared with an abusive parent, predisposing both parent and child to aggression, and which may also elicit negative reactions from the people around him.
156
But whatever the modifying factor is, it does not turn the effects of the low-activity version of the gene upside down. In all the studies, the gene has an aggregate or main effect in the population that could make it a target of selection. Indeed, Moffitt and Caspi (who first discovered that the effect of the gene depends on stressful experiences) suggest that rather than thinking of the low-activity version of the gene as a contributor to violence, we should think of the high-activity version as an inhibitor of violence: it protects people from overreacting to a stressfilled life. Geneticists have discovered statistical evidence of selection for the MAO-A gene in humans, though the evidence does not single out the low- or high-activity variant; nor does it prove that the gene was selected for its effects on aggression.
157
155
It’s hard to pinpoint the exact stressors that have this effect, because stressful lives are often stressful in many ways at once. In fact, the modulating factor may consist of other genes which are shared with an abusive parent, predisposing both parent and child to aggression, and which may also elicit negative reactions from the people around him.
156
But whatever the modifying factor is, it does not turn the effects of the low-activity version of the gene upside down. In all the studies, the gene has an aggregate or main effect in the population that could make it a target of selection. Indeed, Moffitt and Caspi (who first discovered that the effect of the gene depends on stressful experiences) suggest that rather than thinking of the low-activity version of the gene as a contributor to violence, we should think of the high-activity version as an inhibitor of violence: it protects people from overreacting to a stressfilled life. Geneticists have discovered statistical evidence of selection for the MAO-A gene in humans, though the evidence does not single out the low- or high-activity variant; nor does it prove that the gene was selected for its effects on aggression.
157
Other genes that affect dopamine have been associated with delinquency as well, including a version of a gene that affects the density of dopamine receptors (DRD2) and a version of a gene for a dopamine transporter (DAT1) that mops up excess dopamine from the synapse and transports it back into the neurons that release it.
158
Any of them could be fair game for rapid natural selection.
158
Any of them could be fair game for rapid natural selection.
Genetic tendencies toward or away from violence, then, could have been selected during the historical transitions we have examined. The question is, were they? The mere existence of pathways to evolutionary change does not prove that those pathways were taken. Evolution depends not just on the genetic raw material but on factors such as the demography of a population (including both sheer numbers and the degree to which they have absorbed immigrants from other groups), the roll of the genetic and environmental dice, and the dilution of genetic effects by learned adjustments to the cultural milieu.
So is there evidence that the Pacification or Civilizing Process ever rendered the pacified or civilized peoples constitutionally less susceptible to violence? Casual impressions can be misleading. History offers many examples in which one nation has considered another to be peopled by “savages” or “barbarians,” but the impressions were motivated more by racism and observations of differences in societal type than by any attempt to tease apart nature and nurture. Between 1788 and 1868, 168,000 British convicts were sent to penal colonies in Australia, and one might have expected that Australians today would have inherited the obstreperous traits of their founding population. But Australia’s homicide rate is lower than that of the mother country; in fact it is one of the lowest in the world. Before 1945 the Germans had a reputation as the most militaristic people on earth; today they may be the most pacifistic.
What about hard evidence from the revolution in evolutionary genomics? In their manifesto
The 10,000 Year Explosion: How Civilization Accelerated Human Evolution,
the physicist Gregory Cochran and the anthropologist Henry Harpending reviewed the evidence for recent selection in humans and speculated that it includes changes in temperament and behavior. But none of the selected genes they describe has been implicated in behavior; all are restricted to digestion, disease resistance, and skin pigmentation.
159
The 10,000 Year Explosion: How Civilization Accelerated Human Evolution,
the physicist Gregory Cochran and the anthropologist Henry Harpending reviewed the evidence for recent selection in humans and speculated that it includes changes in temperament and behavior. But none of the selected genes they describe has been implicated in behavior; all are restricted to digestion, disease resistance, and skin pigmentation.
159
I know of only two claims of recent evolutionary changes in violence that have at least a soupçon of scientific evidence behind them. One involves the Maori, a Polynesian people who settled in New Zealand about a thousand years ago. Like many nonstate hunter-horticulturalists, the Maori engaged in extensive warfare, including a genocide of the Moriori people in the nearby Chatham Islands. Today their culture preserves many symbols of their warrior past, including the
haka
war dance that fires up New Zealand’s All-Blacks rugby team, and an armamentarium of beautiful greenstone weapons. (I have a lovely battle-ax in my office, a gift of the University of Auckland for giving a set of lectures there.) The acclaimed 1994 film
Once Were Warriors
vividly depicts the crime and domestic violence that currently plague some of the Maori communities in New Zealand.
haka
war dance that fires up New Zealand’s All-Blacks rugby team, and an armamentarium of beautiful greenstone weapons. (I have a lovely battle-ax in my office, a gift of the University of Auckland for giving a set of lectures there.) The acclaimed 1994 film
Once Were Warriors
vividly depicts the crime and domestic violence that currently plague some of the Maori communities in New Zealand.
Against this cultural backdrop, the New Zealand press quickly picked up on a 2005 report showing that the low-activity version of the MAO-A gene is more common among the Maori (70 percent) than among descendants of Europeans (40 percent).
160
The lead geneticist, Rod Lea, suggested that the gene had been selected in the Maori because it made them more accepting of risk during the arduous canoe voyages that brought them to New Zealand, and more effective in their internecine tribal battles thereafter. The press dubbed it the Warrior Gene and speculated that it could help explain the higher levels of social pathology among the Maori in modern New Zealand.
160
The lead geneticist, Rod Lea, suggested that the gene had been selected in the Maori because it made them more accepting of risk during the arduous canoe voyages that brought them to New Zealand, and more effective in their internecine tribal battles thereafter. The press dubbed it the Warrior Gene and speculated that it could help explain the higher levels of social pathology among the Maori in modern New Zealand.
The Warrior Gene theory has not fared well in warfare with skeptical scientists.
161
One problem is that the signatures of selection for the gene could also have been caused by a genetic bottleneck, in which a random assortment of genes that happened to be carried by a few founders of a population was multiplied in their burgeoning descendants. Another is that the low-activity version of the gene is even more common in Chinese men (77 percent of whom carry it), and the Chinese are neither descended from warriors in their recent history nor particularly prone to social pathology in modern societies. A third and related problem is that an association between the gene and aggression has not been found in non-European populations, perhaps because they have evolved other ways of regulating their catecholamine levels.
162
(Genes often act in networks regulated by feedback loops, so in populations in which a particular gene is less effective, other genes may step up their activity to compensate.) For now, the Warrior Gene theory is staggering around with possibly fatal wounds.
161
One problem is that the signatures of selection for the gene could also have been caused by a genetic bottleneck, in which a random assortment of genes that happened to be carried by a few founders of a population was multiplied in their burgeoning descendants. Another is that the low-activity version of the gene is even more common in Chinese men (77 percent of whom carry it), and the Chinese are neither descended from warriors in their recent history nor particularly prone to social pathology in modern societies. A third and related problem is that an association between the gene and aggression has not been found in non-European populations, perhaps because they have evolved other ways of regulating their catecholamine levels.
162
(Genes often act in networks regulated by feedback loops, so in populations in which a particular gene is less effective, other genes may step up their activity to compensate.) For now, the Warrior Gene theory is staggering around with possibly fatal wounds.
The other claim of a recent evolutionary change appeals to a civilizing rather than a pacification process. In
A Farewell to Alms: A Brief Economic History of the World,
Gregory Clark sought to explain the timing and location of the Industrial Revolution, which for the first time in history increased material well-being faster than the increase could be eaten up by population growth (see, for example, figure 4–7, taken from his book). Why, Clark asked, was it England that hosted this onetime escape from the Malthusian trap?
A Farewell to Alms: A Brief Economic History of the World,
Gregory Clark sought to explain the timing and location of the Industrial Revolution, which for the first time in history increased material well-being faster than the increase could be eaten up by population growth (see, for example, figure 4–7, taken from his book). Why, Clark asked, was it England that hosted this onetime escape from the Malthusian trap?
The answer, he suggested, was that the nature of Englishmen had changed. Starting around 1250, when England began to shift from a knightly society to “a nation of shopkeepers” (as Napoleon would sneeringly call it), the wealthier commoners had more surviving children than the poorer ones, presumably because they married younger and could afford better food and cleaner living conditions. Clark calls it “the survival of the richest”: the rich got richer
and
they got children. This upper middle class was also outreproducing the aristocrats, who were splitting heads and chopping off body parts in their tournaments and private wars, as we saw in figure 3–7, also taken from Clark’s data. Since the economy as a whole would not begin to expand until the 19th century, the extra surviving children of the wealthier merchants and tradesmen had nowhere to go on the economic ladder but downward. They continuously replaced the poorer commoners, bringing with them their bourgeois traits of thrift, hard work, self-control, patient future discounting, and avoidance of violence. The population of England literally evolved middle-class values. That in turn positioned them to take advantage of the commercial opportunities opened up by the innovations of the Industrial Revolution. Though Clark occasionally dodges the political correctness police by noting that nonviolence and self-control can be passed from parent to child as cultural habits, in a précis of his book entitled “Genetically Capitalist?” he offers the full-strength version of his thesis:
and
they got children. This upper middle class was also outreproducing the aristocrats, who were splitting heads and chopping off body parts in their tournaments and private wars, as we saw in figure 3–7, also taken from Clark’s data. Since the economy as a whole would not begin to expand until the 19th century, the extra surviving children of the wealthier merchants and tradesmen had nowhere to go on the economic ladder but downward. They continuously replaced the poorer commoners, bringing with them their bourgeois traits of thrift, hard work, self-control, patient future discounting, and avoidance of violence. The population of England literally evolved middle-class values. That in turn positioned them to take advantage of the commercial opportunities opened up by the innovations of the Industrial Revolution. Though Clark occasionally dodges the political correctness police by noting that nonviolence and self-control can be passed from parent to child as cultural habits, in a précis of his book entitled “Genetically Capitalist?” he offers the full-strength version of his thesis:
The highly capitalistic nature of English society by 1800—individualism, low time preference rates, long work hours, high levels of human capital—may thus stem from the nature of the Darwinian struggle in a very stable agrarian society in the long run up to the Industrial Revolution. The triumph of capitalism in the modern world thus may lie as much in our genes as in ideology or rationality.
163
A Farewell to Alms
is filled with illuminating statistics and gripping narratives about the historical precursors to the Industrial Revolution. But the Genetically Capitalist theory has not competed well in the struggle for survival among theories of economic growth.
164
One problem is that until recently, the rich have outreproduced the poor in pretty much
every
society, not just the one that later blasted off in an industrial revolution. Another is that while aristocrats and royals may have had no more legitimate heirs than the bourgeoisie, they more than made up for it in bastards, which could have contributed a disproportionate share of their genes to the next generation. A third is that when institutions change, a nation can vault to spectacular rates of economic growth in the absence of a recent history of selection for middle-class values, such as postwar Japan and post-communist China. And most important, Clark cites no data showing that the English are innately more self-controlled or less violent than the citizenries of countries that did not host an industrial revolution.
is filled with illuminating statistics and gripping narratives about the historical precursors to the Industrial Revolution. But the Genetically Capitalist theory has not competed well in the struggle for survival among theories of economic growth.
164
One problem is that until recently, the rich have outreproduced the poor in pretty much
every
society, not just the one that later blasted off in an industrial revolution. Another is that while aristocrats and royals may have had no more legitimate heirs than the bourgeoisie, they more than made up for it in bastards, which could have contributed a disproportionate share of their genes to the next generation. A third is that when institutions change, a nation can vault to spectacular rates of economic growth in the absence of a recent history of selection for middle-class values, such as postwar Japan and post-communist China. And most important, Clark cites no data showing that the English are innately more self-controlled or less violent than the citizenries of countries that did not host an industrial revolution.
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