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These claims led to the rather dreamy-eyed utopian conclusion that "Just as 'wars begin in the 
minds of men', peace also begins in our minds. The same species who invented war is capable of 
inventing peace. The responsibility lies with each of us." In essence, understanding our biology will not 
contribute to understanding violence and war because we invented war as well as peace, woven out of 
nurture's cloth and her infinite tapestry of cultural potential. These kinds of claims about the role of 
biology in human behavior are at best incoherent, and at worst plain wrong. They are also dangerous 
because they imply a view of human nature that is infinitely plastic, unconstrained by both universal 
features of our biology, as well as individual differences that predispose some to extreme violence and 
others to extreme altruism. 
What makes the Seville Statement, and other claims like it incoherent is a set of false attributions 
to biologists about the role of biology. Statements 2-5 are accurate in that it is incorrect to say that war or 
violence are genetically programmed, subject to stronger selection than other kinds of behaviour, built 
into the brain as a violent brain, and based on instinct with a single, inevitable output. But I don't know 
any biologists who believe statements like these. The biologist Peter Marler famously spoke of singing in 
birds as an instinct to learn, while the evolutionary psychologist Steven Pinker described the Chomskyan 
insight into language as the language instinct. A bird's instinct to learn does not mean that there is a one-
to-one, inflexible mapping between genes or brain circuits and a specific type of song. All songbirds have 
the potential to acquire their species' song, and in some birds, such as mockingbirds and parrots, this 
capacity extends to acquiring the sounds of other animals and even inanimate sounds. But if there is no 
input at all, or if the bird is deafened, the output is deficient in structure, unrecognizable as a species-
specific song. The same holds for the language instinct. Instincts are biological biases that constrain the 
range of potential variation. Biology differentiates songbirds from birds that don't learn their songs. This 
same biology allows some birds to learn one song and use it for life, and allows other birds to acquire a 
variety of different sounds for use in singing. The biology doesn't determine the specific content of a 
song. The content is determined by what the bird hears, constrained by what its bird brain and syrinx will 
process and reproduce. To a large extent, language is no different. Our biology allows us, but not any 
other species, to acquire language. This same biology sets up constraints, due in part to what our brains 
can keep in memory, what our ears can hear, and what our larynx can produce. Like songbirds, the 
specific content of what we say, whether with a French or Vietnamese accent, is determined by where we 
live and who we listen to. 
If there is any intelligible sense of genetically programmed or instinct, whether for violence, 
language, sex, or mathematics, it is that our biology provides us with the capacity to acquire these 
domains of knowledge and expression. This doesn't mean that violence, language, sex or mathematics are 
inevitable or fixed in their expression. There are thousands of languages, ways of having sex, and forms 
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of mathematical expression. There are also thousands of ways of being violent, and equally, ways of 
counteracting such violence. But none of this takes away from the importance of biology, especially its 
role in constraining the form that these expressions take in different environmental settings. To think 
otherwise is just wrong. 
The debate about version 1.5 of lethal aggression gains interest if we restrict the conversation to 
the similarities and differences between chimpanzee and human killing. Similarities speak to our shared 
evolutionary history, including the mechanisms we inherited and the pressures that favored this form of 
violence. Differences speak to both changes in our biology and the environments we confronted and 
created. 
Those who argue that the comparison between human and chimpanzee killing lacks any 
analytical value come from two different camps. On the one hand are anthropologists such as Robert 
Sussman and Brian Ferguson who suggest that chimpanzee killing is infrequent, has little benefit in terms 
of resources or competition, and is restricted to populations that are either artificially provisioned by 
humans or crowded in by us. They also suggest that the archaeological evidence for human warfare 
doesn't really begin until about 12,000 years ago. As Ferguson notes "To argue that war is a result of 
some sort of innate predisposition to wage it requires that war be practiced throughout our prehistoric 
past." This date, so Ferguson continues, is too recent to invoke natural selection as a cause, and leaves 
unexplained why there is no earlier evidence of massive killing if our last common ancestors had this 
capacity. 
These criticisms either fly in the face of contradictory evidence or have little to do with the 
original ideas. Concerning chimpanzee killing, the evidence comes from multiple sites in East and West 
Africa, including sites with no provisioning and no crowding from humans. Further, analyses by 
Wrangham and his colleagues show that humans living as hunter-gatherers or subsistence farmers on the 
continents of Africa and South America, engage in coalitionary killing, using stealthy raids and 
imbalances of power to minimize the costs and maximize the benefits. Looking at 32 different small 
scale societies, calculations of the median death rate were between 164-595 per 100,000 per year. 
Looking at 9 chimpanzee communities spanning 5 populations in Tanzania, Uganda, and Ivory Coast, the 
rate was 69-287 per 100,000 per year. Chimpanzees fall well within the range of human hunter-gatherers 
and subsistence farmers. This evidence not only shows parallels between chimpanzees and human 
societies living under conditions most like our ancestors, but also provides a resounding rejection of the 
view that chimpanzee killing is infrequent and of trivial importance. If the rates of killing are comparable, 
then either they are trivial for both species or trivial for neither. Given that both chimpanzees and human 
hunter-gatherers live in small groups, killing even a few individuals can have a dramatic effect on their 
capacity to defend resources. 
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A further parallel between chimpanzees and small scale human societies comes from analyses of 
two extreme warring societies, the Waorani of New Zealand and the Yanomamo of Venezuela. Though 
violence accounts for between 40-55% of all deaths in these two groups, attackers appeared immune to 
injury, with no more than 5% dying in battle, and often no deaths at all. Chimpanzee attackers are 
likewise immune to injury, due in large part to the strategic use of imbalances of power. 
The parallels between chimpanzees and humans living in small scale societies supports the idea 
that similar pressures favored the capacity for coalitionary killing in both species. Does this mean that 
each of these species should always kill in this way, and thus, as argued by Ferguson, the archaeological 
record should be chock full of deaths by coalitionary attackers? To argue for this position is to 
misunderstand the nature of an adaptation, and the arguments put forth by Wrangham as well as the 
evolutionary psychologists Martin Daly, Margo Wilson, and David Buss. As I discussed earlier on in this 
chapter, adaptations are contingent upon particular environmental circumstances. What is adaptive today 
need not be tomorrow. This is why it is not only unsurprising to see variation in the frequency of 
coalitionary killing among chimpanzee sites, among humans living in small scale societies, and among 
modern day humans who sometimes kill their spouses, stepchildren, and rivals, but predicted by 
evolutionary theory. Adaptations are economically efficient solutions to particular social or ecological 
problems. If those problems or pressures change, the original adaptation may have no impact on survival 
or a negative impact. A hiatus in the archaeological record—assuming this is the last word—is 
interesting with respect to the conditions that might favor or select against coalitionary killing, but in no 
way undermines the logic of an evolutionary adaptation, one shared by chimpanzees and humans. 
The second camp arguing against the parallels between humans and chimpanzees is defended by 
the economist Samuel Bowles and his colleagues. Unlike the anthropologists, Bowles is entirely 
sympathetic to biology but sees fundamental differences in the pattern of human killing and warfare. To 
explain these differences he invokes two important attributes of human societies that have only weak 
parallels in other species: large scale cooperation with unrelated others from the same group, together 
with hatred, symbolic labeling, and the motivation to hurt all others outside the group. These two factors, 
what Bowles calls parochial altruism, may have paradoxically generated both greater levels of 
cooperation within groups and higher rates of warfare between groups. Those groups with the best 
cooperators acquired the greatest resources and experienced the fewest losses due to cheaters and other 
morally corrosive rogues. This power and inward-looking favoritism led to self-defensive emotions and 
behaviors, ultimately leading to lethal aggression toward those with different beliefs and values. Thus 
parochialism and altruism co-evolved, hand in hand, breeding prejudice as a result of group safety. This 
evolutionary handshake resulted in warfare and our unique capacity as killers. 
Bowles' analysis is interesting and consistent with my explanation of how we evolved the 
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capacity for evil. For both Bowles and I, certain aspects of our capacity to harm others emerges as an 
incidental byproduct of other capacities, and once this dynamic emerges, the combination of these 
capacities can evolve and change. What Bowles' analysis misses, however, is the fact that parochial 
altruism could well be true, and so too could our shared capacity for killing with chimpanzees. As noted 
above, rates of killing among chimpanzees and several small scale societies are comparable, and so too 
are the costs and benefits to attackers and victims. This argues in favor of a shared history, and a shared 
adaptation. It does not mean that all aspects of killing in humans are similar, or that the human mind froze 
in a chimpanzee state with regard to its capacity to kill. It most definitely did not freeze. 
Unlike the lethal attacks by chimpanzees that are restricted to cases where groups attack lone 
victims, primarily from neighboring groups, we wreak havoc on a massive scale, with one on one, many 
against many, and one against many, including victims within and outside our core group. Unlike 
chimpanzees, even our young children have an appetite for violence that can be nurtured, as evidenced by 
the brutality of child soldiers around the globe. Unlike chimpanzees, individuals will sacrifice themselves 
for an entire group as evidenced most recently by suicide bombers in the Middle East. Unlike 
chimpanzees, we derive great pleasure from watching others suffer, from watching violent movies, seeing 
other animals fight, and imagining the decimation of an enemy. Our minds also generate ideological 
reasons to motivate violence at extraordinary scales — again, think of suicide bombers taking their lives 
for a God, as well as the reward of an idyllic afterlife. And when our minds break down, or when we are 
afflicted with particular disorders early in life, we are capable of experiencing bizarre appetites for 
violence, including the joy of eating the flesh of murdered victims, having intercourse with dead bodies, 
and asking for bondage and whippings to enhance sexual pleasure. These novel and unanticipated ways 
of harming others are the result of new hardware that has evolved only once in the history of this planet. 
HARMING OTHERS, version 2.0: requires Homo sapiens hardware 
We depart from the pattern of adulticide seen in other animals because of our promiscuous brain. The 
idea is not that our brains evolved for killing in these unique ways, but rather, that our unique style of 
thinking led to novel ways of harming as an incidental consequence. The hardware that is our brain 
enabled new ways of harming others, building on specialized adaptations, some shared with other species 
and some uniquely human. The result is a brain that can develop a peculiar appetite for harming others. 
To see how version 2.0 runs on our distinctively human hardware, let's return to some of the core 
microcontrollers that I discussed a few sections back. Recall that there are hormones like testosterone 
that surge when individuals win a competition, whether this involves the physical fighting of deer using 
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their antlers, humans using their fists, or chess masters using their minds. Along with testosterone's 
increase is an increase in dopamine, a decrease in cortisol and serotonin, and a decrease in frontal lobe 
activity and control. Within the environment of a promiscuous brain, this physiological ballet affects our 
sense of fairness, empathy, moral conscience, attitude toward retribution and justice, as well our 
willingness to engage in lethal aggression. 
Brain imaging studies reveal that the prefrontal cortex plays an essential role in regulating our 
aggressive instincts, when it's working. When individuals respond aggressively to an unfair offer in a 
bargaining game, testosterone levels rise and activity decreases in a part of the prefrontal cortex 
associated with self-control. Thus, testosterone's effectiveness in human aggression is facilitated by a 
loss of control. Damage within this region of the brain causes abnormal aggressive responses to not only 
direct insult, but even such trivial matters as being offered a lowish offer in the ultimatum game discussed 
earlier. Anatomical and functional abnormalities within this region of the frontal lobes are also associated 
with aggressive pathology, such as psychopathology. There are also individual differences in aggressive 
tendencies among healthy people, due in part to differences in the patterns of activity between the right 
and left prefrontal cortices. Heightened activity on the left is associated with greater sensitivity to reward, 
lowered sensitivity to punishment, and considerably stronger aggressive responses to threatening stimuli, 
such as an angry face. This is not simply a correlation, as evidenced by studies that experimentally either 
suppress or increase activity in one hemisphere compared with the other. For example, if you contract 
your right hand you will increase activity in the left hemisphere of the brain; conversely, contracting the 
left hand increases activity in the right hemisphere. Subjects contracting the right hand in an 
experimental setting showed more aggressive responses to insult than did subjects contracting the left 
hand. The next time someone shakes a fist at you, check whether it is the right or left hand. If the person 
is from a different group and holds a fundamentally different suite of ideological beliefs, which hand is 
clenched is the least of your worries. 
Favoritism toward those who are like us, combined with hatred toward those who are not, is 
common in animals. As noted above, in-group favoritism or parochialism can lead to heightened levels of 
cooperation within groups, while simultaneously increasing the level of hostility towards those outside. 
Our promiscuous brain facilitated this co-evolutionary process, inviting the hormone oxytocin into the 
mix. Among mammals, including humans, oxytocin is released in females during labor and breastfeeding, 
and in both males and females during social bonding and parenting. This has led many to think of 
oxytocin as the cuddle hormone or love drug. Floating within the human brain, oxytocin boosts trust in 
games of cooperation, and greatly increases our ethnocentric biases. The Dutch psychologist Carsten De 
Dreu and his colleagues ran a series of experiments that required male subjects to spray oxytocin or a 
control up their noses. When oxytocin shoots up the nose, it goes straight to the brain. Relative to the 
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control group, those who sniffed oxytocin perceived in-group members as more likeable, more human, 
more richly endowed with social emotions such as embarrassment, contempt, humiliation and admiration, 
and more worthy of saving in an emergency. Oxytocin increases our sense of camaraderie toward those 
within the inner sanctum, which can result in greater animosity toward those outside. Oxytocin may 
therefore facilitate our ability to take out the competition even if this means killing another human being. 
Oxytocin is two-faced, cuddling with its left profile and harming with its right. 
This is a small sampling of the ways in which our promiscuous brain enables new forms of harm, 
including killing other adults. We didn't invent lethal aggression. We share this capacity with a small 
group of animals that also kill other adults. But whereas these other species typically restrict their lethal 
attacks to situations in which one group greatly outnumbers another, typically targeting adults from a 
neighboring group, we evolved far beyond this monogamous approach. We adopted the cost-benefit 
analysis that drives killing in other animals and applied it to killing in a virtually limitless space of 
homicidal opportunities. We kill when we outnumber our opponents or are outnumbered by them, 
attacking individuals within and outside our core group. We kill spouses, ex-lovers, stepchildren, those 
who believe in God and those who don't, the wealthy and the poor, kin and non-kin, and even ourselves if 
the cause is good enough. Virtually anything goes. 
Our promiscuous brains opened a Pandora's box of harmful means, including the capacity to 
address a multitude of injustices. This is a capacity that is inherently good, but incidentally bad. It is a 
capacity that evolved in response to growing pressures to balance inequities and take care of those who 
attempt to cheat society. It is a capacity that enabled us to engage in punishment in a broad range of 
contexts, righting wrongs and opening a new path to feeling good about harming others. 
Incidental justice 
Cooperation is ubiquitous in the animal kingdom, occurring in a wide range of situations. 
Humans are no exception. Like ants, humans also bring resources to their queen—think England. Like 
scrub jays, humans work with extended family members to rear the next generation of offspring—think 
Mormons. Like dolphins, human males form super-coalitions to gain access to females—think the 
Yanomami Indians of South America, where men raid neighboring villages to take their women. And like 
chimpanzees, humans cooperate to monitor their borders, often capturing and killing intruders—think 
Palestine and Israel. But human cooperation is distinctive in two ways: we frequently cooperate with 
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large numbers of genetically unrelated strangers, and punish those who cheat by free-riding on others' 
good will. 
The challenge to any cooperative enterprise is to avoid getting suckered by free-riders who cheat 
and add little or no help. As group size grows, the opportunity to cheat and get away with it grows as 
well given the challenges of storing information about reputation. How do individuals and groups avoid 
this sucker problem? 
When nonhuman animals cooperate with members of the same species, they typically target kin. 
Helping kin provides a buffer against the sucker's costs because investing in kin means investing in 
genetic posterity. Helping relatives, even at a cost, translates to helping ones genes move on into the next 
generation—an insight developed by the British evolutionary biologist William D. Hamilton. When 
cooperation involves unrelated others, nonhuman animals attempt to circumvent the sucker's problem by 
working with a small number of familiar others whose reputation is known, targeting contexts where all 
participants benefit more from working cooperatively than working alone. These mutual benefits help 
offset the costs of cooperation. 
By restricting cooperation to relatives or small numbers of unrelated but familiar group members, 
animal societies have buffered themselves from extreme cheating. This is significant because cheaters 
arise in a variety of contexts where there are rules of engagement, including both cooperative and non-
cooperative situations. For example, both lions and chimpanzees cooperate in group defense against 
dangerous neighbors. Some individuals cheat by lagging behind, or failing to join in altogether. In 
societies organized around hierarchies, low ranking animals sometimes cheat by attempting to eat more 
than their fare share or by reproducing when their societal norms explicitly forbid it. Interestingly, 
cheaters in cooperative situations such as those in lions and chimpanzees, never suffer any adverse 
consequences from the dominants. In contrast, cheaters in competitive situations such as those that arise 
in hierarchical societies, are punished. Nonhuman animals thus have the capacity to recognize and 
change rule breakers. And yet, these capacities are not applied in the context of cooperative interactions. 
Monogamous thinking rules. 
When it comes to enforcing fairness in cooperation, the fundamental barrier for animals lies with 
the economics of punishment. Punishment is costly. When a cheater is detected and attacked, there is 
always the possibility that he will retaliate. Cheaters, even when caught, need not surrender without a 
fight. Punishment therefore requires the capacity to surmount an immediate cost, while recognizing the 
possibility that any benefit could be greatly delayed. For example, lashing out against a lion laggard or a 
chimpanzee cheater might cause each to join in on future cooperative ventures, but this is a delayed 
benefit, and it may never materialize. This adds another potential opportunity cost. As Shakespeare so 
deftly noted "Defer no time, delays have dangerous ends." 
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Waiting for a future benefit, whatever its currency, is hard for all animals, humans included. 
Studies of rats, birds, monkeys, apes, human children and adults show that individuals perceive future 
gains as less valuable than immediate, but smaller gains. For example, give any one of these animals a 
choice between one piece of food and ten, and they will pick the ten. More is better than less, except if 
you are a dieting human. Now give them a choice between one piece available immediately and ten pieces 
available after some delay. Virtually every animal shifts to the one piece, with the only comparative 
difference linked to the length of the delay — a few seconds for rats, birds, most monkeys, young 
children, and adults with frontal lobe damage, a few minutes for some apes, older children and some 
adults. Everyone is, to some extent, pulled by the hedonistic now. The future loses its luster when a tasty 
alternative is just within reach. This is in part due to the uncertainty associated with the future, and in 
part, the sheer temptation to take what is in front of us. 
Unique evolutionary changes in the human brain allowed us to exert much greater patience, 
overriding the pull of the hedonistic now. These changes didn't evolve for punishment, but they were 
readily deployed by this system of justice. We rely on creative strategies to place value on the future, 
including putting resources away so that we can't use them —think savings accounts —and making 
verbal commitments that bind us to the future —think about the social embarrassment of failing in front 
of friends. These strategies help diminish the emotional pull of taking what is immediately available, 
allowing future benefits to gain in attractiveness. This is a brain that can wait for the delayed benefits of 
punishment. 
The brain changes that facilitated our capacity to delay gratification were accompanied by others 
that further offset the costs of punishment: our brains reward us with a feel-good feeling when we pay the 
costs of punishment. For the first time in evolutionary history, the physical and psychological costs of 
punishment were at least partially offset by the pleasure of justice served, whether delivered directly or 
witnessed from afar. 
When we punish or get even with those who have acted badly, we feel a hedonic high, an 
experience captured by the metaphor "revenge is sweet but not fattening." As demonstrated by the 
economist Ernst Fehr, this is more than a metaphor. When we hand someone his just deserts, punishing 
someone for cheating, lying, or breaking a promise, our brain responds as if we handed ourselves just 
desserts, activating brain circuitry associated with reward. In one study, two subjects played a monetary 
exchange game in which one player—the donor—decided how much of a pot of money to give to 
another. A third player observed, out of view, the outcome of the exchange. In some cases, observers 
witnessed a fair division of the money and in other cases, an unfair division in which the donor kept a 
disproportionate amount of the total. The observer then faced a difficult decision: leave the two players 
with their take-home earnings or use personal funds to take away money from the donor, returning it to 
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the bank. Taking money away from the donor is a form of costly punishment. It is costly in two ways: the 
punisher loses money he could have kept, and the donor loses money that he unfairly kept in his previous 
exchange. 
When donors kept a significantly larger portion of the original sum, observers punished, paying 
the costs. They also reported feeling good about taking down the cheapskates. Where was this honey hit 
to the brain coming from? 
To figure this out, Fehr and his colleagues put people in a brain scanner and used a technique 
called Positron Emission Tomography or PET. This type of scanning provides a picture of how much 
glucose is used up by different brain areas during a task. Higher glucose consumption occurs when there 
is higher activity in a brain region. When punishers decided to punish a selfish donor, glucose 
consumption increased in a region of the brain associated with reward: the dorsal striatum. This region is 
also active when you eat ice cream, earn money, and solve an unexpected problem. Punishers incurred a 
financial cost, but gained emotional elation and internal reward. It turns out that giving someone his just 
deserts feels like eating dessert, but without the caloric gain. 
When distinctively promiscuous punishment evolved, it transformed our capacity to cooperate 
and to maintain conformity to social norms. It provided us with the tools to not only repair a puncture in 
the system of norms, but to feel good about the costs personally incurred. When we punish, we have 
served justice and served ourselves a helping of the brain's rewards. The fossil record doesn't capture 
when we evolved the capacity to punish, as skulls and bones, stone tools, and even paintings are silent on 
why someone received a spear through the head — perhaps punished for a wrong doing or perhaps an 
enemy, a competitor interested in the same resources, or a suspected lover. No one will ever know. What 
we do know is that other primates never punish cheaters who break the norms of cooperation, whereas 
hunter-gatherers dotting the continents do. Though people living today as hunter-gatherers are not perfect 
replicas of what life was like way back when, they indicate that before we had sophisticated technology, 
agriculture, and permanent residences, we had the capacity to minimize the costs of the sucker's problem. 
Whenever this capacity emerged in human evolution, it provided a critical part of the solution to the 
problem of large-scale cooperation among unrelated strangers. With many eyes on the look out for 
cheaters, and a capacity to take out or ostracize the free riders, cooperation emerged as a stable solution to 
problems that are unsolvable at an individual level, including group defense and the acquisition of costly 
resources. Punishment enabled humans to live in large, stable cooperative societies, many of whom are 
unrelated strangers. 
This momentous event in the history of cooperation carried with it a serious cost, bringing 
aggression and reward into closer proximity, with the costs of attacking others overshadowed by the 
benefits. As I discuss in greater detail in the next chapter, this economic transformation created, as an 
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incidental consequence, a hunger to watch violence and to see it as entertainment. It allowed our feelings 
of inequity and envy to morph into schadenfreude, retaliation, and spite. It allowed us to enjoy violence 
as perpetrators and spectators. It allowed us to put our money on feeling good about righting an injustice. 
Why oh why? 
Why did evil, expressed as excessive harm to innocent others, evolve? The answer lies, so I suggest, in a 
special property of the human brain. Some time after we diverged from a chimpanzee-like common 
ancestor, the human brain was remodeled to allow promiscuous connections between previously 
unconnected circuits. Promiscuity enabled us to explore new problems using a combination of older, but 
nonetheless adaptive parts. Some of these novel explorations led to highly adaptive consequences, as 
when we developed the ability to self-deceive in the service of pumping ourselves up to do better in the 
context of competition; or when we invented new technologies to solve difficult environmental problems, 
such as using spears to capture prey at a distance; or, when we acquired the know-how to stockpile and 
enhance resources such as food, water and fertile land that are critical to individual survival and 
reproduction; or when we evolved the richly textured social emotions of jealousy, shame, guilt, elation, 
and empathy, feelings that motivate individuals to recognize the importance of others' well-being and 
interests and to correct prior wrongs; or, when we tapped into the rich connection between reward and 
aggression to punish cheaters trying to destabilize a cooperative society. But these same adaptive 
explorations also resulted in incidental costs that have destroyed the lives of innocent individuals. The 
capacity to deny others' moral worth enabled us to justify great harms, including self-sacrifice as living 
bombs designed to annihilate thousands of non-believers. The capacity to create advanced weaponry 
enabled us to kill at a distance, thereby avoiding the aversiveness of taking out those staring back. The 
capacity to stockpile resources led to the growth of greed, increasing disparities among members of 
society, the inspiration to steal, and heightened violence both to defend and to obtain. The capacity to 
experience social emotions such as jealousy led to blind rage and a driving engine of homicide, including 
cuckolded lovers who kill their spouses and stepparents who kill their stepchildren. The capacity to feel 
good about harming others enabled us to recruit this elixir in the service of causing excessive harm in any 
number of novel contexts, from ethnic cleanings to bizarre fetishes that include self-mutilation. And the 
list goes on. This is the yin and yang of promiscuous thinking. This is the natural history of evil, its 
ancestry and adaptive significance. This evolutionary explanation sets the stage for unpacking the recipe 
for evil, how it develops within individuals and societies, ingredient by ingredient. 
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Endnotes: Chapter 1 
Recommended books 
Bloom, P. (2010). How Pleasure Works. New York, Norton Press. 
Coyne, J. A. (2009). Why Evolution is True. New York, Viking Press. 
French, P. (2001). The Virtues of Vengeance. University of Kansas Press. 
Goldhagen, D.J. (2009). Worse than War. New York, Public Affairs. 
Kekes, J. (2005) The Roots of Evil. Ithaca, Cornell University Press. 
Kiernan, B. (2007). Blood and Soil: A World History of Genocide and Extermination from Sparta to 
Darfur. New Haven, Yale University Press. 
McCullough, M.E. (2008). Beyond Revenge. John Wiley & Sons. 
Pinker, S. (2011) The Better Angels of Our Nature. New York, Viking Press 
Wrangham, R.W., & Peterson, D. (1996). Demonic Males: Apes and the Origins of Human Violence. 
Boston, Houghton-Mifflin. 
Notes: 
• Smarter and more painful mice: Wei, F., Wang, G.-D., Kerchner, G. A., Kim, S. J., Xu, H.-M., Chen, 
Z.-F., & Zhuo, M. (2001). Genetic enhancement of inflammatory pain by forebrain NR2B 
overexpression. Nature, 4, 2; Tang, Y.-P., Shimizu, E., Dube, G. R., Rampon, C., Kerchner, G. 
A., Zhuo, M., & Tsien, J. Z. (1999). Genetic enhancement of learning and memory in mice. 
Nature, 401: 63-69. 
• Insects that play leaf: Wedmann, S., Bradler, S. & Rust, J. (2007). The first fossil leaf insect: 47 million 
years of specialized cryptic morphology and behavior., Proceedings of the National Academy of 
Sciences 104(2): 565-569. 
• Evolving tameness: Trut, L. N. (1999). Early canid domestication: the farm-fox experiment. American 
Scientist, 87, 160-169; Hare, B., Plyusnina, I., Ignacio, N., Schepina, O., Stepika, A., Wrangham, 
R. W., & Trut, L. N. (2005). Social cognitive evolution in captive foxes is a correlated by-product 
of experimental domestication. Current Biology, 15(3), 226-230; Udell, M., Dorey, N., & Wynne, 
C. (2009). What did domestication do to dogs? A new account of dogs' sensitivity to human 
actions Biological Reviews, 85(2): 327-345; Galibert, F., Quignon, P., Hitte, C., & Andre, C. 
(2011). Toward understanding dog evolutionary and domestication history., Current Reviews in 
Biology 334(3), 190-196; Careau,V., Reale, D., Humphries, M.M., & Thomas, D.W. (2010). The 
pace of life under artificial selection: personality, energy expenditure, and longevity are 
correlated in domestic dogs. American Naturalist 175(6), 753-758. 
• What's religion for? Boyer, P. (2001). Religion explained: The evolutionary origins of religious thought. 
New York, Basic Books; Wilson, D.S. (2002). Darwin's Cathedral. Chicago: University of 
Chicago Press; Sosis, R., & Bressler, E.R. (2003). Cooperation and commune longevity: a test of 
the costly signaling theory of religion. Cross-cultural Research, 37(2), 211-239; Norenzayan, A, 
& Shariff, A. F. (2008). The origin and evolution of religious prosociality. Science, 322(5898), 
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58-62; Banerjee, K., Huebner, B, & Hauser, M.D. (2010). Intuitive Moral Judgments are Robust 
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Peeters, R, Simone, L, Nelissen, K, Fabbri-Destro, M, Vanduffel, W, Rizzolatti, G, & Orban, G 
A. (2009). The representation of tool use in humans and monkeys: common and uniquely human 
features. Journal of Neuroscience, 29(37), 11523-11539; Preuss, Todd M, Caceres, Mario, 
Oldham, Michael C, & Geschwind, Daniel H. (2004). Human brain evolution: insights from 
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Preuss, T., Faber, T., Pagnoni, G., Votaw, J. (2007). A comparison of resting state brain activity 
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being: The foundations of hedonic psychology. Eds., D. Kahnemann, E. Diener & N. Schwarz. 
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den Heuvel, Martijn P, Mandl, Rene C W, Kahn, Rene S, & Hulshoff Pol, Hilleke E. (2009). 
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architecture of the human brain. Human Brain Mapping, 30(10), 3127-3141. 
• Evil wasps, monogamous thinking, and Darwin's worry: Haspel, G., & Libersat, F. (2004). Wasp 
manipulates cockroach behavior by injecting venom cocktail into prey central nervous system. 
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(2006). Host-handling behavior: an innate component of foraging behavior in the parastaoid wasp 
Ampulex compressa. Ethology, 112(7), 699-706; Wedman, S., Bradler, S., & Rust, J. (2007). The 
first fossil leaf insect: 47 million years of specialized cryptic morphology and behavior. 
Proceedings of the National Academy of Sciences, 104(2), 565-569; Harvey, J.A., Brezemer, 
T.M., Gols, R., Nakamatsu, Y., & Tanaka, T. (2008). Comparing the physiological effects and 
function of larval feeding in closely-related endoparasitoids (Braconidae: Microgastrinae). 
Physiological Entomology, 33(3), 217-225; Polidori, C., Matteo, F., Pesarini, C., & Francesco, A. 
(2007). Factors affecting spider prey selection by Sceliphron mud-dauber wasps (Hymenoptera: 
Sphecidae) in northern Italy. Animal Biology, 57(1), 11-28. 
• Fighting but not killing: Bradbury , J. & Vehrencamp, S. (1998) Principles of Animal Communication. 
Sinauer Associates; Espmark, Y., Amundsen, T., & Rosenqvist,G. (2000). Animal Signals. Tapir 
Academic Press; Hauser, M.D. (1996) The Evolution of Communication. MIT Press; Krebs, J., & 
Dawkins, R. (1984). Animal signals: mind-reading and manipulation. In: Behavioural ecology, 
pp. 380-402, Sinauer Press; Maynard Smith, J. & Harper, D. (2003) Animal signals. Oxford 
University Press; Searcy, W.A. & Nowicki, S. (2010). The Evolution of Animal Communication. 
Princeton University Press; Zahavi, A. & Zahavi, A. (1999). The Handicap Principle. Oxford 
University Press. 
• An elite group of killers: Beckerman, S., Erickson, P., Yost, J., Regalado, J., Jaramillo, L., Sparks, C., 
Iromenga, M.(2009). Life histories, blood revenge, and reproductive success among the Waorani 
of Ecuador. Proceedings of the National Academy of Sciences, 106(20), 8134-8139; Durrant, R. 
(2011). Collective violence: An evolutionary perspective. Aggression and Violent Behavior, 
16(5), 428-436.; Durrant, R., & Ward, T. (2011). Evolutionary explanations in the social and 
behavioral sciences: Introduction and overview. Aggression and Violent Behavior, 16(5), 361-
370. Elsevier Ltd.; Ferguson, R. B. (2006). Tribal, "Ethnic," and Global. In: The Psychology of 
Resolving Global Conflict: From war to peace, volume 1, Nature vs Nurture (Ed. M. Fitzduff, 
C.E. Stout; Lawrence Erlbaum Ass), 1-15; Ferguson, R. B. (2011). Tribal Warfare. In: The 
Encyclopedia of War (ed. G. Martel, Blackwell Publ, Oxford, pp. 1-13; Heinze, J., & Weber, M. 
(2010). Lethal sibling rivalry for nest inheritance among virgin ant queens. Journal of Ethology, 
29(1), 197-201; Kelly, R. C. (2005). The evolution of lethal intergroup violence Proceedings of 
the National Academy of Sciences of the United States of America, 102(43), 15294-15298; 
Roscoe, P. (2007). Intelligence, Coalitional Killing, and the Antecedents of War. American 
Anthropologist, 109(3), 485-495; Sussman, R.W. 1999. The myth of man the hunter, man the 
killer and the evolution of human morality (evolutionary and religious perspectives on morality). 
Zygon 34: 453-472.Tobena, A. (2011). Suicide Attack Martyrdoms: Temperament and Mindset 
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Watts, D. P., Muller, M., Amsler, S. J., Mbabazi, G., & Mitani, J. C. (2006). Lethal intergroup 
aggression by chimpanzees in Kibale National Park, Uganda. American journal of primatology, 
68(2), 161-180; Wrangham, R. (1999). Evolution of coalitionary killing. Yearbook of Physical 
Anthropology 42: 1-30; Wrangham, R. W. (2011). Chimpanzee violence is a serious topic: A 
response to Sussman and Marshak's Critique of Demonic Males: Apes and the Origins of Human 
Violence. (Vol. 1, pp. 29-50). Global Nonkilling Working Papers; Wrangham, R. W., & 
Glowacki, L. (2011). Intergroup aggression in chimpanzees and war in nomadic hunter-gatherers: 
evaluating the chimpanzee model Human Nature, pp. 1-53; Wrangham, R. W., & Wilson, M. L. 
(2006). Collective Violence: Comparisons between Youths and Chimpanzees. Annals of the New 
York Academy of Sciences, 1036(I), 233-256; Wrangham„ R. W., Wilson, M. L., & Muller, M. 
N. (2005). Comparative rates of violence in chimpanzees and humans. Primates, 47(1), 14-26. 
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• The evolution of cooperation and punishment in animals: Clutton-Brock, T.H (2009). Cooperation 
between non-kin in animal societies. Nature, 462, 51-57; Clutton-Brock, T.H., & Parker, G.A. 
(1995). Punishment in animal societies. Nature, 373, 209-216; Hauser, M.D. (2006). Moral 
Minds. Harper Collins; Jensen, K. (2010). Punishment and spite, the dark side of cooperation. 
Philosophical Transactions of the Royal Society B: Biological Sciences, 365(1553), 2635-2650; 
Stevens, J.R, & Hauser, M.D. (2004). Why be nice? Psychological constraints on the evolution of 
cooperation. Trends in Cognitive Sciences, 8(2), 60-65. 
• Cooperation, punishment, and feeling good, see: Boyd, R, Gintis, H, & Bowles, S. (2010). Coordinated 
punishment of defectors sustains cooperation and can proliferate when rare. Science, 328(5978), 
617-620; Boyd, R., Gintis, H., Bowles, S., & Richerson, P.J. (2003). The evolution of altruistic 
punishment. Proceedings of the National Academy of Sciences, USA, 100, 3531-3535; Boyd, R., 
& Richerson, P.J. (1992). Punishment allows the evolution of cooperation (or anything else) in 
sizeable groups. Ethology and Sociobiology, 113, 171-195; Carlsmith, K. M., Wilson, T. D., & 
Gilbert, D. T. (2008). The paradoxical consequences of revenge. Journal of Personality and 
Social Psychology, 95 (6), 1316-1324; de Quervain, D.J-F., Fischbacher, U, Treyer, V., 
Schellhammer, M., Schnyder, U., Buck, A., & Fehr, E. (2004). The neural basis of altruistic 
punishment. Science, 305, 1254-1258; Fehr, E., & Gachter, S. (2002). Altruistic punishment in 
humans. Nature, 415, 137-140; Gachter, S, Renner, E, & Sefton, M. (2008). The long-run 
benefits of punishment. Science, 322(5907), 1510; Henrich, J., & Boyd, R. (2001). Why people 
punish defectors weak conformist transmission can stabilize costly enforcement of norms in 
cooperative dilemmas. Journal of Theoretical Biology, 208(1), 79-89; Henrich, J., Ensminger, J., 
McElreath, R., Barr, A., Barrett, C., Bolyantz, A., Ziker, J. (2010). Markets, religion, community 
size, and the evolution of fairness and punishment. Science, 327(5972), 1480-1484; Henrich, J., 
McElreath, R., Barr, A., Ensminger, J., Barrett, C., Bolyantz, A., Ziker, J. (2006). Costly 
punishment across human societies. Science, 312, 1767-1770; Herrmann, B, Thoni, C, & 
Gachter, S. (2008). Antisocial punishment across societies. Science, 319(5868), 1362-1367; 
Rockenbach, B., & Milinski, M. (2006). The efficient interaction of indirect reciprocity and 
costly punishment. Nature, 444(7120), 718-723; Sigmund, K., Hauert, C., & Nowak, M. (2001). 
Reward and punishment. Proceedings of the National Academy of Sciences, 98(19), 10757-
10762; Trivers, R.L. (1971). The evolution of reciprocal altruism. Quarterly Review of Biology, 
46, 35-57. 
Quotes 
Watts, D. P., Muller, M., Amsler, S. J., Mbabazi, G., & Mitani, J. C. (2006). Lethal intergroup aggression 
by chimpanzees in Kibale National Park, Uganda. American journal of primatology, 68(2), 161-180; 
quote: p.g., 166 
Seville Statement on Violence, Spain, 1986: http://portaLunesco.orgieducationtenlev.php-
URIJD=3247&URL_DO=DO_TOPIC&URL_SECTION=201.html 
Ferguson, R. B. (2006). Tribal, "Ethnic," and Global. In: The Psychology of Resolving Global Conflict: 
From war to peace, volume 1, Nature vs Nurture (Ed. M. Fitzduff, C.E. Stout; Lawrence Erlbaum Ass), 
1-15.; quote: p.g. 45 
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Chapter 2: 
Runaway desire 
The desire of being believed, or the desire of persuading, of leading and 
directing other people, seems to be one of the strongest of all of our desires. 
—Adam Smith 
In 1999, an investment officer in a management firm had a gut feeling that something was wrong in a 
corner of the securities market. Based on calculations from the stated investment strategy for the fund, the 
returns were not only orbiting outside the financial stratosphere, but were mathematically impossible. The 
officer contacted the Securities and Exchange Commission, outlined the problem, and asked them to look 
into it. No reply. Year after year, the officer continued to contact the SEC about this case, explaining that 
it was potentially lethal, that he had no personal investments in the fund, and had never been an 
employee. No reply. Then, in 2007, he sent the SEC a 17-page report, showing why there was only one 
plausible conclusion: the stated strategy for the fund was a fraudulent cover up for a massive money-
making scheme. Eventually, in 2008, the brains behind this scheme was ousted, escorted to a life in prison 
as number 61727-054, and welcomed to his new home by a community of like-minded white-collar 
criminals. 
Meet HARRY MARKOPOLOS — the investment officer — MADOFF SECURITIES — the seductive 
investment opportunity— and BERNARD MADOFF— the genius behind one of the most spectacular 
Ponzi schemes in recorded history. Whistleblowers started warning the SEC as early as 1992, but no one 
listened. Madoff was making money hand over fist by pocketing new investments and if needed, using 
some of these to pay off individuals wishing to redeem their funds. Ponzi schemes work as long as new 
investments exceed the number of investors wishing to redeem their own investments. And for Madoff, 
this balancing act worked for 16 years. Then, as in the Bible's Book of Joshua, the walls came tumbling 
down, with Jericho riding in to hand Madoff a 150-year prison term for the financial murder of his 
trusting clients. Altogether, these innocent and trusting people lost approximately $65 billion dollars, all 
because one man allowed his desire for wealth and power to run out of control. Or so it seemed. 
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Were those who put their money in Madoff securities entirely innocent? Or, like Madoff, did 
they too allow their desire for more wealth to run wild? Was Madoff's desire for extreme wealth that far 
off the curve of human variation? And if he was off, who or what do we blame? Could some quirky 
feature of Madoff's genome, together with the brain states it orchestrates, have pushed him over the edge 
into a universe of unreasonable and even irrational desires? Does it matter that Madoff grew up in a world 
in which the rich get richer despite the direct or indirect harm caused to innocent others? 
Many have speculated answers to these questions, based on little or no evidence, and certainly no 
scientific evidence. But what matters most about case studies such as this is that they raise the kinds of 
questions that science can answer. What matters is that it is possible to run a Ponzi scheme. It is possible 
because there are humans like Madoff who are driven by the desire to accumulate great wealth, despite 
the personal risks and costs to others. It is possible because the world is populated by people who are 
willing to throw critical reasoning to the wind in the face of a tempting offer to make a huge amount of 
money. It is possible because people will take risks either without considering the potentially horrific 
consequences to innocent others, or by trivializing them. 
As far as we know, Madoff never intended to put his friends and family members in a state of 
financial ruin. As far as we know, many invested in his securities knowing that investments can fail. As 
far as we know, some must have been suspicious, at least for a while, about how their investments could 
consistently yield such over the top returns when nothing else has or seemingly could. Madoff is certainly 
to blame for creating a fraudulent investment opportunity, but so too are the many who trusted him 
without question, happy to make absurdly high returns. 
Did Madoff cause excessive harm? Yes. Were those harmed innocent victims? Not entirely. 
None were forced to invest, and all invested with at least some knowledge that the promised returns were 
off the charts. This is not innocence. This is desire run amuck while self-control and reason fly standby. 
Madoff was morally wrong, but not evil, at least not on my accounting of the ingredients of evil. 
Madoff's case illustrates the power of desire to stampede reason. As the essayist James Thurber 
remarked, "Love is blind, but desire just doesn't give a good goddamn." Madoff didn't give a good 
goddamn. How does a system like this get going, and then sometimes go wrong, very wrong? It all starts 
with the elements of pleasure. 
P for pleasure 
Imagine that scientists have just announced the discovery of a center in the brain that manages our 
experience of pleasure. Imagine further that they have invented a consumer device that, for only $49.99, 
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enables you to ramp up or down the activity in the pleasure center. Want to get a bit more out of your 
dinner, a movie, tennis stroke, work, or sex? Flip the switch. Want to buffer yourself from the pain of 
ostracism, a romantic break up, or a colonoscopy? Flip the switch. Would you buy it? If so, what would 
you use it for, and would you be a habitual user? Would you worry about any side effects? Might using 
this device become addictive, or worse, either destroy the feeling of pleasure altogether or push you into a 
never-ending quest for satisfaction, with each dollop of pleasure leaving you wanting a bigger dollop next 
time around. This may seem like science fiction, but it's closer to non-fiction. 
Over fifty years ago, scientists working with rats, implanted electrodes into in a region of the 
brain called the nucleus accumbens. The electrodes were connected to a switch. If the rat pressed the 
switch, it activated the electrode and thus stimulated this brain area. The rats indeed pressed, over and 
over again, some at a rate of 2000 presses per hour, with no external reward or threat of punishment. 
Pressing the switch was the reward, or at least the vehicle to what appeared to be the experience of 
reward. Pressing the switch was addictive. These scientists had discovered a critical part of the circuitry 
of pleasure, in rats! The rats discovered a pleasure switch, something they wanted to experience over and 
over and over again. 
Soon after this discovery, clinicians started using deep brain stimulation to treat individuals with 
neurological complications, including Parkinson's patients suffering from loss of motor control, patients 
experiencing sustained pain, Tourette's patients suffering from motor tics and obsessive-compulsive 
problems, and even a patient in a coma who had lost, but then slowly recovered the capacity to name and 
grasp objects. Similar to the rat work, the technique involves implanting an electrical pulse generator 
within a targeted brain region. When the generator is turned on, it stimulates activity in previously 
malfunctioning regions. But as with genetic manipulations and the unknown space that characterizes the 
genomic universe, so too is the situation unchartered in the neuronal universe. Two patients suffering 
from chronic pain were implanted with impulse generators and subsequently developed profound 
addictions to the stimulation. In addition to relatively successful pain reduction, both patients experienced 
an enhanced desire for sex, including erotic feelings. One of these patients self-stimulated so often that 
she forgot to wash, change clothes, and adhere to family commitments. What happened? Did the 
clinicians treating these patients hit the pleasure center? When stimulated, what actually changed in the 
minds of these patients? Did they simply want more sexual arousal, suggesting that stimulation turned up 
the gain on their desire for sex? Or, did stimulation change what they like, a shift in the sense of pleasure 
that accompanies sex, and in this case, the anticipation of sex? Or perhaps it changed both wanting and 
liking, especially since we often want things we like? 
These observations of how humans and rats respond to brain stimulation show that particular 
areas of the brain are linked to pleasure, especially the motivation to obtain rewarding experiences. These 
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observations also reveal that when these areas are activated, they can result in addictive, uncontrollable 
behaviors that are toxic to self and others. They suggest that areas of the brain associated with desire can 
run out of control. But to understand how the brain motivates us to want some things but not others, how 
it creates the experience of liking, and how it enables us to want things we like by learning about the 
world, we must turn to experiments on nonhuman animals, brain scans of healthy humans, the mechanics 
of mind-altering drugs, conscious and unconscious influences on our choices, and the sad stories of 
individuals with uncontrollable urges to eat, drink, snort, shoot up, and gamble. This is the evidence that 
scientists, especially the American cognitive neuroscientist Kent Berridge, have gathered to explain the 
three core elements of pleasure: wanting, liking, and learning. 
Often, but not always, the experiences we want or desire are the experiences we like. Often, but 
not always, the experiences we like are rewarding and good for our health. Often, but not always, we are 
aware of the experiences we like, and make rational plans to experience them again. Often, but not 
always, our rational plans to experience what we like makes us happy. Often, but not always, we find 
ways to maintain our happiness by using self-control to moderate our exposure to pleasurable 
experiences. The often refrain refers to the fact that the systems involved in wanting, liking, and learning 
work in this way most of the time. The but not always refrain is a tip off to exceptions that speak to 
interesting aspects of the machinery. For example, though we most often want things we like, we can 
want things we don't like and like things we don't want. I want to lift weights, do push-ups and sit-ups to 
stay fit, but I don't really like doing any of these exercises. I like the Porsche Boxster sportscar, but don't 
want one because it is a gas guzzling environmental nightmare and inconveniently small for a family car. 
The fact that wanting and liking can mount a unified front or conflict with each other shows that it is 
important to look at the glue between them, which is learning. Learning allows us to work out our values, 
setting up a preference profile for what we desire in the people we interact with, the places we visit, the 
objects we handle, and the events we experience. Learning allows us to predict situations that generate 
pleasure or pain. Though we are conscious of many of these processes, and can actively influence them, 
unconscious processes are also at work. These systems of the brain link us to our evolutionary past, and to 
animals without language but clearly expressed likes and dislikes. 
Evolution has endowed all animals with unconscious wanting systems—brain circuitry that 
motivates individuals to seek resources that enable survival and reproduction. For young mammals, still 
dependent upon parental care, their desires are simple: milk and warmth from their mothers, and 
depending on the species, additional warmth and protection from fathers; for the record, the human father 
is an oddity among primates, as no other ape and virtually no other monkeys express a paternal instinct. 
With growth and independence, desires shift to other resources that can satisfy hunger and thirst, 
accompanied by sexual and social status desires. Though driven by unconscious operations, the behaviors 
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that result are often strategic, dependent upon changes in climate, other competitors, who happens to be in 
a bad mood, and who is sexually active. In a variety of species, from dung beetles to deer, biologists have 
developed mathematical models that accurately predict how long an individual should wait for a sexual 
partner or feed in a food patch. The accuracy of these models shows that individuals' desires for resources 
linked to survival and reproduction are captured by lawful principles or rules. This is important because it 
means we understand how the machine underlying behavior works. It means we understand how animals 
make certain choices. 
Understanding how wanting works is straightforward. In both humans and nonhuman animals, 
we can measure what individuals approach when we give them a choice, as well as how much effort they 
are willing to exert while approaching and gaining access to a particular object or experience. For 
example, in studies that explore whether captive animals are provided with sufficient housing conditions, 
an experimenter presents individuals with a choice of rooms, one consisting of the typical housing 
environment and the others by the addition of goods believed to be of interest. To enter a given room 
requires opening a door. To determine how much an individual really wants what is in another room, the 
experimenter ramped up the difficulty of opening each door. In studies of captive hens and mongoose, 
individuals exerted considerable effort to open some doors but not others. Hens rammed into doors 
opening onto a chipped wood floor, whereas mongoose did the same for a pool of water. These are items 
they want, but do not get in captivity. 
What about liking? It may seem, at first blush, that because liking is a subjective experience, that 
there are no clear objective ways to measure it. My likes are my own. You can't possibly know what it 
is like to be me. If you can't know what it is like to be me, then we can't possibly know what it is like to 
be a mongoose, mouse or monkey. There are, however, ways of measuring liking and disliking that are 
reliable, objective, and consistent across species. In many animals, including human babies who can't 
speak and human adults who have lost this capacity due to brain injury, there are distinctive behaviors 
that are consistently linked to positive experiences and others linked to negative ones. For example, in 
mice, monkeys, and human babies, tasting something sweet like sugar causes a lot of lip licking, whereas 
tasting something bitter such as quinine causes mouth gaping, nose twitching, and arm flailing. These 
similarities show that evolution has been conservative, maintaining the same underlying mechanisms for 
handling likes and dislikes. These similarities have enabled scientists to understand how the brain systems 
involved in wanting and liking can change together or separately, even though they can't help us 
understand the harder problem of what, in particular, it is like for a given mouse, monkey or man to like 
something. 
To understand the machinery that drives rodent wants or desire, Susana Pecifia and Kent Berridge 
took advantage of the genetic technique that Joe Tsien —mentioned in the last chapter— used to create 
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smart mice. Recall that Tsien jazzed up a gene's expression to improve memory and learning. In 
contrast, Pecifia and Berridge quieted a gene that controls the amount of dopamine floating around in 
between neurons. With this gene silenced, dopamine levels increased. Compared with normal mice, these 
dopamine-plus mice consumed twice as much food and water, and learned much faster where food was 
located within a maze. But when it came to measuring licking as liking, the dopamine-plus mice were no 
different from normal mice. Dopamine is therefore essential for the wanting system, but not the liking 
system. This conclusion has been supported by many other studies, of mice and men, in the context of 
eating and drug addiction —two topics that I will shortly revisit. 
To understand what rodents like, Pecifia and Berridge injected an opioid drug —similar to opium 
from poppy plants—into two brain regions associated with reward —the nucleus accumbens and the 
ventral pallidum. Not only have studies of rodents, monkeys, and humans revealed that these areas are 
associated with reward —recall the brain stimulation studies — but they contain sub-regions known as 
hedonic hotspots— zones tuned to particular kinds of stimulation, designed to jazz up the liking element 
of pleasure. Following injection, individuals licked four times more often in response to sugar as the non-
injected individuals, but did not show a difference in wanting. The opioid injections also caused a 
decrease in the aversiveness of bitter quinine, as evidenced by a decrease in mouth gaping. Turning on 
these hedonic hotspot ramped up the pleasure of sweets, and diminished the displeasure from bitters. 
Together, the Pecina and Berridge studies highlight the independence of wanting and liking, and the ways 
in which the brain — or a clever experimenter playing with it —regulates the elements of pleasure. 
How does the brain figure out what's hot and what's not, delicious or disgusting? It's one thing 
to desire a particular experience, and another to derive pleasure from the experience. But the world is not 
set up with labels that indicate which objects and events are pleasurable and which distasteful. Every 
object and event has particular properties that, depending on the animal's sensory ability, can be seen, 
heard, tasted, smelled, or touched. For all organisms, there are receptors within each of the sensory 
modalities that are biased to prefer some things over others. This is why no human baby has to be taught 
to dislike bitter things and like sweet things. From the very first encounter, sugary solutions trigger 
tongue protrusions and licking, whereas bitter solutions trigger a gaping mouth. We have evolved, as have 
other animals, sensory systems that are tuned to prefer some things and dislike others, right from the start. 
These initial biases guide the learning process, facilitating acquisition of new knowledge in some cases, 
making it almost impossible in others, and setting a course to self-destruction in yet others. Try teaching 
a young child that the taste of sweet chocolate is disgusting while the taste of bitter endives is delicious. 
Try teaching cocaine addicts to turn off the magnetic pull of white powder, or convincing alcoholics that 
the clinking sound of ice in a glass isn't meaningful. Try teaching rogue soldiers involved in genocide 
Hauser 
Chapter 2. Runaway desire 
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