The Savanna Principle - The Nature and Limitations of the Human Brain

The Intelligence Paradox: Why the Intelligent Choice Isn't Always the Smart One - Satoshi Kanazawa 2012

The Savanna Principle
The Nature and Limitations of the Human Brain

In this chapter I will focus on the human brain as an evolved organ and talk about how evolution has shaped and designed the human brain to have certain limitations and constraints. I will introduce you to a very fundamental observation in evolutionary psychology, which I call “the Savanna Principle.”1

The Savanna Principle

Evolutionary adaptations, whether they are physical or psychological, are designed for and adapted to the conditions of the ancestral environment, during the period of their evolution, not necessarily to the conditions of the current environment.2 Evolution cannot anticipate or foresee the future; it can only respond to conditions in the past. So it is impossible for it to design adaptations that will suit conditions that have not yet existed. This is easiest to see in the case of physical adaptations, such as the vision and color-recognition system.

What color is a banana? A banana is yellow in the sunlight and in the moonlight. It is yellow on a sunny day, on a cloudy day, on a rainy day. It is yellow at dawn and at dusk. The color of a banana appears constantly yellow to the human eye under all these conditions, despite the fact that the actual wavelengths of the light reflected by the surface of the banana under these varied conditions are different. Objectively, they are not the same color all the time. However, the human eye and color-recognition system can compensate for these varied conditions because they all occurred during the course of the evolution of the human vision system, and the visual cortex can perceive the objectively varied colors as constantly yellow.[3]

So a banana looks yellow under all conditions, except in a parking lot at night. Under the sodium vapor lights commonly used to illuminate parking lots, a banana does not appear natural yellow. This is because the sodium vapor lights did not exist in the ancestral environment, during the course of the evolution of the human vision system, and the visual cortex is therefore incapable of compensating for them. Evolution, which designed the human eye, only “knew” about the sun, the moon, and possibly open fire as the only sources of illumination. It could not have anticipated the sodium vapor lights or any other type of artificial illumination, like fluorescent lamps, which is why things look unnatural under fluorescent light.

Fans of the 1989 James Cameron movie The Abyss may recall a scene toward the end of the movie, where it is impossible for Ed Harris's character (a deep-sea diver) to distinguish colors under artificial lighting in the otherwise total darkness of the deep oceanic basin. Regular viewers of the TV show Forensic Files (formerly known as Medical Detectives) and other real-life crime documentaries may further recall that eyewitnesses often misidentify the colors of cars on freeways, leading the police either to rule in or rule out potential suspects incorrectly. This happens because highways and freeways are often lit with sodium vapor lights and other evolutionarily novel sources of illumination, which distort colors to the human eye.

The same principle that holds for physical adaptations like the color recognition system also holds for psychological adaptations. Pioneers of evolutionary psychology4 all explicitly recognized that the evolved psychological mechanisms are designed for and adapted to the conditions of the ancestral environment, not necessarily to those of the current environment. In 2004, I systematized these observations into what I call the Savanna Principle.5

Savanna Principle: The human brain has difficulty comprehending and dealing with entities and situations that did not exist in the ancestral environment.

Other evolutionary psychologists call the same observation the evolutionary legacy hypothesis6 or the mismatch hypothesis.7 The names may vary, but the observation remains the same. We are stuck with the stone-age brain which assumes that we are still hunter-gatherers on the African savanna, and responds to the environment as if it were the African savanna.[8] There are many manifestations of the Savanna Principle in our daily life.

TV Friends

Here is one illustration of the Savanna Principle in action. In 2002, I discovered that individuals who watched certain types of TV shows were more satisfied with their friendships, just as they were if they had more friends or socialized with them more frequently.9 And this finding was later replicated by others.10 From the perspective of the Savanna Principle, this may be because realistic images of other humans, such as those found in TV, movies, videos, photographs, and DVDs, did not exist in the ancestral environment, where all realistic images of other humans were other humans. As a result, the human brain may have implicit difficulty distinguishing their “TV friends” (the characters they repeatedly see on TV shows) and their real friends. So by being repeatedly exposed to their “TV friends,” that is, by watching TV shows with their familiar characters, they feel like they are actually with their friends, and their satisfaction with friendships increases.

The Savanna Principle suggests that, because TV and other realistic electronic depictions of other human beings did not exist in the ancestral environment, our brain cannot really comprehend TV. A lot of people get angry when I say this, and they vehemently deny that their brain fails to distinguish between “TV friends” and their real friends. They adamantly insist that they do know how TV works.

Well, you do and you don’t. At the conscious level, you do know how TV programs are produced. You do know that the people you see on TV are actors, who are hired and paid millions of dollars to play certain roles written by screenwriters in their scripts. You consciously know that TV shows and movies aren't real.

But your brain doesn’t. If it does, why did you cry when Julia Roberts's character died at the end of Steel Magnolias? Don't you know that she's just an actor who was paid a lot of money (reportedly, $90,000 in 198911) to play the role of a dying woman? Don't you know that she's not really dead? Why did you get scared when Freddie Kruger slashed many teenagers in A Nightmare on Elm Street? Don't you know that the actor who played Freddie Kruger (Robert Englund) is really a nice person and has never killed anyone? Don't you know that none of the teenagers who were murdered in the movie actually died, nor were they in any real danger because they were surrounded by dozens of crew at every moment? Your brain doesn't truly comprehend any of these things, and that's why you are able to enjoy watching movies and TV shows. If your brain truly did comprehend movies and TV shows, you would never be able to enjoy them.

This is what I mean by comprehension in the Savanna Principle (and later in the Savanna-IQ Interaction Hypothesis, discussed in Chapter 4). Comprehension means true, logical, and scientifically and empirically accurate understanding of how something works. Your brain (as opposed to you) truly comprehends something when your reaction or behavior in response to it is consistent with a scientifically and empirically accurate understanding of how it works.

So true comprehension of a TV show is that a large number of professional actors are paid to enact certain scripted roles but the characters they play on the show do not really exist in real life. Untrue comprehension of it includes, among others, that the stories portrayed on the show are real and that you know the characters on the show personally and they know you personally. The studies discussed above suggest that your brain (as opposed to you) does not always have true comprehension of TV shows because your reaction to them—increased satisfaction with your friendships—suggests otherwise.


Pornography—in particular, the vast sex differences in its consumption and reactions to it—is another illustration of the Savanna Principle at work.

An overwhelming majority of consumers of pornography worldwide are men.12 Given their greater desire for sexual variety, it is understandable why men would consume more pornography and seek out sexual encounters with numerous women in pornographic photographs and videos, just as they do when they contract prostitutes in search of greater sexual variety.

Such desire for sexual variety on the part of men is evolutionarily adaptive. A man who has sex with 1,000 women in a year can potentially produce 1,000 children (or more if there are multiple births); more realistically, he can expect to father 30 children (given that the probability of conception per coital act is .03).13 In sharp contrast, a woman who has sex with 1,000 men in a year can still only expect to have one child (barring a multiple birth) in the same time period, which she can achieve by having regular sex with only one man. So there is very little reproductive benefit for women in seeking a large number of sex partners, as there is for men.

However, unlike consorting with prostitutes, watching pornography does not lead to actual sexual intercourse. So why do men like to consume pornography?

The Savanna Principle suggests that a man's brain does not really know that he cannot copulate with the women he sees in pornographic photographs and videos. When men see images of naked and sexually receptive women, their brains cannot truly comprehend that they are artificial images of women whom they will likely never meet, much less have sex with, because no such images existed in the ancestral environment. Every single naked and sexually receptive woman that our male ancestors saw throughout human evolutionary history was a potential sex partner.

As a result, their brains think that they might have actual sexual encounters with these women. Why else would men have an erection when they view pornographic photographs and videos, when the only biological function of an erection is to allow men to have intercourse with women? If men's brains truly comprehended that they would likely never have sex with the naked women in pornography, they would not get an erection when they watch it.

The same principle holds in strip clubs and peep shows, even though these involve real live women, not their photographic and electronic images. In the ancestral environment, there were no women who were paid to dance around naked in front of men, and pretended to be sexually aroused and interested in them, but would never actually have sex with them. So men's brains cannot truly comprehend strippers and lap dancers. That is why they get an erection at strip clubs and peep shows, when they consciously know that they would not actually have sex with the naked women dancing in front of them.

The failure of men's brains to comprehend images of naked women nevertheless has some real consequences. In one experiment, men who viewed Playboy centerfolds subsequently found their own girlfriends physically less attractive and expressed less love for them.14 Why would men love their girlfriends less after viewing pictures of naked women in Playboy unless their brains implicitly assume that they could potentially date the Playboy centerfolds instead of their current girlfriends, most of whom pale in comparison to them?

But it's not only men's brains that fail. The Savanna Principle applies equally to women as it does to men; women's brains have the same evolutionary constraints and limitations as those of men. This is why women do not consume pornography nearly as much as men do, even though women enjoy having sexual fantasies as much as men do.15 Women do not seek sexual variety because their reproductive success does not increase by having sex with a large number of partners. In fact, given the limited number of children they can have in their lifetimes, the potential cost of having sex with the wrong partner is far greater for women than it is for men. This is why women are far more cautious about having sex with someone they do not know well16 and tend to require a much longer period of acquaintance before agreeing to have sex than men do.17

So it makes perfect evolutionary sense for women to avoid casual sex with anonymous strangers, and their brains cannot really tell that there is no chance that they might copulate with, or, worse yet, be impregnated by, a large number of the naked and sexually aroused men they see in pornography. Women's brains cannot fully comprehend that they will not get pregnant by watching pornography, just as men's brains do not know that they cannot copulate with women in pornography. Women avoid pornography for the same reason that men consume it; in both cases, their brains cannot really distinguish between real sex partners and the imaginary ones, because the latter did not exist in the ancestral environment.

The human brain (of both men and women), incidentally, also implicitly assumes that all sex potentially leads to reproduction. That is why we still experience sex with contraception as pleasurable, and we are motivated to pursue it. The human brain, adapted to and designed for the ancestral environment, cannot truly comprehend modern contraceptives that did not exist in the ancestral environment. If it did, we would not find sex with contraception physically pleasurable. The only contraception that existed in the ancestral environment was abstinence, and, consequently, this is the only form of contraception that the human brain can truly comprehend. This why men and women do not experience abstinence (not having sex) as pleasurable, but still experience sex when the woman is on the pill as pleasurable, even though both have the same reproductive consequences.

Cooperation in One-shot Prisoner's Dilemma Games

In a Prisoner's Dilemma game, two players make simultaneous decisions without knowing what the other player decides. Each player can decide to “cooperate” with the other player, or to “defect” on the other player. Cooperative choice benefits the other player, whereas the defective choice hurts the other player. Given the particular payoffs in Prisoner's Dilemma games, it is always rational to defect on the other player as long as the game is one-shot and not repeated infinitely. Regardless of what the other player does, you get a higher payoff by defecting than cooperating.

In one-shot games, there is nothing the other player can do to punish you for defecting. When the game is repeated infinitely, however, it becomes rational for you to cooperate in a Prisoner's Dilemma game because then the other player can punish you on future rounds for defecting. But there are no such concerns for possible future retaliation in a one-shot game. In typical experimental settings for Prisoner's Dilemma games, the two players interact via computers and never face each other in person. In addition, the researchers make sure that the two players will never run into each other before, during and after the experiment, so the two players will remain completely anonymous to each other. Under these experimental conditions, it is always rational to defect on the other player and receive higher payoffs. There are no negative consequences for defection.

Yet experiment after experiment conducted in the last half century show that roughly half of the players of one-shot Prisoner's Dilemma games make the theoretically irrational decision to cooperate.18 This has been one of the longstanding unsolved mysteries in game theory for more than 50 years. There are some ideas, but no one yet knows for sure exactly why half the people make the irrational decision to cooperate in one-shot Prisoner's Dilemma games in clear contradiction to the prediction of their elegant mathematical models. Microeconomics assumes that all human actors are rational, yet the evidence from these experiments seems to suggest that half of them are not. And microeconomics cannot explain why.

From the perspective of the Savanna Principle, it may be because the two conditions that are theoretically necessary for the prediction of universal defection in one-shot Prisoner's Dilemma games—complete anonymity and noniteration—did not exist in the ancestral environment. There was no such thing as anonymous exchange in the ancestral environment because there were no computer-mediated interactions that would make it possible; all exchanges and interactions in the ancestral environment were face-to-face. And very few, if any, social exchanges were one-shot. Our ancestors lived in a small band of about 150 related individuals all their lives. Everyone in their band was a relative, friend, or ally for life.[19]

So the Savanna Principle suggests that the human brain may have difficulty truly comprehending one-shot games and completely anonymous exchanges, because there were no such things in the ancestral environment. As a result, some individuals may act as if the anonymous one-shot games are face-to-face repeated games, the only kind that existed in the ancestral environment, and decide to cooperate, because it is rational to cooperate in nonanonymous repeated games.

This may be why as many as half the people in one-shot Prisoner's Dilemma games make the irrational choice to cooperate. Further, the Savanna-IQ Interaction Hypothesis, an idea that I introduce in Chapter 4, can potentially explain which 50% of the people are likely to cooperate in one-shot Prisoner's Dilemma games.

When Inclusion Costs and Ostracism Pays, Ostracism Still Hurts

An incredibly ingenious experiment recently conducted by a couple of social psychologists provides yet another illustration of the Savanna Principle in operation.20

Humans are a highly social species, and they rely and depend on each other for survival. For this reason, humans have always lived in social groups. Because humans are highly dependent on others in their groups, ostracism—being excluded from their social groups and the benefits they provide—has always been costly throughout human evolutionary history, and their very survival has often depended on being included in their groups.

It is therefore not surprising at all that humans have evolved psychological mechanisms that incline them to seek group affiliation and avoid ostracism. Studies examining the human brain using the fMRI (functional magnetic resonance imaging) technology have revealed that being ostracized activates the same region of the brain that lights up when individuals experience physical pain.21 In other words, humans are designed to feel physical pain when they are ostracized. Given how dangerous being excluded from the group is for human survival and how very costly ostracism is, especially in the ancestral environment of the African savanna, this makes perfect evolutionary sense. Our ancestors who did not mind being ostracized and didn't feel any pain about it probably didn't live long enough to produce many children.

But what if ostracism was not costly at all? What if, instead, being included is costly and being excluded is beneficial? Would people then come to enjoy being excluded and fear being included? This is the question that motivated Ilja van Beest and Kipling D. Williams to conduct their ingenious experiment in their 2006 article “When Inclusion Costs and Ostracism Pays, Ostracism Still Hurts” published in the premier journal in social psychology, Journal of Personality and Social Psychology.

In their experiment, van Beest and Williams use a variant of a multi-player computer game called Cyberball. An individual plays Cyberball with two other players. Each player can see the other two players on the computer screen, but they are in other locations; each player is alone in the room. The game is very simple. The three players toss a ball on the computer screen back and forth with each other. If you get the ball, you can toss it to either of the other two players, and whoever receives the ball tosses it to one of the other players. Each player has a choice of two players to toss the ball to.

Figure 2.1 is a screenshot from Cyberball (courtesy of Kipling D. Williams). If you are a player in this game, the left hand that you see at the bottom of the screen is yours, and you see the other two players in the game in front of you. In the screen shot, you are observing one of the other players (on the left) throwing the ball to the other player (on the right). You are therefore not involved in this particular ball toss. The player on the left has chosen the other player, not you, to whom to toss the ball.

Figure 2.1 Cyberball


Unbeknownst to the human player, however, the other two players on the screen are simulated actors programmed by the researchers to behave in certain ways. The experiment has a 2 (inclusion vs. exclusion) x 2 (gain vs. loss) design. In some games, the human player is included in a fair share of the ball tosses. This is the “inclusion” condition. In other games, after a couple of tosses at the beginning, the human player is completely excluded from the ball toss, and watches the other two players toss the ball back and forth with each other, completely ignoring and excluding the human player. This is the “exclusion” condition.

In some games, in both “inclusion” and “exclusion” conditions, the human players earns 50 cents every time they touch the ball (when they are included in the ball toss). This is the “gain” or “inclusion pays” condition. In other games, in both the “inclusion” and “exclusion” conditions, the human players lose 50 cents every time they touch the ball; in other words, in this condition, people are financially better off if they are excluded from the ball tosses. This is the “loss” or “exclusion pays” condition.

Van Beest and Williams's experimental design makes these two factors completely independent of each other. Some subjects gain money while being included, some subjects gain money while being excluded. Other subjects lose money while being included, still others lose money while being excluded. Then, after the Cyberball game is over, the researchers measure the subjects’ satisfaction and mood.

It makes perfect sense that human players who were excluded from the ball toss in the “inclusion pays” condition were hurt by being excluded. They would have earned more money if they were included in the game, but they were not, so they felt hurt. No surprises here. What is surprising is that people in the “exclusion pays” condition were also hurt when they were excluded. These people made more money by being excluded from the game, yet they were equally hurt by not being included in the ball toss by the other two players. How could this be? How could people feel hurt when they were doing better?

The Savanna Principle can offer one potential answer. Throughout the course of human evolution, exclusion was always costly and inclusion was always beneficial. These two things always went together, because there were no evil experimental psychologists in the ancestral environment to manipulate these variables independently. There were no such things as “beneficial exclusion” and “costly inclusion.” Our ancestors were never in the “exclusion pays” condition. The human brain therefore cannot comprehend such a thing. The human brain implicitly and unconsciously assumes that all ostracism is costly, just as it assumes that all realistic images of people whom they see on a regular basis (and who don't try to kill or harm them in any way) are their friends, even when these people are on TV.

Microeconomic theory, or any other theory of human behavior which assumes that human behavior is rational and based on carefully calculated cost-benefit analysis, cannot explain van Beest and Williams's remarkable findings that humans are happy to lose money and sad to make money. Without the Savanna Principle, it would be difficult to explain why ostracism makes people sad when it pays. This is one of the many reasons why evolutionary psychology is superior to microeconomics (or any other theory) as an explanation for human behavior, even when we are not talking about sex differences.22

The fundamental insight of evolutionary psychology, expressed in the Savanna Principle, is that the human brain responds to the environment as if it were still the African savanna in the ancestral environment (for the most part, during the Pleistocene Epoch, 1.6 million to 10,000 years ago). You the person may consciously know that this is the 21st century, and you are a stockbroker in New York, an artist in Seattle, a housewife in San Francisco, or a student in Kansas City, but your brain doesn't know that. Your brain, unconsciously and implicitly, still thinks that you are a hunter-gatherer living on the African savanna more than 10,000 years ago, where there was no TV or psychology experiments or virtually anything else you see around you today. As you can imagine, implications of this fundamental observation of evolutionary psychology for our modern life are significant and widespread.


1. Kanazawa (2004a)

2. Tooby and Cosmides (1990)

[3]. Cosmides and Tooby (1999, pp. 17—19); Shepard (1994). Some vision researchers disagree, however, and claim that perception of color is distorted under moonlight (Khan and Pattanaik, 2004) or reduced illumination (Shin, Yaguchi and Shioiri, 2004). If so, this is potentially due to the fact that our ancestors did not engage in many nocturnal activities (see Chapter ). They likely woke up when the sun rose, and went to sleep when the sun set. If the human vision system has difficulty accurately perceiving color under moonlight or reduced illumination, it may be because the need to do so is evolutionarily novel

4. Crawford (1993); Symons (1990); Tooby and Cosmides (1990)

5. Kanazawa (2004a)

6. Burnham and Johnson (2005, pp. 130—131)

7. Hagan and Hammerstein (2006, pp. 341—343)

[8]. Most evolutionary psychologists and biologists concur that humans have not undergone significant evolutionary changes in the last 10,000 years, since the end of the Pleistocene Epoch, as the environment during this period has not provided a stable background against which natural and sexual selection could operate over many generations (Miller and Kanazawa, 2007, pp. 25—28). Evolution cannot operate against a moving target. This is the assumption behind the Savanna Principle. More recently, however, some scientists have voiced opinions that human evolution has continued and even accelerated during the Holocene Epoch in the last 10,000 years (Cochran and Harpending, 2009; Evans et al., 2005). While their studies conclusively demonstrate that new alleles (varieties of genes) have indeed emerged in the human genome since the end of the Pleistocene Epoch, the implications and importance of such new alleles for evolutionary psychology are not immediately obvious. In particular, with the sole exception of lactose tolerance, it is not clear whether these new alleles have led to the emergence of new evolved physiological and psychological adaptations in the last 10,000 years.

9. Kanazawa (2002)

10. Derrick, Gabriel and Hugenberg (2009)


12. Held (2006); Malamuth (1996); Symons (1979, pp. 170—184)

13. Pérusse (1993, pp. 273—274)

14. Kenrick et al. (1989)

15. Ellis and Symons (1990)

16. Clark and Hatfield (1989); Hald and Høgh-Olesen (2010)

17. Buss and Schmitt (1993)

18. Sally (1995)

[19]. Fehr and Henrich (2003) suggest that one-shot encounters and exchanges might have been common in the ancestral environment. In their response to Fehr and Henrich, Hagen and Hammerstein (2006) point out that, even if one-shot encounters were common in the ancestral environment, anonymous encounters could not have been common, and the game-theoretic prediction of defection in one-shot games requires both noniteration and anonymity. A lack of anonymity can lead to reputational concerns even in nonrepeated exchanges.

The available molecular genetic evidence suggests that our ancestors practiced female exogamy. It means that, when girls reached puberty, they left their natal group to marry into neighboring groups, in order to avoid inbreeding, whereas boys stayed in their natal group their entire lives. So all men in a hunter-gatherer band were genetically related to each other, whereas women were not. But they mostly stayed in the group they married into, so they became friends and allies for the rest of their lives. Seielstad, Minch and Cavalli-Sforza (1998).

20. van Beest and Williams (2006)

21. Eisenberger, Lieberman and Williams (2003)

22. Kanazawa (2006d)