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Cristin Clark, Gina Galier,  Katie Schlageter


Title: A Natural History of Rape: Biological Bases of Sexual Coercion

Chapter 3: Why Do Men Rape?

Chapter 4: The Pain and Anguish of Rape

Authors: Craig T. Palmer & Randy Thornhill (2000)


Most people would agree that “the males of most species – including humans – are usually more eager to mate than the females” (Palmer & Thornhill, 53). These intense male instincts/drives are a result of sexual selection. Indeed, selection favors males who advertise and females who choose wisely. In other words, males compete with one another through intrasexual competition for access to fertile females in hopes of being chosen as a mating partner. “But getting chosen is not the only way to gain access to females. In rape, the male circumvents the female’s choice” (Palmer & Thornhill, 53).

            Researchers have suggested that, in addition to mate choice and intrasexual competition, “sexual coercion is best conceptualized as a third type of sexual selection” (Palmer & Thornhill, 54). Sexual coercion is a broader term than rape and is defined as “obtaining sexual access by intimidation, harassment and/or physical force” (Palmer & Thornhill, 54). Although sexual coercion is being considered a form of sexual selection, it is distinctively separate from the other two forms. That is, “a sexually coercive male may succeed in the competition for mates by coercing mating even though he loses in male-male competition for females and is not chosen as a mate by a female” (Palmer & Thornhill, 54). If not for a male’s instinct to sexually coerce, it is possible that his genes could become extinct.

            Although sexual coercion ensures males access to females, it presents significant reproductive and/or survival costs to the females (Palmer & Thornhill, 54). Therefore, females have been selected with a variety of traits that have evolved to reduce these costs. For instance, females’ social behavior includes female-female alliances and pair bonding with males. Perhaps these behaviors have evolved as a way to offer females a sense of security and protection against sexual coercion (Palmer & Thornhill, 54).


Human Rape: Adaptation or By-Product

            Palmer and Thornhill apply the evolutionary theory to rape on both logical and evidentiary grounds. They support the notion that, “nothing in biology makes sense except in light of evolution” (Palmer & Thornhill, 55). Therefore, evolutionary history is applicable to human rape.

Although there may be many potential ultimate causes of human rape, there are only two reasonable explanations that are compatible with modern research and evolutionary history. In essence, human rape is either the result of adaptation or a by-product of other psychological adaptations.

            Adaptations have evolved to help species overcome obstacles to individual reproductive success. Males have had to evolve adaptations to overcome the obstacle of female choosiness. Rape, then, may be viewed as “an adaptation that was directly favored

by selection because it increased male reproductive success by way of increasing mate number” (Palmer & Thornhill, 59). To support this claim, a phenotypic mechanism or feature, designed specifically for the purpose of rape, must be identified.

“Mechanisms merely allowing (or even increasing) a male’s ability to rape are not necessarily designed for that purpose. For instance, the larger average size of human males certainly makes it easier for them to commit rape; however, it cannot be considered an adaptation designed by sexual selection to promote rape,

because such sexual dimorphism is much more parsimoniously explained as an adaptation for intra-male competition in polygynous species” (Palmer & Thornhill, 62).


To illustrate the concept of a phenotypic feature designed specifically to assist males in raping, the notal organ of male scorpionflies should be examined. The notal organ is a “clamp located on the top of the abdomen, behind the wings” (Palmer & Thornhill, 63). “It functions to secure a mating with an unwilling female and to retain her in copulation for the period needed for full insemination” (Palmer & Thornhill, 64). All evidence indicates that the notal organ is specifically designed for rape because it is not used for any other purposes. Other insects such as the waterstrider and the sagebrush cricket also have special-purpose male adaptations that assist males in rape.

            The capacity to rape adaptively can be viewed as a solution to environmental obstacles that adversely affect the reproductive success of a species. Thus, selection shapes the adaptations necessary to survive.

            The second possible explanation of human rape suggests that it is a by-product of other psychological adaptations. More specifically, rape is thought to be a by-product of the psychological adaptations that function to produce “the sexual desires of males for multiple partners without commitment” (Palmer & Thornhill, 60). There are many example of human behavior that are clearly by-products of the intense sexual desires of males and the sexual choosiness of females. For example, sexual abuse of children, frottage (rubbing a woman’s body through her clothing, usually in crowded quarters), genital exhibition and masturbation all demonstrate a male’s attempt to achieve sexual gratification without being chosen by a female. Reproductive pressures have not selected these behaviors and therefore, they are not adaptations. Instead, they appear to be mere by-products of the adaptations that govern male sexual desires (Palmer & Thornhill, 60). From this perspective, rape is also viewed as a by-product of male sexual instincts because it promotes a male’s access to females with the intention of reproductive success.

            To further illustrate the notion of human rape as a by-product, Palmer and Thornhill stated that:

“The primary adaptations responsible for the occurrence of rape were the mechanisms involved in the human male’s greater visual sexual arousal, greater autonomous sex drive, reduced ability to abstain from sexual activity, much greater desire for sexual variety per se, greater willingness to engage in impersonal sex and less discriminating criteria for sexual partners” (64).


These sexually selected adaptations are designed to increase a male’s mating success by increasing the number of sexual partners he could acquire. Therefore, rape is a by-product of these adaptations because “none of the evolved mechanisms were selected specifically for rape” (Palmer & Thornhill, 62).


Evaluation of Vulnerability

            There are six potential adaptations for human rape.  The first is evaluation of vulnerability. Rape is expected to occur only when its potential benefit (production of an extra offspring) exceeds its potential cost (energy expended and risk taken) (Palmer & Thornhill 66).  Men are more likely to rape when costs are low.  This suggests that there may be a psychological mechanism in men designed to evaluate female vulnerability in regards to rape.  Men are more likely to rape when rape’s proximate benefits exceed the chances of injury and punishment (Palmer & Thornhill 66).  This is evident across cultures that enforce laws and rules against rape. 


Males’ lack of resources and/or Lack of Sexual Access to Females

The second possible adaptation is males’ lack of resources and/or lack of sexual access to females.  A lack of alternative reproductive options correlates with the male’s inability to acquire resources (Palmer & Thornhill 67).  This is referred to as the mate-deprivation hypothesis, and is supported by evidence that rape is disproportionately committed by males with lower socioeconomic status (Palmer & Thornhill 67).  Higher rape rates are seen in lower-income houses and areas, and the lower the income, the higher the rate.  Women of lower socioeconomic levels are more fearful of rape (Palmer & Thornhill 67).  Rapists are also characterized by backgrounds of repeated frustration and failed romantic and sexual relationships.  However, rape is not an exclusive act of low-status males and the correlation between low status and crime is not limited to rape.  Rape by men with high status and abundant resources suggests that a lack of sexual restraint may correlate with the likelihood that they will commit a rape. 

            Related to the mate-deprivation hypothesis is the belief that male sexual impulsiveness and risk taking are set during development through experiences related to the environment of upbringing.  The developmental experiences felt to be most important are reduced parental investment (resulting from poverty or from the absence of the father) and a rearing environment in which social relationships in general are not enduring or committed (Palmer & Thornhill 68).  This suggests that rape may reflect a particular developmental adaptation associated with low probabilities of forming long-term relationships with women. 

            Research has found that men with a history of many partners are more physically attractive and report more rape-related behavior.  However, even if women sexually desire physically attractive men, they may receive few material benefits from mating with them.  A female’s unwillingness to mate may increase a man’s perception of her reproductive value.  If a woman’s display of reluctance is truly effective, a man who achieves copulation with her will perceive that he achieved it by force (Palmer & Thornhill 70).


Choosing Victims

            A third possible adaptation is choosing victims.  The chance that a one-time copulation will result in a viable offspring correlates with the female’s fertility.  Therefore, selection may have produced a psychological mechanism influencing males to be more likely to rape highly fertile females.  The “beauty-detection” mechanism suggests that males prefer to rape females at the age of peak fertility (in their early to mid twenties) (Palmer & Thornhill 71).  In contrast, the by-product-of-evolved-sexual-differences hypothesis predicts that men’s preference with regard to rape victims appears to be closer to the age of peak reproductive value (the mid to late teens).  Both views agree that rapists primarily target females of fertile ages.

            In most data, young women are greatly overrepresented and girls and older women are greatly underrepresented as victims of reported rape.  Research has shown that the ages of female rape victims range from 12 to 96, with the median age being 21 and 92% of victims under the age of 40 (Palmer & Thornhill 72-73).  Rapes and other sexual assaults of males by males constitute only about 1-3% of sexual assaults.



Sperm Counts

The fourth possible adaptation is sperm counts.  “Recent advances in the study of sperm competition may offer a way to search for an adaptation in human males designed specifically to promote reproduction via rape.  Such a mechanism would cause ejaculates produced during rape to differ from those produced during sex with consenting partners in a manner conducive to high probability of fertilization during rape.  Men possess ejaculatory mechanisms that adjust sperm volume in response to how long they have been apart from pair-bond mates, a variable that affects the potential for sperm competition.  If the hypothesized adaptation exists, rapists are predicted to deliver large ejaculates because rape would consistently have been consistently have been associated with high sperm competition in human evolutionary history.  The woman’s resistance during rape is expected to be perceived by the rapist as indicating that she has an investing consensual mate” (Palmer & Thornhill 74).


Patterns of Sexual Arousal

            The fifth possible adaptation is patterns of sexual arousal, which involve changes in male arousal patterns during rape relative to during other sex acts.  Males may have been selected for quicker penile arousal and ejaculation during rape than during consensual sex (Palmer & Thornhill 75).  Gaining physical control over an unwilling sexual partner could be sexually arousing to men because it facilitates rape.  However, this does not prove that men are sexually aroused by violence per se.  Rapists rarely engage in gratuitous violence, and many normal men are significantly sexually aroused by depictions of coercive sex, including depictions involving physical aggression. 

Marital Rape as a Sperm-Competition Tactic 

The final possible adaptation is marital rape as a sperm-competition tactic.  “Rapes and other sexual assaults by husbands, former husbands, boyfriends, and former boyfriends make up about a forth of all offenses in this category.  The risk to women of being raped by a current or a past mate may be at least as high as that of being raped by an acquaintance or a stranger” (Palmer & Thornhill 77-78).  Raping an unwilling married mate may be a male tactic of sperm competition.  “A woman’s sexual unreceptivity may suggest to her partner that she is having consensual intercourse with another male.  Because men associate sexual unwillingness and resistance in their long-term mates with infidelity, sexual unwillingness may lead to sexual jealousy and sometimes to rape as a sperm-competition tactic” (Palmer & Thornhill 77).  Rape of a long-term partner is likely to occur during or after a breakup that results from implied infidelity.  There is also a strong relationship between husbands who beat their wives and their sexual jealousy.

            Rape by a male of any species has a developmental background and is condition dependent, meaning that it arises from gene-environment interactions that, during development, construct the psychological adaptations that are involved (Palmer & Thornhill 79).  The conditions that affect adoption of rape by men may include limited resource holdings, social disfranchisement, limited sexual access, few rewarding romantic relationships, low phenotypic or genetic quality, and rape opportunities with high benefits and low costs to male reproductive success during human evolutionary history (Palmer & Thornhill 80).  Offenders are viewed as compensating for genes that interfere with development of psychosocial skills.  Psychopathy in men is associated with high levels of exploitation of others and with criminal behavior, including rape (Palmer & Thornhill 81-82).

            “It is theoretically possible that female sexual resistance to rapists in some species reflects an evolved mechanism for evaluation of the rapist’s ‘genetic quality’ in order to secure genes that will promote the mating success of their sons.  Pre-mating resistance on the part of a female may indirectly assess male heritable quality by testing the would-be rapist’s strength, endurance, and vigor” (Palmer & Thornhill 83).

Evolutionary theory can help us understand why rape is such a distressing occurrence.   Rape interfered with the reproductive success of our ancestors.  This interference took three forms: reduction of victim's fitness by circumventing mate choice; reduction of mate's fitness by lowering paternity security; and reduction of the fitness of relatives because of the two above factors. 


Psychological Pain

Psychological pain is the "…mental state of feeling distraught." (Palmer & Thornhill 85)  Thornhill and Thornhill (1989) hypothesized that our ancestors adapted psychological pain to focus attention on actions that decreased reproductive success.  The result of psychological pain was that our ancestors directed their attention toward avoiding situations that decreased sexual reproductivity.  Two predictions can be made from this hypothesis: psychological pain was triggered by events threatening reproductive success and the greater the threat the greater the psychological pain. 



Rape Victimization in Evolutionary Context

            There were four ways rape resulted in decreased female reproductive success.  The first was that the victim could have been physically injured.  The second was that rape limited a female's ability to choose her mate.  If the rape resulted in offspring, she invested her parental effort in offspring of lower genetic quality.  The third way rape resulted in decreased female reproductive success was that it prevented females from securing resources from a male.  Finally for a pair-bonded woman, it decreased the quality of her mate's protection and parental care.

            Human reproductive success is highly dependent on both males and females.  Rape affects this success because it makes men less willing to care for the offspring of others.  Actual or attempted rape lowers a male's confidence that previous and future offspring are his own.  Males even questioned the loyalty of women who found themselves in situations where an attempted rape occurred. 

            A woman’s perspective on rape can best be understood by considering the negative effect rape has on her reproductive success.  Psychological pain was been selected to help females end the detrimental affect rape had on their reproductive success.  The outward manifestations of psychological pain may also have developed to show mates and relatives how strong the females’ negative feelings were about rape.

Before moving on to the predictions and findings of the psychological-pain hypothesis formed by Thornhill and Thornhill (1989), it is important to have information about their research data.  The data was obtained from the Joseph Peters Institute in Philadelphia.  It included 790 rape victims (age ranging from 2 months to 88 yeas old).  The victims were all females and had reported their sexual assault.  The sample was not representative of females in the United States. 

“The females were asked to evaluate fear of being on the street alone, fear of being home alone, change in social activities, change in eating habits, change in sleeping habits, frequency of nightmares, change in non-sexual relationships with men, change in feelings toward known men, change in relations with husband or boyfriend, change in sexual relations with partner, and insecurities concerning sexual attractiveness.” (Palmer &Thornhill 88-89)


The responses to these twelve variables were measured to indicate the degree of

impact the rape had on each woman and her ability to cope psychologically.


Predictions and Findings:

Age of Victim

            The psychological-pain hypothesis predicts that women of reproductive age (12-44 years old) will suffer greater psychological distress than women who are not of reproductive age (1-11; 45-88 years old). Psychological pain should be experienced in direct relation to the amount of detriment the rape has on the female's reproductive success.  The study showed that reproductive age victims suffered significantly more psychological distress than the non-reproductive age victims.


Mateship Status

            The psychological-pain hypothesis predicts that rape adversely affects a victim’s relationship with her husband or boyfriend.  Increased psychological pain stems from the mate’s withdrawal of material support.  The study found that married women were in fact more traumatized by the rape than unmarried women.  Regardless of whether the rapist was a stranger, friend, or family member, women of reproductive age and married women in the study suffered greater psychological pain.  These results indicate that age and marital status regulate psychological pain.


Force and Violence

            Thornhill and Thornhill (1989) also hypothesized that reproductive age victims are subjected to more violent attacks.  This prediction was based on two factors.  The first is that reproductive age women are more likely to fight back because their reproductive success is at stake.  The second factor is that rapists are more highly motivated to rape a reproductive age female so are willing to use more drastic actions.

             McCahill et al. (1979) performed another study and concluded that a violent rape actually decreases the amount of physiological pain a woman endures.  The occurrence of violence during a rape actually moderates the trauma a committed reproductive-age victim feels.  Victims experience less blame and anger from their mates when visible signs of violence and trauma are apparent.  This is also consistent with the view that outward manifestations of pain were selected to communicate to the victim’s mate and relatives a rape had occurred.  These results support the earlier prediction that mateship status is one of the causes of psychological pain.


Sexual Behaviors

            Another issue examined in the study was the influence of the nature of the sexual behavior on the amount of the victim's psychological pain.  It was predicted that the women of reproductive age were more likely to be the victims of penile-vaginal intercourse, ejaculation in the reproductive tract, and repeated intercourse.  All three of these predictions were supported by the data.  It appears that men are more sexually motivated to rape reproductive age women.  Psychological pain was also expected to be greater in reproductive age women who suffered these three types of sexual behaviors. Penile-vaginal intercourse, ejaculation, and repeated intercourse cheat a reproductive woman from mate selection.  The data concluded that penile-vaginal intercourse and ejaculation increase psychological pain; repeated intercourse appeared to have no significant effect.


Proximate Causes of Psychological Pain after Rape: An Overview

            The results of this study indicate that a rape victim's psychological pain is affected by a woman's age, marital status, the amount of violence or force used by the perpetrator, and the character of the sexual behavior.  It is hypothesized that rape has been a significant enough obstacle to female reproductive success to lead to the evolutionary selection of mechanisms to avoid rape.  "There is growing evidence that psychological pain is an adaptation that defends against the circumstances that reduced the reproductive success of individuals in human evolutionary history." (pg. 95) A review by Paul Watson and Paul Andrews reveals that during periods of psychological pain an individual is completely focused on the problems or circumstances causing that pain.  (pg. 95) The individuals become more objective about themselves and their social status and become better problem-solvers.



Future Research on Psychological Pain of Rape Victims

            No research examining the psychological-pain hypothesis for rape victims from an evolutionary perspective has been reported since 1991.  Predictions from the psychological-pain hypothesis can be derived about the various emotions reproductive age women and non-reproductive age women express as time elapses.  Predictions can be made about their various behaviors, the changes that occur in the social activities, and their mateship status.  Also, if psychological pain is an evolutionary adaptation that increases  problem solving, what are the effects of psychotropic drugs?  Predictions can be made according to the psychological-pain hypothesis.  Finally predictions can be made about the factors related to psychological pain in male rape victims.  The evolutionary hypothesis about psychological pain holds great promise for discovering important cues about alleviating pain and suffering.


Female Adaptation against Rape across Species

            Evolutionary conflict over when and where rape will occur has led to the adaptation of female strategies to avoid male sexual coercion.  Rape is costly to females of many species.  Rape can be so serious as to result in injury or death.  Adaptations vary widely across species.  Some strategies include mating alliances with males, mating with males for convenience, forming alliances with male "friends," female-female coalitions, avoidance, resistance, and physiological counters.

            An example of a non-human female adaptation against rape can be seen in waterstriders.  Female waterstriders have a pair of abdominal spines that are an anti-rape adaptation.  The longer the spine is the more adept a female is at avoiding rape.  A female waterstrider can also do somersaults to get away from a male that grabs her.  These somersaults are energy expensive so he fact that they exist emphasizes the evolutionary importance of female mate selection.  Female scorpionflies also have adapted behaviors to prevent insemination by rapists.  When raped, a scorpionfly returns to sexual receptivity twice as fast as after consensual intercourse with a mate.

Ejaculate dumping is a behavior used by a variety of species to reduce the possibility of insemination by a rapist.  "Flowback" is a rejection of sperm typically connected to the lack of orgasm associated with rape.  It is possible that the capacity to orgasm in a woman may have developed in response to reducing a rapist's chance to inseminate her and to increase the chance of insemination from consensual intercourse.


Men's Evolutionary Counter-Strategies

            The clearest evidence of male counter-strategies is the high rate of fertilization that occurs during rape warfare.  In Rwanda's recent civil war, 35% of 304 rape victims were impregnated.  The number of pregnancies that result from peacetime rape varies from 1% to 33%.


Women's Rape Avoidance Behavior

            Numerous predictions can be made about women's rape avoidance behavior according to evolutionary theory and hypotheses.  In general young, fertile women are the most adept at detecting rape-relevant cues.  The menstrual cycle also influences a women's ability to assess rape risks.  During the mid-cycle of menstruation women are more likely to avoid activities that would put them at risk of rape.  It has also been hypothesized that the social and economic advancement of women has fueled the focus of the women's liberation movement on the prevention of sexual assault.  Women have less male protection and greater mobility currently so there may be more of a need to socially protect one another from the occurrence of rape.



            Females show evolutionary adaptations against rape.  Psychological pain appears to be an adaptation against the loss of reproductive success that is caused by rape.  Evidence for this derives from studies that show the degree of psychological pain depends on a woman's age, her mateship status, the nature of the sex act, and whether or not there is physical evidence that the copulation was not consensual.  Rape victims can lose the material and emotional support of their mate. Women in pain have enhanced problem-solving abilities, are more fearful of rape, and are more likely to avoid risky situations.










CHAPTER 3: Why Do Men Rape?


I. Rape circumvents the female’s choice of a mating partner

A.     Selection pressures favor traits in males that increase their sexual access to


B.  Rape overrides the mating obstacle of female choosiness.

II. Rape as a type of Sexual Selection

A.  Sexual coercion is the third type of sexual selection (besides mate choice and

                 intrasex competition).

B.     A variety of female traits have evolved in order to reduce the costs of sexual


III. Human Rape: Adaptation or By-Product?

            A. Human Rape as an Adaptation

1. Rape may be an adaptation that was favored by selection because it

    increased male reproductive success.

2. In order for this to be true, however, there needs to be evidence of a

    phenotypic feature in males designed specifically to rape.

3. Example: the notal organ of the scorpionfly

B. Human Rape as a By-Product

1. Rape may be a by-product of other psychological adaptations.

2. It could be a by-product of the intense sexual desires of human males      

    and the sexual choosiness of human females.                                  


IV.  Potential Adaptations for Human Rape


A.     Evaluation of vulnerability


1.      Rape is expected to occur when potential benefits exceed potential costs


B.     Males’ lack of resources and/or lack of sexual access to females


1.      Mate-deprivation hypothesis: Lower socioeconomic status causes men to rape


2.      Rapists have frustration and failed romantic and sexual relationships


C.     Choosing victims


1.      Men are more likely to rape females at peak fertility or at peak reproductive value


2.      Median age of female rape victims is 21;  92% of victims are under age 40


D.     Sperm counts


1.      Men may ejaculate differently during rape than during consensual sex


E.      Patterns of sexual arousal


1.  May have quicker penile arousal and ejaculation during rape than consensual sex   


F.      Marital rape as a sperm-competition tactic


1.      Rape by husbands or boyfriends make up ¼ of sexual assaults


2.      Female unreceptivity to sex may suggest infidelity and lead to jealousy and rape


V.  Human Rape as a Conditional Tactic


A.     Rape is “condition dependent” -  many things cause it


VI.  Does Female Choice Favor Rape in Some Species?


A.     Female resistance to rape may reflect evaluation of the rapist’s genetic qualities




I.  Rape as an Interference in Reproductive Success


            A.  Circumventing female mate selection


            B.  Lowering male's paternity security


            C.  Reduction in fitness of offspring, mate, and relatives


II.  Psychological Pain


            A.  Definition


            B.  Hypothesis


            C.  Result


            D.  Predictions


III.  Rape Victimization in Evolutionary Context


            A.  4 ways rape decreases female reproductive success


            B.  Male perspective


            C.  Data of Study


                        1.  790 victims (2 months to 88 years old)


                        2. Not representative of females in the U.S.


                        3.  12 variable measured


D.  Predictions and Findings


                        1. Age of victim


a. Prediction… reproductive age women suffer greater

                                        psychological distress


                                    b.  Data confirms prediction





2.  Mateship status


                                    a.  Prediction … rape adversely affects relationship with mate


                                    b.  Mates remove support


                                    c.  Data confirms prediction


                        3.  Force and violence


a.       Prediction… reproductive age victims suffer more violent



                                    b.  Why?


                                                i.  Reproductive age women fight back


                                                ii.  Rapist more sexually motivated


c.       McCahill study: violence decreases psychological pain


d.      Results confirm all three previous predictions (force, mateship


status, and age).


                        4.  Sexual Behaviors


a.  Prediction… women of reproductive age are more likely to be   

     the victim of penile-vaginal intercourse, ejaculation into the

     reproductive tract, and repeated intercourse


b.  Results confirm first two parts of prediction


IV.  Proximate Causes of Psychological Pain after Rape:  An Overview


            A.  Age, mateship status, violence, behavior


            B.  Psychological pain an adaptation to rape


B.     Psychological pain: more objective about self and society and better problem  



V.  Future Research on Psychological Pain of Rape Victims


A.     Future research comparing victims of  reproductive age to victims of non-  

reproductive age women


            B.  Emotional, behavioral , social, and sexual comparisons


            C.  Male rape victims?


VI.  Female Adaptation to Rape across Species


            A.  Possible adaptation strategies


            B.  Waterstriders and scorpionflies


            C.  Ejaculate dumping and "flowback"


VII.  Men's Evolutionary Counter Strategies


            A.  Warfare vs. peacetime rape


VIII.  Women's Rape Avoidance Behavior


            A.  Young, fertile women adept at recognizing rape cues


            B.  Menstrual cycle


IX.  Summary












A Natural History of Rape: Biological Bases of Sexual Coercion

Critical Review


Interesting points:


(a )       From a female perspective, the notion that rape is explained and validated by

evolutionary history is an interesting, yet frightening thought. Palmer and Thornhill justify a male’s desire to sexually coerce a female. Whether it is the result of an adaptation or a by-product of a psychological adaptation, rape is designed to ensure reproductive possibilities to males who would otherwise be overlooked by females. Regardless of this reasonable explanation, rape remains to be a terrifying thought for any human female.


(a)                The point that a female’s unwillingness to mate may increase a man’s perception of her reproductive value.


(a)        I found the study conducted by Palmer & Thornhill to be particularly interesting and a little disturbing.  These researchers concluded that violence accompanying rape actually decreases the level of psychological pain the victim feels.  It was a surprising result even for the researchers who had expected a positive relationship between violence and trauma.  It was predicted that a victim with physical evidence of a rape occurring had less difficulty convincing others that the experience was not consensual.  My immediate reaction would be to think that the more violent the experience the more traumatized I would be.  However,  researchers believe that psychological pain actually decreases because their husbands or boyfriends are less blaming and judgmental. 





Points of disagreement:


(b)              According to Palmer and Thornhill, males are more likely to rape when potential costs are low. This theory of rape is not necessarily true, however, because rape is far too common in our society. Men do not wait until they believe the costs associated with forced copulation are ideally low. Instead, they act upon their intense sexual instincts. Of course, this is not true for all human males but limiting the occurrence of rape to only situations of lowered potential costs is a mistake. Perhaps Palmer and Thornhill should elaborate more on this claim.


(b)        The statement that female sexual resistance to rapists in some species reflects an evolved mechanism for evaluation of the rapist’s genetic quality.  It seems as though there would not be any reason for a female to want to be raped.  There are many other ways for a female to assess the reproductive qualities of males.


(b)        I believe the case made by the authors regarding the development and cause of

psychological pain was not a complete explanation of why women would feel that way if raped.  I understand the theory that  historically women were trying to protect their reproductive success.  However, I feel when we are applying this hypothesis to modern humans a little consideration needs to be paid to the fact that emotionally and mentally rape is a devastating experience.  A woman is robbed of control of her own body.  I wish that the authors had made some sort of reference to these issues when arguing their evolutionary, Darwinian hypothesis.

Points that could have been explained further:


(c)        In regards to the discussion of potential ultimate causes of human rape, located in Chapter 3, one might question why Palmer and Thornhill chose to include these null hypotheses into their argument. Ultimate causes are broken down into phylogenetic holdovers and evolutionary agents. The holdover view of rape is invalid because it describes a pattern of evolutionary continuance of a trait, but it doesn’t identify the ultimate cause of the continuance. Similarly, the evolutionary agents are not clearly defined or detailed in this section of the book. This portion of the text was very vague and difficult to comprehend. Perhaps it was even a waste of space because the authors later explain that there can only be two likely candidates for ultimate causes of human rape: adaptation or by-product.


(c)        The concept that there may be a mechanism that would cause males to ejaculate differently during rape than during consensual sex, resulting in a high probability of fertilization.


(c)        After discussing the proximate causes of  psychological pain caused by rape, the authors went on to describe a study that claimed individuals in psychological pain are more focused on their problems, more objective, and better problem solvers.  I would like to have this claim further explained because I think it would be very interesting to understand how such clarity and strength results from depression and pain.



1.   Evaluation of vulnerability

2.   Males’ lack of resources and/or lack of sexual access to females

3.   Choosing victims

4.   Sperm counts

5.   Patterns of sexual arousal

6.  Marital rape as a sperm-competition tactic














                                                                                                            Overhead Slide






























                                                                                    Overhead Slide







·      Rape overrides the mating obstacle of female




·      Sexual Coercion



·      Human Rape as an Adaptation



·       Human Rape as a By-Product












                                                                                                       Overhead Slide


A Natural History of Rape: Biological Bases of Sexual Coercion





·       Rape is validated from an evolutionary perspective


·       Female unwillingness to mate increases her reproductive



·       Violence accompanying rape decreases the victim’s

 psychological pain





·       Males rape when potential costs are low


·       Female sexual resistance to rape is an evolved mechanism for evaluation of the rapist’s genetic quality


·       Psychological pain theory





·       Potential ultimate causes of human rape


·       Males ejaculate differently during rape


·       Individuals in psychological pain are more objective and

focused on their problems

                                                                                                            Overhead Slide