A Natural History of Rape
Biological Bases of Sexual Coercion
By RANDY THORNHILL and CRAIG T. PALMER
The MIT Press
Read the Review
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
· 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
· 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
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
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
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
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
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
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
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
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
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;
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
There are three reasons why evolutionary psychologists argue that the
human brain must be composed of many specialized, domain-specific
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
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
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."
(C) 2000 Massachusetts Institute of Technology All rights reserved. ISBN: 0-262-20125-9