A Natural History of Rape: Biological Bases of Sexual Coercion available in Paperback
A biologist and an anthropologist use evolutionary biology to explain the causes and inform the prevention of rape.
In this controversial book, Randy Thornhill and Craig Palmer use evolutionary biology to explain the causes of rape and to recommend new approaches to its prevention. According to Thornhill and Palmer, evolved adaptation of some sort gives rise to rape; the main evolutionary question is whether rape is an adaptation itself or a by-product of other adaptations. Regardless of the answer, Thornhill and Palmer note, rape circumvents a central feature of women's reproductive strategy: mate choice. This is a primary reason why rape is devastating to its victims, especially young women. Thornhill and Palmer address, and claim to demolish scientifically, many myths about rape bred by social science theory over the past twenty-five years. The popular contention that rapists are not motivated by sexual desire is, they argue, scientifically inaccurate.
Although they argue that rape is biological, Thornhill and Palmer do not view it as inevitable. Their recommendations for rape prevention include teaching young males not to rape, punishing rape more severely, and studying the effectiveness of "chemical castration. " They also recommend that young women consider the biological causes of rape when making decisions about dress, appearance, and social activities. Rape could cease to exist, they argue, only in a society knowledgeable about its evolutionary causes.
The book includes a useful summary of evolutionary theory and a comparison of evolutionary biology's and social science's explanations of human behavior. The authors argue for the greater explanatory power and practical usefulness of evolutionary biology. The book is sure to stir up discussion both on the specific topic of rape and on the larger issues of how we understand and influence human behavior.
About the Author
Randy Thornhill is Regents' Professor and Professor of Biology at the University of New Mexico.
Craig T. Palmer is Instructor of Anthropology at the University of Colorado.
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Rape and Evolutionary Theory
Not enough people understand what rape is, and, until they do ... , not enough will be done to stop it.
rape victim, quoted in Groth 1979 (p. 87)
By one intuitive and relevant definition, rape is copulation resisted to the best of the victim's ability unless such resistance would probably result in death or serious injury to the victim or in death or injury to individuals the victim commonly protects. Other sexual assaults, including oral and anal penetration of a man or a woman under the same conditions, also may be called rape under some circumstances.
In one study, 13 percent of the surveyed American women of ages 18 and older reported having been the victim of at least one completed raperape having been defined as "an event that occurred without the woman's consent, involved the use of force or threat of force, and involved sexual penetration of the victim's vagina, mouth or rectum" (Kilpatrick et al. 1992, p. i). Other surveys using slightly different definitions or different data-collection procedures have found high rates too, especially when the survey procedures have given researchers access to victims of alleged rapes not reported to the police. Kilpatrick et al. (ibid., p. 6) estimate the percentage of rapes of women not reported at between 66 and 84. Of women who had experienced a rape involving penile-vaginal intercourse, from 37 to 57 percent experienced post-traumatic stress syndrome afterwarda frequency higher than that associated with any other crime against women, including aggravated assault,burglary, and robbery (Kilpatrick et al. 1987; Resnick et al. 1993).
We suggest two answers to the question of why humans have not been able to put an end to rape:
· Most people don't know much about why humans have the desires, emotions, and values that they have, including those that cause rape. This is because most people lack any understanding of the ultimate (that is, evolutionary) causes of why humans are the way they are. This lack of understanding has severely limited people's knowledge of the exact proximate (immediate) causes of rape, thus limiting the ability of concerned people to change the behavior.
· For 25 years, attempts to prevent rape have not only failed to be informed by an evolutionary approach; they have been based on explanations designed to make ideological statements rather than to be consistent with scientific knowledge of human behavior.
One cannot understand evolutionary explanations of rape, much less evaluate them, without a solid grasp of evolutionary theory. Failure to appreciate this point has caused much valuable time to be wasted on misplaced attacks on evolutionary explanations.
Assuming that the main interest of most readers of this book is the subject of rape rather than evolutionary theory per se, we now present some questions about rape that an evolutionary approach can answer:
· Why are males the rapists and females (usually) the victims?
· Why is rape a horrendous experience for the victim?
· Why does the mental trauma of rape vary with the victim's age and marital status?
· Why does the mental trauma of rape vary with the types of sex acts?
· Why does the mental trauma of rape vary with the degree of visible physical injuries to the victim, but in a direction one might not expect?
· Why do young males rape more often than older males?
· Why are young women more often the victims of rape than older women or girls (i.e., pre-pubertal females)?
· Why is rape more frequent in some situations, such as war, than in others?
· Why does rape occur in all known cultures?
· Why are some instances of rape punished in all known cultures?
· Why are people (especially husbands) often suspicious of an individual's claim to have been raped?
· Why is rape often treated as a crime against the victim's husband?
· Why have attempts to reform rape laws met with only limited success?
· Why does rape exist in many, but not all, species?
· Why does rape still occur among humans?
· How can rape be prevented?
The question "What is man?" is probably the most profound that can be asked by man. It has always been central to any system of philosophy or of theology. We know that it has been asked by the most learned humans 2000 years ago, and it is just possible that it was being asked by the most brilliant australopithecines 2 million years ago. The point I want to make now is that all attempts to answer that question before 1859 are worthless and that we will be better off if we ignore them completely. Simpson 1966, p. 472
Intelligent life on a planet comes of age when it first works out the reason for its own existence. If superior creatures from space ever visit Earth, the first question they will ask, in order to assess the level of our civilization, is: "Have they discovered evolution yet?" Living organisms had existed on Earth, without ever knowing why, for more than three billion years before the truth finally dawned on one of them. His name was Charles Darwin. To be fair, others had inklings of the truth, but it was Darwin who first put together a coherent and tenable account of why we exist.Dawkins 1976, p. 1
Many social scientists (and others) have dismissed claims such as these as evidence of a somehow non-scientific "messianic conviction" (Kacelnik 1997, p. 65). Although these quotes indicate considerable enthusiasm, the important question is whether they accurately describe the implications of the theory of evolution by natural selection. Simpson's and Dawkins's enthusiasm is warranted by the tremendous success of evolutionary theory in guiding the scientific study of life in general and of humans in particular to fruitful ends of deep knowledge.
Cause, Proximate and Ultimate
A friend of ours once told us that after a movie she returned with her date to his car in an isolated parking lot. Then, instead of taking her home, the man locked the doors and physically forced her to have sexual intercourse with him. The question addressed in this book, and the question asked us by our friend, is: What was the cause of this man's behavior?
In both the vernacular sense and the scientific sense, cause is defined as that without which an effect or a phenomenon would not exist. Biologists study two levels of causation: proximate and ultimate. Proximate causes of behavior are those that operate over the short termthe immediate causes of behavior. These are the types of causes with which most people, including most social scientists, are exclusively concerned. For example, if, when reading our friend's question concerning the cause of the man's behavior, you said to yourself it was because he hated women, felt the need to dominate someone, had been abused as a child, had drunk too much, had too much testosterone circulating in his body, was compensating for feelings of inadequacy, had been raised in a patriarchal culture, had watched too much violence on television, was addicted to violent pornography, was sexually aroused, hated his mother, hated his father, and/or had a rare violence-inducing gene, you proposed a proximate cause of his behavior. You probably didn't ask why your proposed proximate cause existed in the first place. That is, you probably didn't concern yourself with the ultimate cause of the behavior.
Because they refer to the immediate events that produce a behavior or some other phenotypic (i.e., bodily) trait, proximate causes include genes, hormones, physiological structures (including brain mechanisms), and environmental stimuli (including environmental experiences that affect learning). Proximate explanations have to do with how such developmental or physiological mechanisms cause something to happen; ultimate explanations have to do with why particular proximate mechanisms exist.
Proximate and ultimate explanations are complements, not alternatives. For example, the claim that millions of years of selection caused the human eye to have its current form (an ultimate explanation) is in no way contradictory to the claim that a series of rods and cones enable the eye to relay visual information to the brain (a proximate explanation). Similarly, the claim that learning affects men's rape behavior (i.e., that it is a proximate cause) does not contradict the view that the behavior has evolved.
Identifying ultimate causes, however, is important, because certain proximate explanations may be incompatible with certain ultimate explanations. This is because certain ultimate explanations specify the existence of certain types of proximate mechanisms. For example, the ultimate explanation that the human eye evolved by natural selection because it increased our ancestors' ability to detect light requires the existence of proximate light-detection mechanisms in the eye.
No aspect of life can be completely understood until both its proximate and its ultimate causation are fully known. To understand how ultimate causes can be known, one must understand how natural selection leads to adaptations.
Natural Selection and Adaptations
Adaptations are phenotypic features (morphological structures, physiological mechanisms, and behaviors) that are present in individual organisms because they were favored by natural selection in the past. Darwin sought to explain the existence of adaptation in terms of evolution by selection. Initially, he observed the action of selection on living things in naturea fact of natural history that is inescapable in view of the high rates of reproduction and mortality in all organisms. Later, he realized just how creative selection could be when extended over the long history of life on Earth. This retrospection is evident in the following eloquent passage from On the Origin of Species:
Natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working.... We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages. (Ridley 1987, p. 87)
The biologist George Williams, in his 1966 book Adaptation and Natural Selection, clarified what Darwin meant when he wrote of natural selection's rejecting all that was "bad" and preserving all that was "good." First, Williams noted, these words were not used in a moral sense; they referred only to the effects of traits on an individual's ability to survive and reproduce. That is, "good" traits are those that promote an individual's reproductive interests. We evolutionists use the term reproductive success to refer to these reproductive interests, by which we mean not the mere production of offspring but the production of offspring that survive to produce offspring (Palmer and Steadman 1997). A trait that increases this ability is "good" in terms of natural selection even though one might consider it undesirable in moral terms. There is no connection here between what is biological or naturally selected and what is morally right or wrong. To assume a connection is to commit what is called the naturalistic fallacy. In addition, Williams clarified that natural selection favors traits that are "good" in the sense of increasing an individual's reproductive success, not necessarily traits that are "good" in the sense of increasing a group's ability to survive.
The idea that selection favors traits that increase group survival, known as group selection, had become very popular before the publication of Williams's bookespecially after the publication of Animal Dispersion in Relation to Social Behavior, an influential book by the ornithologist V. C. Wynne-Edwards (1962). Williams's rebuttal of the concept of group selection convinced almost every biologist who read it that Wynne-Edwards was mistaken. However, the idea that selection favors traits that function for the good of the group appears to have been too attractive for many non-scientists to give up. Not only does it remain popular among the general public; it continues to have a small following among evolutionary biologists (Wilson and Sober 1994; Sober and Wilson 1998).
One cannot grasp the power of natural selection to "design" adaptations until one abandons both the notion that natural selection favors traits that are morally good and the notion that it favors traits that function for the good of the group. Only then can one appreciate the power of natural selection to design complex traits of individuals.
The human eye's many physiological structures exist because they increased the reproductive success of individuals in tens of thousands of past generations. Although there are four agents of evolution (that is, four natural processes that are known to cause changes in gene frequencies of populations), selection is the only evolutionary agent that can create adaptations like the human eye. The other evolutionary agents (mutation, drift, and gene flow)cannot produce adaptations; they lack the necessary creativity, because their action is always random with regard to environmental challenges (e.g., predators) facing individuals. Selection, when it acts in a directional, cumulative manner over long periods of time, creates complex phenotypic designs out of the simple, random genetic variation generated by the three other evolutionary agents. Selection is not a random process; it is differential reproduction of individuals by consequence of their differences in phenotypic design for environmental challenges. An adaptation, then, is a phenotypic solution to a past environmental problem that persistently affected individuals for long periods of evolutionary time and thereby caused cumulative, directional selection. Evolution by selection is not a purposive process; however, it produces, by means of gradual and persistent effects, traits that serve certain functionsthat is, adaptations.
Adaptations do not necessarily increase reproductive success in current environments if those environments differ significantly from past environments. The seeds of a tree that fall on a city sidewalk are complexly designed adaptations, formed by selection over many generations in past environments, yet they have essentially no chance of surviving or reproducing in the current environment of the sidewalk. Similarly, the North American pronghorn antelope shows certain social behaviors and certain locomotory adaptations (e.g., short bursts of high speed) for avoiding species of large cats and hyenas that are now extinct (Byers 1997).
The difference between current and evolutionary historical environments is especially important to keep in mind when one is considering human behavioral adaptations. Today most humans live in environments that have evolutionarily novel components. (Modern contraception is one such component that obviously influences the reproductive success of individuals in an evolutionarily novel way.) Therefore, human behavior is sometimes poorly adapted (in the evolutionary sense of the word) to current conditions.
Evolutionary functional explanations also differ from the nonevolutionary functional explanations familiar to most social scientists. In fact, evolutionary functional explanations overcome a problem that has plagued non-evolutionary functional explanations. Non-evolutionary functional explanations are unable to explain why a particular trait has come to serve a certain function when alternative traits could also serve that function (Hempel 1959). For example, Emile Durkheim, one of the founders of sociology, tried to explain religion by stating that it functioned to maintain the social group (Durkheim 1912). That explanation, however, is unable to account for why religion, instead of numerous alternative institutions (e.g., political governments, non-religious social organizations and ideologies), fulfills this particular function. The concept of evolution by natural selection helps overcome this problem. Any gene that happens to arise by random mutation, and happens to have the effect of increasing an organism's reproductive success, will become more frequent in future generations. Eventually, additional random mutations will also happen to occur in future generations and will also be favored by natural selection. Over time, this process results in functionally designed traits. Randomness (in the form of mutations) and the non-random process of natural selection combine to answer the question of why a particular trait has evolved instead of other imaginable traits that conceivably could have served the same function.
There is also the important fact that selection works only in relation to what has already evolved. The process does not start anew each time. Thus, there are many features that seem poorly designed relative to what might be imagined as a better solution. For example, the crossing of the respiratory and digestive tracts in the human throat can cause death from choking on food. It would be better designmuch safer in terms of survivalif our air and food passages were completely separate. However, all vertebrates (backboned animals) from fishes to mammals on the phylogenetic tree (the tree connecting all life to a common ancestor) have crossing respiratory and digestive tracts. The human respiratory system evolved from portions of the digestive system of a remote invertebrate ancestral species, and the food and air passages have been linked in non-functional tandem ever since (Williams 1992). The crossing of passages is a historical legacy of selection's having built respiratory adaptations from ancestral digestive system features. Not itself an adaptation, it is a by-product of selection's having molded respiratory adaptation from what came before.
Similarly, any new mutation, through its bodily effect, is assessed by selection in relation to how well it performs in the evolved environment of other individuals in the population as well as in the evolved environment of the various body forms that characterize the developmental pathway of traits. Thus, what has evolved (including the existing developmental adaptations) may constrain what can evolve, or may establish certain evolutionary paths as more likely than others.
Because selection is the most important cause of evolution, and because it is the only evolutionary agent that can produce adaptations, the ultimate approach seeks to provide explanations for these seemingly purposefully designed biological traits of individuals in relation to the causal selective forces that produced them. Thus, the adaptationist approach focuses on how an adaptation contributed to successful reproduction of its bearers in the environments of evolutionary history. The challenge in applying an ultimate or evolutionary analysis is not to determine whether an adaptation is a product of selection; it is to determine the nature of the selective pressure that is responsible for the trait. That selective pressure will be apparent in the functional design of the adaptation.
By-Products of Selection
Not all aspects of living organisms are adaptations. Indeed, Williams (1966, pp. 4-5) emphasized that "adaptation is a special and onerous concept that should be used only where it is really necessary," and the evolutionists that Williams inspired have been well aware that a trait's mere existence does not mean that it was directly favored by natural selection. Nor is a demonstration that a trait or a character increases an individual's reproductive success sufficient evidence that the trait is an adaptation.
Not only may an increase in reproductive success be due to some evolutionarily novel aspect of the environment; an increase in reproductive success in evolutionary environments may be only a beneficial effect rather than an evolutionary function. To illustrate this point, Williams cited a fox walking through deep snow to a henhouse to catch a chicken, then following its own footprints on subsequent visits to the henhouse. This makes subsequent trips to the henhouse more energy efficient for the fox, thus potentially increasing its reproductive success. Following its own footprints back may well involve adaptations in the brain of the fox, but there is no known feature of the fox's feet that exhibits design by natural selection to pack snow. The fox's feet are clearly designed for walking and running, but they are not clearly designed for snow packing. Hence, even though snow may have been part of the past environments of foxes, there is no evidence that it acted as a sufficient selective pressure to design the feet of foxes for efficient snow packing. Snow packing and any associated reproductive success appear to be fortuitous effects of the structure of the fox's feet. That is, snow packing is not a function of any known aspect of the fox's feet. Symons (1979, p. 10) noted that "to say that a given beneficial effect of a character is the function, or a function, of that character is to say that the character was molded by natural selection to produce that effect." Williams (1966, p. 209) stated that "the demonstration of a benefit is neither necessary nor sufficient in the demonstration of function, although it may sometimes provide insight not otherwise obtainable," and that "it is both necessary and sufficient to show that the process [or trait] is designed to serve the function."
As Williams emphasized, the concept of adaptation should be used only where really necessary; however, it is essential to consider the concept of adaptation in all cases of possible phenotypic design, because only then can it be determined if a trait has been designed by natural selection. Williams (ibid., p. 10) proposed that plausibly demonstrating design by natural selection requires showing that a trait accomplishes its alleged function with "sufficient precision, economy, and efficiency, etc." Following Williams's criteria, Symons (1979, p. 11) stated that "[a] function can be distinguished from an incidental effect insofar as it is produced with sufficient precision, economy, and efficiency to rule out chance as an adequate explanation of its existence." Hence, according to the doctrine of parsimony, "if an effect can be explained adequately as the result of physical laws or as the fortuitous byproduct of an adaptation, it should not be called a function" (ibid.).
Similarly, drift and mutation can be ruled out as explanations of the evolutionary history of a trait when the trait shows evidence of functional design. Drift may apply only to traits that do not adversely affect reproductive success: if there are such effects, then selection will determine a trait's fate. Few traits meet the criterion of no cost to reproductive success; thus, as the biologists Richard Alexander (1979) and Richard Dawkins (1986) have explained, drift is a matter of interest primarily in the cases of phenotypic traits that do not attract adaptationists' attention in the first place.
Most mutations are deleterious and thus are in a balance with selection (selection lowering the frequency and mutation increasing it). Selection is stronger because mutation rates are very low. Thus, mutation, as an evolutionary cause for traits, may apply only to those traits that are only slightly above zero frequency in the population. Because selection is the most potent of the evolutionary agents, any explanation of the evolutionary history of a trait based on mutation or on drift must be fully reconciled with the potency of selection to bring about trait evolution.
Further evidence of adaptation may come from cross-species comparisons. First, "if related species [i.e., those sharing a recent common ancestral species] come to occupy different environments where they are subject to different selection pressures, then they should evolve new traits as adaptive mutations occur that confer a reproductive advantage under the new conditions" (Alcock 1993, p. 222). Variation among the beaks of different species of the finches Darwin found on the Galápagos Islands would be an example of such "divergent evolution." The beak types are different adaptations for eating different, species-typical foods (Weiner 1994). Second, if two distantly related species "have been subjected to similar selection pressures," they "should have independently evolved similar behavioral traits through convergent evolutionif the trait truly is an adaptation to that selection pressure" (Alcock 1993, p. 222). Convergent evolution is responsible for the similar shapes of fishes and marine mammals that have evolved by natural selection in the context of mobility in water.
Hence, the diversity of life has two major components: adaptations and the effects of adaptations. The latter are known as by-products. Adaptations are traits formed directly by selective pressures; by-products are traits formed indirectly by selective pressures.
In addition to snow packing by fox feet, another example of a by-product is the red color of human arterial blood (Symons 1987a,b). This trait did not arise because of selection in the context of blood-color variation among individuals. That is, redness of arterial blood did not cause individuals with arterial blood of that color to become more frequent in succeeding generations. Instead, selection acting in other contexts gave rise to the trait as an epiphenomenon of adaptations. Human arterial blood is red for two proximate reasons: the chemistry of oxygen and hemoglobin in blood, and human color vision. Hence, the ultimate causation of the color of blood lies in the selective pressures that produced the chemical composition of human blood and human color vision.
Another example of a by-product is the higher death rate of males relative to females among humans of all ages (Alexander 1979; Trivers 1985; Wilson and Daly 1985; Geary 1998). The higher male mortality is not an adaptation; it is an incidental effect of sex-specific adaptations. The adaptations are in males' and females' bodies, including their brains. For example, various traits motivate male humans, relative to female humans, to engage in riskier activities. The ultimate cause of these male adaptations is a human evolutionary history of stronger sexual selection acting on males than on females.
When one is considering any feature of living things, whether evolution applies is never a question. The only legitimate question is how to apply evolutionary principles. This is the case for all human behaviorseven for such by-products as cosmetic surgery, the content of movies, legal systems, and fashion trends.
The crucial legitimate scientific debate about the evolutionary cause of human rape concerns whether rape is a result of rape-specific adaptation or a by-product of other adaptations. That is, does rape result from men's special-purpose psychology, and perhaps from associated nonpsychological anatomy, designed by selection for rape, or is rape an incidental effect of special-purpose adaptation to circumstances other than rape? We two authors, having debated this question for more than a decade (Palmer 1991, 1992a,b; Thornhill and Thornhill 1992a,b), agree that it may eventually be answered by determining whether or not rape is the result of special-purpose psychological mechanisms that motivate and regulate men's pursuit of rape in itself. We also agree that enough now is known about the ultimate evolutionary causes of human rape that an evolutionary approach can contribute significantly to prevention of the act.
But how can an ultimate explanation of why men rape help prevent future rapes? The answer is that ultimate evolutionary explanations have unique power in both a theoretical and a practical sense. In terms of theory, only selection can account for the creation and the maintenance of adaptations. Even complete identification of all proximate causes of an adaptation could not explain the genesis and the persistence of that adaptation. However, an ultimate explanation of a biological phenomenon can account for all proximate causes influencing the phenomenon, whether the phenomenon is an adaptation or an incidental effect of an adaptation. Thus, ultimate explanations are more general in that they are more inclusive of causation. As a result, ultimate explanations have enormous practical potential: if evolution by individual selection is truly the general theory of life, it should lead to the best insights about proximate causes, and identifying proximate causes is the key to changing human behavior (e.g., eliminating rape).
That an ultimate evolutionary approach can serve as a guide for research into proximate causes has been shown repeatedly in investigations of nonhuman organisms. Indeed, this approach has revolutionized those investigations (Krebs and Davies 1993; Alcock 1997). It is also revolutionizing the study of human behavior (Alexander 1987; Wright 1994; Pinker 1997; Geary 1998; Buss 1999).
Evolutionary theory contributes to the study of proximate causation in two ways.
First, it leads to the discovery of new biological phenomena whose proximate causes are unknown. For example, the evolutionary psychologists Leda Cosmides and John Tooby (1992) have found that the human brain contains a mechanism designed specifically to detect cheating in social exchanges. The discovery of such a "cheater-detection" mechanism was the result of an understanding of the evolutionary concept of reciprocal altruism originally developed by the biologist Robert Trivers (1971). Similarly, evolutionary theory has led to the discovery of specific patterns of nepotism. This knowledge has resulted from studies directed by the fundamental evolutionary concept of kin selection: individuals perpetuate their genes not only by producing offspring but also by aiding relatives, including offspring (Hamilton 1963, 1964; Alexander 1987; Chagnon and Irons 1979; Betzig et al. 1988; Betzig 1997; Crawford and Krebs 1998). Relatives contain a high proportion of identical genes, and the closer the kinship relationship the higher the genetic similarity. What are the proximate cues by which individuals identify their relatives and distinguish categories of relatives? "Social learning" is the general answer (Alexander 1979; Palmer and Steadman 1997). Children are taught who their relatives are by their parents and their other relatives and through association with them during upbringing, and are encouraged by their adult relatives to be altruistic toward them (especially close kin). But what is the precise nature of the learning schedules involved in the ontogeny (development) of an individual's nepotistic behavior? This question would never have been asked had not evolutionists first successfully predicted the patterns of nepotistic behavior. After the social-learning aspects of nepotism are understood, the proximate physiological mechanisms in the brain that cause humans to feel closer to and more generous toward close relatives can be investigated. Also, we may someday know the locations of human genes (another category of proximate causation), which, in conjunction with the environment, construct proximate mechanisms of kin recognition and discriminative nepotism.
The second way in which evolutionary theory interacts with the identification of proximate causes is even more direct and important. Evolutionary theory can tell investigators what proximate mechanisms are most likely to be found, and therefore where any investigation of proximate causation should begin. For example, evolutionary theory has provided unique directions for investigations of child abuse, child neglect, and infanticide (Daly and Wilson 1988). Evolutionary predictions regarding parental investment have directed researchers to multiple proximate causes of child maltreatment: resources available for successfully rearing offspring; paternity certainty and genetic relatedness of parent to offspring generally; health, sex, and status of offspring; age of parent; birth order.
The example of child abuse also demonstrates the ability of an evolutionary approach to identify the proximate causes of both adaptations and by-products. In this case, it is not child abuse or infanticide per se that was favored by selection in human evolutionary history. The adaptations concern what Daly and Wilson (1988) call "child-specific parental solicitude" or "discriminative parental solicitude," which evolved because they increased the number of surviving offspring in a parent's lifetime relative to parents who invested indiscriminately in children generally. These are species-wide psychological adaptations that cause some parents to show love to all their children more or less equally, or to love some children and neglect (or even abuse or kill) others. The power of an evolutionary approach in identifying these factors is illustrated by Daly and Wilson's observation (1995, p. 22) that "living with a stepparent has turned out to be the most powerful predictor of severe child abuse risk yet discovered, but two decades of intensive child abuse research conducted without the heuristic assistance of Darwinian insights never discovered it." We suggest that the evolutionary approach can make a similar contribution to the identification of the proximate causes of rape. Specifically, we suggest that an understanding of the evolved differences between male and female sexuality can lead to identification of the proximate causes of rape. Indeed, the ability of an ultimate evolutionary approach to direct research to the proximate causes of rape may be the key to lowering the frequency of rape.
Adaptations Are Functionally Specific
An understanding of the ultimate cause of adaptations can provide specific ways of preventing rape because adaptations are themselves specific.
In a paper titled "If we're all Darwinians, what's the fuss about?" Donald Symons (1987a) pointed out that the difference of opinion between traditional social scientists and the evolutionary anthropologists, biologists, and psychologists who were inspired by Williams's book Adaptation and Natural Selection does not concern whether or not the brain is designed by selection. The idea of psychological (brain) adaptation is almost certainly compelling to anyone who accepts that the rest of the human body has evolved by Darwinian selection. Indeed, the notion that the rest of the body could have been designed by selection without selection's simultaneously acting on the brain and the nervous system that control the body is absurd. To those who accept the notion of evolution, it is clear that the human brain must contain evolved structures that process environmental information in a manner that guides feelings and behavior toward ends that were adaptive in past human environments. Similarly, a moment's reflection on the evolution of the human opposable thumbwhose name implies both a structure and the movement (behavior) of that structureshould resolve any remaining controversy as to whether human physical behavior (muscle-induced motion) has evolved. All this means that the explanations of human behavior put forth by the social scientists who accept evolution (the vast majority) are implicitly evolutionary explanations. Hence, according to Symons (p. 126), "perhaps the central issue in psychology is whether the mechanisms of the mind are few, general, and simple, on the one hand, or numerous, specific, and complex, on the other." Symons goes on to say that "for all their differences, theories that purport to explain human affairs in terms of learning, socialization, or culture, and so on seem to have one thing in common: they assume that a few generalized brain/mind mechanisms of association or symbol manipulation underpin human action" (p. 139). We suggest that one reason that many social scientists have not learned evolutionary theory is that they have mistakenly assumed that adaptations are so general as to be of little significance.
Special-Purpose and General-Purpose Adaptations
Defined more precisely than above, adaptations are mechanisms that Darwinian selection "designed" because they provided solutions to environmental problems faced by ancestors (Williams 1966, 1992; Symons 1979; Thornhill 1990, 1997a). Providing these solutions is the "function" of adaptations (Williams 1966).
Although most people consider physical traits to be distinct from psychological (or mental) traits, this is a mistake. The brain, even if one calls it the psyche, is a physiological component of the body. In fact, the brain is the component of physiology and anatomy that controls the rest of physiology and anatomy via environmental information processing. Hence, when evolutionary psychologists speak of evolved "psychological mechanisms," they are actually postulating physiological mechanisms in the nervous system that, at the present stage of scientific knowledge, can only be inferred from patterns of behavior (Palmer 1991, 1992a,b).
Psychological mechanisms can be characterized as either special-purpose or general-purpose on the basis of the kind of information they process to accomplish their function. Information that is domain-specific (for example, that will help an individual acquire a proper diet or a mate with high reproductive potential) is, by definition, special-purpose. If the information processed to accomplish a goal is ecologically general, the mechanism is, by definition, general-purpose. Thus, we can imagine a general-purpose mechanism that evaluates a broad range of items (food items, potential mates, rocks) in terms of their quality.
Hypothetically, adaptations could range from very general to very specific. For example, a mechanism that used the same information to obtain a good diet and a mate with high reproductive potential would not be as general-purpose as a mechanism that used the same information to solve those problems and also the problem of finding safe places to sleep. On the other hand, finding a mate with high reproductive potential might involve a number of even more specific mechanisms. For example, among humans there seem to be separate, specific psychological mechanisms that have evolved to discriminate health, age-related cues, and parenting ability in a potential mate (Symons 1979, 1995; Thornhill and Moller 1997; Townsend 1998).
Hence, what is at question is not whether psychological mechanisms are general-purpose or special-purpose; it is their degree of specificity. Many social scientists believe that humans possess only a few very general psychological mechanisms; evolutionary psychologists posit many very specific mechanisms. This evolutionary perspective is akin to many cognitive scientists' long-standing assumption of the modularity of mind (Gazzaniga 1995).
There are three reasons why evolutionary psychologists argue that the human brain must be composed of many specialized, domain-specific adaptations.
The first is that the environmental problems our evolutionary ancestors faced were quite specific. Since adaptations are solutions to these specific environmental problems that impinged on ancestors during evolutionary history, they should be equally specific. Selection should have led to special-purpose adaptations because such adaptations can better solve specialized problems.
Any environmental problem that is typically solved by organisms could be used to illustrate the issue of specificity. Vision, for example, may at first appear to present only the very general problem of viewing one's environment. However, "vision" and "environment" are actually general words for complex phenomena. "Vision" entails solving many specific problems: color, black and white, depth, edges, distance, available light, and so on. Which of these problems an organism solves, and in what manner, will depend on very specific variables in the environment in which the organism's ancestors lived. Hence, the eyes, brains, and nervous systems of various species respond only to certain colors, shapes, and movements, and these vary greatly among species in correspondence to the features of the environments that impinged on the past reproductive success of individuals of the various species. For example, some cells in the European toad's eye "respond most to long, thin objects that move horizontally across the toad's visual field," and this specific design "becomes clear if one imagines how they would respond to a nearby moving worm" (Alcock 1993, pp. 134, 135). Furthermore, an individual animal's environment often is specific not only to the species but also to the individual's age and sex. Vision stems from many specialized psychological adaptations, each designed to process specific environmental information. An eye is a collection of many special-purpose psychological adaptations. Evolutionary psychologists expect the same to be true of an organism's other adaptations.
The second reason why human psychological adaptations are expected to be special-purpose is that much of successful human behavior depends on environmental circumstances that are variable (Symons 1987a).
The existence of environmentally dependent behavioral flexibility is often mistakenly used by social scientists to argue against the existence of specialized brain structures. "Many writers seem to believe that behavioral flexibility somehow implies the existence of simple, amorphous mental structures," Symons (1987a, p. 127) notes. He continues: "There is a litany in the literature of anthropology that goes something like this: Human beings have no nature because the essence of the human adaptation is plasticity, which makes possible rapid behavioral adjustments to environmental variations. This litany, however, has the matter backwards: Extreme behavioral plasticity implies extreme mental complexity and stability; that is, an elaborate human nature. Behavioral plasticity for its own sake would be worse than useless, random variation suicide. During the course of evolutionary history the more plastic hominid behavior became the more complex the neural machinery must have become to channel this plasticity into adaptive action."
A facultative response to the environment (that is, a conditional response that depends on specific environmental variables) evolves when the environment changes within the lifetime of an individual in a way that significantly influences reproductive success. The capacity to learn is one such response. The human social environment is one of change, and the portion of human psychology that is involved with social learning is large. This is probably an evolutionary outcome of selection in the context of changing social conditions within the lifetimes of individuals, coupled with an inability to solve a learning task by experimentation or trial-and-error learning; under this scenario, social learning evolves (Humphrey 1980; Alexander 1989). However, learning will generate maladaptive behaviors (behaviors that decrease the reproductive success of the individual) unless special-purpose mental mechanisms guide and bias learning and behavior along paths that are adaptive.
We humans are social strategists par excellence (Wright 1994), and our social behavior is apparently unique in the degree of its plasticity. This unique behavioral plasticity requires not only that human psychology consists of many specialized mechanisms but also that it be much more diverse and complex in structure than the psychology of any other organism.
The third reason that human psychological adaptations are expected to be special-purpose rather than general-purpose is that our knowledge of the functional design of non-psychological adaptations indicates that they are special-purpose. The human body, for example, is not a single general-purpose adaptation; it is a bundle of innumerable specific adaptations designed to solve specific challenges to reproduction in past environments. Indeed, those who accept the reality of evolution realize that species-specific non-psychological adaptations are what allow biologists to distinguish species morphologically, physiologically, and developmentally. If adaptations were general-purpose, differences among species (including differences in behavior) would not exist, and thus the discipline of taxonomy (the classification of organisms) would not exist. It is also sex-specific adaptations, psychological and otherwise, that allow researchers to describe sex differences, and it is age-specific adaptations, psychological and otherwise, that make the field of developmental biology possible.
Many social scientists apparently fail to realize that it is species-specific psychological adaptations that allow biologists to distinguish species behaviorally. Not only is it unreasonable to think that the human psyche will be an exception to the general pattern of specific adaptations; there is increasing evidence from behavioral studies and from neuroscience that the human psyche is composed of adaptations that process specialized information.
In 1989 the cognitive neuroscientist Michael Gazzaniga reviewed the evidence that aspects of human cognition are structurally and functionally organized into discrete units ("modules") that interact to produce mental activity. Gazzaniga summarized his review as follows: "... when considering the various observations reported here, it is important to keep in mind the evolutionary history of our species. Over the course of this evolution efficient systems have been selected for handling critical environmental challenges. In this light, it is no wonder there are specialized systems (modules) that are active in carrying out specific and important assignments." (1989, p. 951) As is evident from this summary, Gazzaniga had been led by empirical evidence to the conclusion that the human psyche is made up of many specialized adaptations.
Of course, to demonstrate the implausibility of the assumption that there are only a few very general psychological adaptations is not to demonstrate the existence of very specialized adaptations. Similarly, the existence of specialized adaptations in the frog brain is not evidence that similar specialized adaptations exist in the human brain. But evidence of specialized adaptations in the human brain is abundant. Symons (1987b, 1992), Cosmides and Tooby (1987, 1989), Barkow et al. (1992), Buss (1994, 1999), Gazzaniga (1995), Pinker (1997), and many others have amassed human behavioral evidence that the specific nature of ecological problems applies to environmental information-processing problems as much as it applies to other related problems, and thus that human psychological mechanisms appear to be domain-specific in function.
Although evolutionists debate the exact degree of specificity of the psychological mechanisms of the human brain (Symons 1987b, 1992; Alexander 1990; Turke 1990), essentially all of them are in agreement that the brain is much more specialized than is implied by a certain class of social scientists. As the evolutionary anthropologist Paul Turke (1990, p. 319) notes, "with the exception of some outdated behaviorists, ... we all have been working towards understanding the nature of the more or less specific mechanisms that constitute the human psyche."
Table of Contents
|1||Rape and Evolutionary Theory||1|
|2||The Evolution of Sex Differences||31|
|3||Why Do Men Rape?||53|
|4||The Pain and Anguish of Rape||85|
|5||Why Have Social Scientists Failed to Darwinize?||105|
|6||The Social Science Explanation of Rape||123|
|7||Law and Punishment||153|
|8||Social Influences on Male Sexuality||169|
|11||Treatment and Recovery||187|
What People are Saying About This
This book brings much-needed perspective and analysis to a topic that is emotionally charged. It is a welcome addition to the literature.
By dispelling the most dangerous social science myths attempting to explain rape, Randy Thornhill and Craig Palmer have
scientifically unveiled one of the most hideous scourges of the human condition for what it really isand thus armed us with the
intelligence to finally prevent victimization. This book should be read by every woman and by every man who cares about a
(Michael P. Ghiglieri, Professor of Physical Anthropology, Northern Arizona University and author of The Dark Side of Man: Tracing the Origin of Male Violence and East of the Mountains of the Moon: Chimpanzee Society in the African Rainforest)
Rape can cost males very little, and females very much. That's why men do it,
and women don't. It took a horde of humanists to obscure that fact. It took a biologist and an anthropologist--Thornhill and
Palmer--to clear it up. If we're ever to stop rape, the facts will have to be faced. That makes this an important book.
(Laura Betzig, editor of Human Nature)
By dispelling the most dangerous social science myths and attempting to explain rape, Randy Thornhill and Craig T. Palmer have scientifically unveiled one of the most hideous scourges of the human condition for what it really is- and this armed us with the intelligence to finally prevent victimization. This book should be read by every woman and by every man who cares about a woman.Michael P. Ghiglieri, PH.D., Professor of Physical Anthropology, Northern Arizona University and author of The Dark Side of Man: Tracing the Origin of Male Violence and East of the Mountains of the Moon: Chimpanzee Society in the African Rainforest
This is a courageous, intelligent, and eye-opening book with a noble goalto understand and eliminate a loathsome crime. Armed with logic and copious data, Thornhill and Palmer will force many intellectuals to decide which they value more: established dogma and ideology, or the welfare of real women in the real world. Steven Pinker , Professor of Psychology, Massachusetts Institute of Technology, and author of How the Mind Works and Words and Rules
By dispelling the most dangerous social science myths and attempting to explain rape, Randy Thornhill and Craig T. Palmer have scientifically unveiled one of the most hideous scourges of the human condition for what it really is- and this armed us with the intelligence to finally prevent victimization. This book should be read by every woman and by every man who cares about a woman. Michael P. Ghiglieri , PH. D. , Professor of Physical Anthropology, Northern Arizona University and author of The Dark Side of Man: Tracing the Origin of Male Violence and East of the Mountains of the Moon: Chimpanzee Society in the African Rainforest
This is a courageous, intelligent, and eye-opening book with a noble goal - to understand and eliminate a loathsome crime.
Armed with logic and copious data, A Natural History of Rape will force many intellectuals to decide which they value more:
established dogma and ideology, or the welfare of real women in the real world.
(Steven Pinker, Professor of Psychology, MIT, and author of How the Mind Works and Words and Rules)
Thornhill and Palmer's A Natural History of Rape is a must read for anyone who truly wishes to understand this sobering
phenomenon. Their balanced and thorough scientific treatment of the subject will provide the spark needed to generate
reconsideration of the standard social-science explanation of rape.
(David C. Geary, Professor of Psychology, University of Missouri at Columbia and author of Male, Female: The Evolution of Human Sex Differences)
Most Helpful Customer Reviews
My professors absolutely hate this book, which is perhaps part of the reason I like it. As a feminist who's also interested in evolution, I have no choice but to take this book seriously. As a woman, I must say that I recommended it to a friend who's been raped, and she told me that this book helped her a lot more than any of the counseling she's went to. Perhaps that's the only recommendation needed.
¿The ultimate causes of human rape are clearly to be found in the distinctive evolution of male and female sexuality. The evidence demonstrates that rape has evolved as a response to the evolved psychological mechanisms regulating female sexuality, which enabled women to discriminate among potential sex partners. If human females had been selected to be willing to mate with any male under any circumstances, rape would not occur.¿ 'pg. 84' These few sentences accurately describe what the authors take 200 pages trying to argue and prove. Could they not just have easily stated that if human males had been selected to not need to reproduce with as many partners as possible and be able to control themselves that rape would not occur? In short, they argue that men want to reproduce as much as possible and women want an investment. They could make this argument persuasive had they given more credible evidence than basing it on their studies of other animals that rape. Their solutions are fairly simple and quite sexist: create ¿programs educating women on attractiveness, how to dress, and put on makeup to decrease their likelihood of being the victim of rape¿ and ¿be made aware of the costs associated with attractiveness.¿ I do not care if a girl/woman is walking around practically naked, that does not give anyone the right to rape her nor should it be her fault. This book is very well written and clearly presents ¿evidence¿ to support their theory on the causes of rape. However, it is very obvious that they only chose evidence that supported their theory and failed to address ¿evidence¿ of the contrary. While humans are technically animals, it is not accurate to use animals to explain our current behavior. Humans have something all other animals do not: a complex brain. The authors also fail to address the many instances of rape that do not align with their theory. They do not explain men raping men. If men really evolved to reproduce as much as possible, why do many rapists use condoms? They also do not explain why some men rape woman too old to have kids, nor do they explain pedophilia which clearly is not going to produce offspring. Randy Thornhill and Craig T. Palmer blatantly show that they are sexist and bigoted. Throughout this book, they distinctly put all of the blame on women. Why should women have to make themselves less attractive just because men can¿t control themselves? Why is the way that women evolved the cause of rape, when it could just as easily be the way men evolved? They even claim that ¿women tell more lies, false stories and gossip, calling the credibility of their allegations into question.¿ So not only are women the one¿s to blame for rape occurring in the first place, but they are also chronic liars and story tellers. The worst part of this book is that it ultimately supports and perpetuates an ongoing problem in the United States concerning rape cases. Study any criminal justice or law material and you will find what most of us know through common sense: all too often in rape cases the women are on trial for whether or not they are lying, rather than the man being on trial for whether or not he is guilty. Essentially, the victim is on trial and not the defendant. If you are sexist and chauvinistic, then you will love this book. If you want to read some absolutely unbelievable material, then I recommend this book just for the shock value. Otherwise, don¿t bother.
The title of my review should be the battle-cry for the majority of the critics of this book. I'm an undergraduate who has read this book and taken the accompanying course taught by one of the authors. I signed up for it with a skeptical attitude and went in to it doubting everything that was presented and expecting an explanation. The course followed the text very closely and I got the explanation that I had wanted. I realize that this is a sensitive topic for many people and that most who criticize the book strongly have probably either never read it or have preexisting biases due to some life experience and thus read it with a closed mind. That is unfortunate and sets back the entire field of science. To call this a 'sales pitch for evolutionary biology' is a severely misguided attack. Those who would put forth such a view might as well join the hardline religious camp that spouts that the earth is only 4,000 years old and evolution is a myth proposed by satan. All new ideas must spurn discussion and debate to be successful, even if they are not well liked. By the many vehement reviews and public discussion this book has caused I believe it has been a smashing success. I will not say that it is a flawless work, nor that all the ideas put forth in it must be 100% correct. It is, however, a different perspective on rape and one that will hopefully make people consider the topic in depth. How to prevent rape is a difficult question but at least the authors are promoting some ideas and applying scientific reasoning where others have failed.