Evolutionary psychology is the attempt to understand our mental faculties in light of the evolutionary processes that shaped them. It often investigates the adaptive functions of cognitive and emotional systems - how natural selection "engineered" them to solve the kinds of problems faced by our ancestors in their struggle to survive and reproduce.


Natural selection operates over thousands of generations. For ninety-nine percent of human existence, people lived as foragers in small nomadic bands. Our brains are adapted to that long-vanished way of life, not to brand new agricultural and industrial civilizations. They are not wired to cope with anonymous crowds, schooling, written language, government, police, courts, armies, modern medicine, formal social instiutions, high technology, and other newcomers to the human experience. Since the modern mind is adapted to the Stone Age, not the computer age, there is no need to strain for adaptive explanations of everything we do. Our ancestral environment lacked the instiutions that now entice us to nonadaptive choices, such as religoius orders, adoption agencies, and pharmaceutical companies, so until very recently there was never a selection pressure to resist the enticements. Had the Pleistocene savanna contained trees bearing birth-control pills, we might have evolved to find them as terrifying as a venomous spider.

Fears in modern-city dwellers protect us from dangers that no longer exist, and fail to protect us from dangers in the world around us. We ought to be afraid of guns, driving fast, riving without a seatbelt, lighter fluid, and hair dryers near bathtubs, not of snakes and spiders. Public safety officials try to strike fear in the hearts of citizens using everything from statistics to shocking photographs, usually to no avail. Parents scream and punish to deter their children from playing with matches or chasing a ball into the street, but when Chicago schoolchildren were asked what they were most afraid of, they cited lions, tigers and snakes, unlikely hazards in the Windy City.

Natural selection is not the only process that changes organisms over time. But is the only process that seemingly designs organisms over time.


This book... is about the human mind. I will try to explain what the mind is, where it came from, and how it lets us see, think, feel, interact, and pursue higher callings like art, religion, and philosophy. On the way I will try to throw light on distinctively human quirks. Why do memories fade? How does makeup change the look of a face? Where do ethnic stereotypes come from, and when are they irrational? Why do people lose their tempers? What makes children bratty? Why do fools fall in love? What makes us laugh? And why do people believe in ghosts and spirits?

The mind, like the Apollo spacecraft, is designed to solve many engineering problems, and thus is packed with high-tech systems each contrived to overcome its own obstacles.

Let's take a look at an everyday miracle: getting a body from place to place. When we want a machine to move, we put it on wheels. The invention of the wheel is often held up as the proudest accomplishment of civilization. Many textbooks point out that no animal has evolved wheels and cite the fact as an example of how evolution is often incapable of finding the optimal solution to an engineering problem. But it is not a good example at all.
Even if nature could have evolved a moose on wheels, it surely would have opted not to. Wheels are good only in a world with roads and rails. They bog down in any terrain that is soft, slippery, steep, or uneven. Legs are better. Wheels have to roll along an unbroken supporting ridge, but legs can be placed on a series of separate footholds, an extreme example being a ladder. Legs can also be placed to minimize lurching and to step over obstacles.
Even today, when it seems as if the world has become a parking lot, only about half of the earth's land is accessible to vehicles with wheels or tracks, but most of the earth's land is accessible to vehicles with feet: animals, the vehicles designed by natural selection.

Knowing who is a bachelor is just common sense, but there's nothing common about common sense. Somehow it must find its way into a human or robot brain. And common sense is not simply an almanac about life that can be dictated by a teacher or downloaded like an enormous database. No database could list all the facts we tacitly know, and no one ever taught them to us.
You know that when Irving puts the dog in the car, it is no longer in the yard. When Edna goes to church, her head goes with her. If Doug is in the house, he must have gone in through some opening unless he was born there and never left. If Sheila is alive at 9 A.M. and is alive at 5 P.M., she was also alive at noon. Zebras in the wild never wear underwear. Opening a jar of a new brand of peanut butter will not vaporize the house. People never shove meat thermometers in their ears. A gerbil is smaller than Mt. Kilimanjaro.

An intelligent system, then, cannot be stuffed with trillions of facts. It must be equipped with a smaller list of core truths and a set of rules to deduce their implications. But the rules of common sense, like the categories of common sense, are frustratingly hard to set down. Even the most straightforward ones fail to capture our everyday reasoning.

For all its moonstruck madness, love is no bug or crash or malfunction. The mind is never so wonderfully concentrated as when it turns to love, and there must be intricate calculations that carry out the peculiar logic of attraction, infatuation, courtship, coyness, surrender, commitment, malaise, philandering, jealousy, desertion, and heartbreak. And in the end, as my grandmother used to say, every pot finds a cover; most people--including, significantly, all of our ancestors--manage to pair up long enough to produce viable children. Imagine how many lines of programming it would take to duplicate that!

Another expansion of our vista comes from the startling similarities between identical twins, who share the genetic recipes that build the mind. Their minds are astonishingly alike, and not just in gross measures like IQ and personality traits like neuroticism and introversion. They are alike in talents such as spelling and mathematics, in opinions on questions such as apartheid, the death penalty, and working mothers, and in their career choices, hobbies, vices, religious commitments, and tastes in dating.
Identical twins are far more alike than fraternal twins, who share only half their genetic recipes, and most strikingly, they are almost as alike when they are reared apart as when they are reared together. Identical twins separated at birth share traits like entering the water backwards and only up to their knees, sitting out elections because they feel insufficiently informed, obsessively counting everything in sight, becoming captain of the volunteer fire department, and leaving little love notes around the house for their wives.

The complex structure of the mind is the subject of this book. Its key idea can be captured in a sentence: The mind is a system of organs of computation, designed by natural selection to solve the kinds of problems our ancestors faced in their foraging way of life, in particular, understanding and outmaneuvering objects, animals, plants, and other people. The summary can be unpacked into several claims.
The mind is what the brain does; specifically, the brain processes information, and thinking is a kind of computation. The mind is organized into modules or mental organs, each with a specialized design that makes it an expert in one arena of interaction with the world. The modules' basic logic is specified by our genetic program. Their operation was shaped by natural selection to solve the problems of the hunting and gathering life led by our ancestors in most of our evolutionary history.
The various problems for our ancestors were subtasks of one big problem for their genes, maximizing the number of copies that made it into the next generation. On this view, psychology is engineering in reverse. In forward-engineering, one designs a machine to do something; in reverse-engineering, one figures out what a machine was designed to do.
In the seventeenth century William Harvey discovered that veins had valves and deduced that the valves must be there to make the blood circulate. Since then we have understood the body as a wonderfully complex machine, an assembly of struts, ties, springs, pulleys, levers, joints, hinges, sockets, tanks, pipes, valves, sheaths, pumps, exchangers, and filters. Even today we can be delighted to learn what mysterious parts are for.
Why do we have our wrinkled, asymmetrical ears? Because they filter sound waves coming from different directions in different ways. The nuances of the sound shadow tell the brain whether the source of the sound is above or below, in front of or behind us. The strategy of reverse-engineering the body has continued in the last half of this century as we have explored the nanotechnology of the cell and of the molecules of life.
The rationale for reverse-engineering living things comes, of course, from Charles Darwin. He showed how "organs of extreme perfection and complication, which justly excite our admiration" arise not from God's foresight but from the evolution of replicators over immense spans of time. As replicators replicate, random copying errors sometimes crop up, and those that happen to enhance the survival and reproduction rate of the replicator tend to accumulate over the generations. Plants and animals are replicators, and their complicated machinery thus appears to have been engineered to allow them to survive and reproduce.
Only in the past few years has Darwin's challenge been taken up, by a new approach christened "evolutionary psychology" by the anthropologist John Tooby and the psychologist Leda Cosmides. Evolutionary psychology brings together two scientific revolutions. One is the cognitive revolution of the 1950s and 1960s, which explains the mechanics of thought and emotion in terms of information and computation. The other is the revolution in evolutionary biology of the 1960s and 1970s, which explains the complex adaptive design of living things in terms of selection among replicators. The two ideas make a powerful combination.
Cognitive science helps us to understand how a mind is possible and what kind of mind we have. Evolutionary biology helps us to understand why we have the kind of mind we have. This book is about the brain, but I will not say much about neurons, hormones, and neurotransmitters. That is because the mind is not the brain but what the brain does, and not even everything it does, such as metabolizing fat and giving off heat.
The 1990s have been named the Decade of the Brain, but there will never be a Decade of the Pancreas. The brain's special status comes from a special thing the brain does, which makes us see, think, feel, choose, and act. That special thing is information processing, or computation.

Human thought and behavior, no matter how subtle and flexible, could be the product of a very complicated program, and that program may have been our endowment from natural selection. The typical imperative from biology is not "Thou shalt ...," but "If ... then ... else."


Perhaps we should rejoice that people's emotions aren't designed for the good of the group. Often the best way to benefit one's group is to displace, subjugate or annihilate the group next door. Ants in a colony are closely related, and each is a paragon of unselfishness. That's why ants are one of the few kinds of animals that wage war and take slaves. When human leaders have manipulated or coerced people into submerging their interests into the group's, the outcomes are some of history's worst atrocities. People's desire for a comfortable life for themselves, their family and their friends may have braked the ambitions of many an emperor.

Parental love causes the fundamental paradox of politics: no society can be simultaneously fair, free and equal. If it is fair, people who work harder will accumulate more. If it is free, people will give their wealth to their children. But then it cannot be equal, for some people will inherit wealth they did not earn. Ever since Plato called attention to these tradeoffs in 'The Republic', most political ideologies can be defined by the stance they take on which of these ideals should yield.

The family is a subversive organisation. Every political and religious movement in history has sought to undermine the family. Leninism, Nazism, and other totalitarian ideologies always demand a new loyalty higher than, and contrary to, family ties. So have religions from early Christianity to the Moonies.

Legal monogamy historically has been an agreement between more and less powerful men, not between men and women. Its aim is not so much to exploit the customers in the romance industry (women) as to minimize the costs of competition among the producers (men). Under polygamy, men vie for extra-ordinary Darwinian stakes - many wives versus none - and the competition is literally cut-thraot. Many homicides amd most tribal wars are directly or indirectly about competition for women. Leaders have outlawed polygamy when they needed less powerful men as allies and when they needed their subjects to fight an enemy instead of fighting one another.

Ironically for the men-are-slime theory, an eye for nubile young women may have evolved in the service of marriage and fatherhood, not one-night stands. Among chimpanzees, where a father's role ends with copulation, some of the wrinkled and saggy females are the sexiest.

Could we be equipped with an innate eye for beauty? What about the natives in National Geographic who file their teeth, stretch their necks with stacks of rings, burn scars into their cheeks, and put plates in their lips? What about the fat women in the Rubens paintings and Twiggy in the 60s? Dont they show that standards of beauty are arbitary and vary capriciously? They do not. Who says that everything people do to their bodies is an attempt to look sexy?
People decorate their bodies for many reasons - to look rich, to look well connected, to look tough, to look "in". Sexual attractiveness is different. People outside a culture usually agree with the people inside about who is beautiful and who is not, and people everywhere want good-looking partners. Even three-month-old infants prefer to look at a pretty face.

Beauty is not, as some feminists have claimed, a conspiracy by men to objectify women. The really sexist societies drape women in chadors from head to foot. Throughout history the critics of beauty have been powerful men, religous leaders, sometimes older women, and doctors, who can always be counted on to say that the latest beauty craze is hazardous to women's health. The enthusiasts are women themselves. The explanation is simple economics and politics. Women in open societies want to look good because it gives them an edge in competing for husbands, status, and the attention of powerful people.

Many pundits blame violence against women on this or that feature of American society, such as circumcision, war toys, James Bond or football. But it happens worldwide, including in foraging socieities. Among the Yanomamo, a man who suspects his wife of infidelity might slash her with a machete, shoot her with an arrow, hold an ember against her, cut off her ears, or kill her. Rivalry over women is the leading cause of violence, homicide and warfare among foraging peoples.
None of these points "condone" the violence or imply that "its not the man's fault," as it is sometimes claimed. Those non sequitirs could be attached to any explanation, such as the common feminist theory that men are brainwashed by media images that glorify violence against women.

The chaotic cycles of style and fashion, in which the chic look of one decade becomes dowdy or slutty, nerdy or foppish in the next has been explained as a conspiracy of clothing makers, an expression of nationalism, a reflection of the economy, and much else. But Quentin Bell, in his classic analysis of fashion, 'On Human Finery', showed that only one explanation works : people follow the rule, "Try to look like the people above you; if you're at the top, try to look different from the people below you."

People have an emotional response that seems designed to weed out fair-weather friends. Hard times show you who your real friends. That is because the point of friendship, in evolutionary terms, is to save you in hard times when it's not worth anyone else's trouble.
The design of friendship emotions may explain the alienation and loneliness that so many people feel in modern society. The comfortable environment that makes us physically more secure may make us emotionally less secure, because it minimizes the crises that tell us who our real friends are.

No account of human relationships could be complete without a discussion of war. War is not universal, but people in all cultures feel that they are members of a group (a band, tribe, clan or nation) and feel animosity toward other groups. And warfare itself is a major fact of life for foraging tribes. Yanomamo villages raid one another endlessly. 30% of the men are killed by other men. 44% of the men have killed someone.
In primitive warfare, mobilization was more complete, battles more frequent, casualties higher, prisoners fewer, and weapons more damaging. War has always been hell.
The homicide rate in the most vicious American urban jungles are 20 times lower than in many foraging societies. Modern Britons are 20 times less likely to be murdered than their medieval ancestors.


Free will is an enigma. How can my actions be a choice for which I am responsible if they are completely caused by my genes, my upbringing and my brain state? Some events are determined, some are random, how can a choice be neither?
Free will is an idealization of human beings thats makes the ethics game playable. A human being is simultaneously a machine and sentient free agent, depending on the purpose of the discussion.

So should we all just take poison now and be done with it? Some people think that evolutionary psychology claims to have discovered that human nature is selfish and wicked. But they are flattering the researchers and anyone who would claim to have discovered the opposite. No one needs a scientist to measure whether humans are prone to knavery. The question has been answered in the history books, the newspapers, the ethnographic records, and the letters to Ann Landers. But people treat it like an open question, as if someday science might discover that it's all a bad dream and we will wake up to find that it is human nature to love one another.


Adaptive design must be a product of natural selection. Complex organs like eyes have many precise parts in exacting arrangements, and the odds are astronomically stacked against their having arisen fortuitously from random genetic drift or as a byproduct of something else. Second, the brain, like the eyes and the feet, shows signs of good design. The adaptive problems it solves, such as perceiving depth and color, grasping, walking, reasoning, communicating, avoiding hazards, recognizing people and their mental states, and juggling competing demands in real time are among the most challenging engineering tasks ever stated, far beyond the capacity of foreseeable computers and robots. Put the premises together - complex design comes from natural selection, and the brain shows signs of complex design - and we conclude that much of the brain should be explained by natural selection.

It is archeological and ethnographic evidence that leads evolutionary psychologists to infer that the ancestral environment (the environment of evolutionary adaptedness for humanity) lacked agriculture, contraception, high-tech medicine, mass media, mass-produced goods, money, police, armies, communities of strangers, and other modern features - absences with profound implications for the minds that evolved in such an environment.

# EXTERNAL RESOURCES has an online version of the opening chapter of the book.


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