Thornhill.1992.rape

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Unformatted text preview: BEHAVIORAL AND BRAIN SCIENCES (1992) 15, 363-421 Printed in the United States of America , The evolutionary psychology of men’s coercive sexuality Randy Thornhill- and Nancy Wilmsen Thornhillb -Depam-nenr of Biology and bDepanmenrs of Biology and Anthropology, University of New Mexico. Albuquerque, NM 871314091 Electronic mail: [email protected] Abstract: Psychological adaptation underlies all human behavior. Thus, sexual coercion by men could either arise from a rape-specific psychological adaptation or it could be a side—effect of a more general psychological adaptation not directly related to rape. Determining the specific environmental cues that men’s brains have been designed by selection to process may help us decide which of these rival explanations is correct. We examine six testable predictions against existing data: (1) Both coercive and noncoercive sex will be associated with high levels of sexual arousal and performance in men. (2) Achieving physical control of a sexually unwilling woman will be sexually arousing to men. (3) Young men will be more sexually coercive than older men. (4) Men of low socioeconomic status will likewise be more sexually coercive. (5) A man’s motivation to use sexual coercion will be influenced by its efiects on his social image. (6) Even in long-term relationships men will be motivated to use coercion when their mates show a lack of interest in or resistance to sex because these are interpreted as signs of sexual in fidelity. Current data support all six predictions and are hence consistent with the rape-specific hypothesis, but this does not eliminate the side-efl'ect hypothesis, which is likewise compatible with the findings, as well as with the further evidence that forced matings increased the fitness of ancestral males during human evolution. We suggest some research that may help decide between the two hypotheses. Keywords: aggression; comparative psychology; evolution; psychological adaptation; rape; sex difi'erences; sexual coercion; sexual dimorphism; sexnal selection; socialization; sociobiology 1. Introduction Is rape just a side-effect of psychological adaptations to circumstances other than rape, such as a desire for sex coupled with a general coercive tendency, or does it arise directly from an evolutionary adaptation to sexual coer- cion itself?1 The answer to this question has practical as well as theoretical consequences: It can help us identify the kinds of social contexts that discourage coercive sex. We hypothesize that sexual coercion by men reflects a sex—specific, species-typical psychological adaptation to rape: Men have certain psychological traits that evolved by natural selection specifically in the context of coercive sex and made rape adaptive during human evolution. The hypothesis that there is a psychological adaptation specific to sexual coercion by men was implicit in some earlier evolutionary writings (Alexander 1979a; Alex- ander & Noonan 1979; Shields & Shields 1983; Thornhill 45: Thornhill 1983). These authors focused directly on rape and dealt only superficially with the mechanisms that regulate men’s sexual arousal and their inclination to use force. Sexual coerciveness alone is not evidence of a rape— specific psychological adaptation, nor do men have any specialized morphology that might have evolved for rape. One form of positive evidence would be if environmental cues that were specific to the costs and benefits of sexual coercion during human evolution still regulated contem- porary coerciveness. The rape-specific hypothesis is examined in the light of e 1999 Cambridne University Press 0140-525X/92 $5.00+.OO information on the natural history of human sexual be- havior and published laboratory studies of men’s sexual arousal in response to sexually—explicit stimuli. The hy- pothesis is not contradicted by current information on human coercive sexuality, but neither is the rival side- effect hypothesis. We outline some research that may help decide between the two. 2. Adaptationism In this target article and in theoretical biology in general adaptation refers to compleny organized, purposefully designed, phenotypic features of individual organisms that exist because they solved a specific environmental problem during evolutionary history (e.g., Burian 1983; Darwin 1859; Dawkins 1986; Symons 1979; Williams 1966; see R. Thornhill 1990 for review). Four natural processes are known to cause evolution or changes in gene frequencies of populations, but selection is the only one that can create an adaptation. The other three — mutation, drift, and gene flow — lack the necessary creativity because their action is random relative to indi- viduals’ environmental problems. Selection, when it acts in a directional, cumulative way over long periods of geological time, creates complex phenotypic designs out of the simple, random genetic variation generated by the other three natural processes. Selection itself is not a random process. It consists of differential survival and 363 ,-_,, _______ -- -__-________. ._ . --_~.-.. -- -v.._-. vvv.v-v.. reproduction by individuals because of differences in their phenotypic design. 3. Psychological adaptationism and rape Human psychological adaptations are information- processing mechanisms providing solutions to problems that influenced the survival and reproductive perfor- mance of individuals during our species’ evolutionary history. Human psychological adaptations are specially engineered to process specific environmental information and to guide feelings, emotions, learning, and behavior toward specific reproductive ends.2 Perception, memory storage and retrieval, cognitive analysis, and so forth are evolved information-processing mechanisms and must be characterized in functional evolutionary terms, that is, by the kinds of information they are designed by selection to process rather than by neurophysiology or neuroanatomy (also see Cosmides & Tooby 1987; 1989; Tooby & Cosmides 1989). Psychologi- cal adaptation must somehow (nusally underlie all human feelings, emotion, learning, and behavior (see Cosmides 6t Tooby 1987; Symons 1987a). To determine whether or not there is a rape-specific adaptation and to begin to characterize its design we must examine how men ’5 sexual information-processing mech- anisms regulate their sexual arousal and behavior in the context of sexual coercion. An empirical focus on rape behavior rather than on rape psychology is not appropri- ate, because neither the fact that men do rape nor the coercive and sexual components of an act of rape can be taken as evidence for (or against) the hypothesis that men are adapted to rape per se. There is nothing about rape behavior itself (in humans or nonhuman species) that could indicate whether or not there is rape-specific de-‘ sign. The physical force associated with human rape did not evolve in the context of forced copulations; aggressive behavior is not limited to rape and is used by children and adults of both sexes. Likewise, it is unlikely that men’s tendencies to use threats of physical force or other forms of coercion to gain sexual access to unwilling partners were specifically selected because they provided a re- productive advantage in the context of rape. Nor, as noted earlier, do men have a morphological specialization that is a candidate for rape adaptation. Phenotypic features specific to sexual coercion have been demonstrated in males of only a few species of nonhuman animals, and in all cases these involve morphology (e.g., an abdominal clamp for holding the wings of sexually unwilling females, Thomhill 1984a; see Thomhill 6: Sauer, 1991, for review). Whether- or not men are specifically adapted to do it, rape cannot be fully understood without explicitly exam- ining men’s sexual psychology because the act of rape involves psychological changes (emotions, arousal, etc.) and behavior, and all psychological changes and behavior are the result of information processing by the nervous system. The discovery of the environmental cues that men’s brains are designed to process will help us under- stand the environmental conditions influencing all as- pects of men’s sexuality. We are implicitly assuming that men and women differ in sexual psychological adaptation. It is often assumed in 364 BEHAVIORAL AND BRAIN SCIENCES (1992) 15:2 the social and behavioral scienccs that human psychology is the same for both sexes and consists of only a few general-purpose learning adaptations (Symons 1987a). However, given current knowledge of the fimctional specificity of the vast numbers of adaptations whose design is understood (e.g., the human heart is specially designed to pump blood in a human body), as well as theoretical advances in understanding how selection works in molding adaptations (they are specific solutions to specific environmental problems), it is most likely that psychological adaptation to sexuality is sexually di- morphic and includes many, highly specialized adapta- tions. Some involve learning and are designed to process information specific to the very different sexual problems faced by males and females in human evolutionary history (see Symons 1987a; 19871), for a detailed analysis and critique of the idea that human sexual psychology is monomorphic and consists only of a few general-purpose adaptations). 3.1. Rape and social learning. “Learning” is often invoked in the social science literature on rape as if it were a complete explanation for men’s and women's sexual be- havior (for a useful review, see Ellis 1989; also see Russell 1982; 1984). Such a view is based on the misunderstand- ing of two fundamean facts of developmental biology: (I) “Learned” behaviors result from causal gene—environment interactions during ontogeny. (2) A vast number of experi- ences necessarily occur during the ontogeny of any be- havior; only a very small number of these would be “learned.” The term “learned” is far too simplistic to warrant its use as the major or only causal explanation of any human behavior (see also Alexander 1990; Cosmides & Tooby 1987; Daly 6: Wilson 1987; 1988; Symons, in press). Furthermore, learning and socialization are never alternatives to evolutionary hypotheses about psychologi- cal adaptation (e.g., see Cosmides & Tooby 1987). The “social learning theory of rape” and the “feminist theory of rape" are very similar (for a review of literature, see Ellis 1989). According to both, rape is primarily or solely caused by arbitrary differences in the way men and women are socialized about heterosexual conduct. If their socialization were the same, they would be equally likely to rape. Both theories posit that rape results from general sex differences — such as men being more motivated than women to use aggression and to be noncommittal in sexual relations — and that these general sex differences are solely or primarily caused by contingent differential socialization. But general sex differences similar to those outlined by these theories of rape — for example, that males are more aggressive, sexually assertive and eager to copulate, and less discriminating of mates - occur not only in humans, but in all animal species with an evolu- tionary history of polygyny (see Daly & Wilson 1983; Thomhill 1986; Thomhill & Alcock 1983; Trivers 1972; 1985).3 In the vast majority of these species there is no sexual training of juveniles by other members of the group (e.g., all polygynous invertebrates), and none have the extensive sexual socialization seen in humans (see Low 1989). Hence the human sex differences common to polygynous species cannot be parsimoniously fully ex— plained by a difference in sexual socialization in humans. The hypothesis of rape-specific adaptation does not assume that there is no learning or sex-specific socializa- tion in the development of human sexuality. Indeed, learning might be important during the ontogeny of the rape-specific adaptation itself. If so, the learning would not be arbitrary but would be guided by special-purpose psychological adaptations that influence perception, cog- nition, memory, and information evaluation in a way that is specific to rape. 3.2. Rape's current effects on reproduction. The extent to which sexual coercion currently promotes the reproduc— tive success of men is not of central importance to the approach emphasized in the target article. Indeed, data about offspring production by victims of sexual coercion are irrelevant to the rape-adaptation hypothesis. Thus our approach is very different from that of Ellis (1989) and Thiessen (1989), who consider the current reproductive consequences of rape to be of fundamental importance. An adaptation is identified and characterized by its evolu- tionary function, what it has been designed by selection to do. The relationship between an adaptation and current reproduction depends on the similarity between the environment in which the adaptation is expressed cur- rently and the environmental features that generated the selection that designed the adaptation. Often this correla- tion no longer exists for contemporary organisms (perhaps especially humans). As a result, adaptations are not distin- guished from nonadaptations by their effects on current reproduction: A phenotypic trait can influence current reproduction negatively but still be an adaptation — a product of long-term directional selection for a solution to a fitness-related ecological problem — and a trait can influence current reproduction positively and not be an . adaptation. In addition, the effect of an adaptation on current reproduction does not identify the evolutionary function of the adaptation because the current environ— ment may difier from the evolutionary environment that generated the selection that designed the adaptation (see also Williams 1966; R. Thornhill 1990). 3.3. Rape in nonhuman animals. One cannot derive evi- dence for or against the existence of human rape adapta- tion from male sexual adaptations in nonhuman species because the selective environments that produced hu- man psychological adaptations were unique to our spe- cies. This is why we do not discuss here the few nonhu- man species in which there is evidence for the existence of rape adaptation (e.g. , certain morphological structures in male scorpionflies, Thornhill 1984a; Thornhill & Sauer 1991). The nonhuman evidence shows that selection in the context of sexual coercion by males can, under some conditions, produce rape adaptation, but it cannot dem‘ onstrate that such selection was effective in designing rape adaptations in other species (e.g., Homo sapiens) in which rape adaptation has not been demonstrated. The only way to provide evidence about the selection pres- sures that designed humans is to identify and characterize human adaptations (see also Symons 1982; Tooby 8r Devore 1987). 3.4. Heritability. In the literature on human rape there is considerable interest in rape heritability (see Ellis 1989, eSpecially pp. 86—88). Heritability refers to the extent to which the variation in a phenotypic trait among individ- Thomhill 6r Thornhill: Evolution or sexual coercron uals is caused by genetic as opposed to environmental variation.4 The concept of heritability is of value primarily in programs to improve domestic plants and animals by artificial selection (Falconer 1981; also see R. Thornhill 1990). [See also Wahlsten “Insensitivity of the Analysis of Variance to Hereditary-Environment Interaction" BBS 13(1) 1990; Plomin & Bergman “The Nature of Nurture" BBS 14(3) 1991.] We emphasize that the rape-adaptation hypothesis does not imply that rape is heritable. That is, it does not predict that variation among men in inclination to rape reflects genetic difierences. Instead, it implies that men have psychological traits whose underlying genes are virtually fixed or invariant in the human gene pool; this is what is meant by “sex—specific" and “species-typical." Thus, we predict that the heritability of the psychological adaptation for rape will be near or at zero. This is not to say that male personality features that may contribute to rape (e. g. , aggressiveness) are not heritable; some may be (see Ellis 1989, pp. 86-97), but the heritability of person- ality features is not relevant to the question of whether there is a specific adaptation to rape. According to the rape- daptation hypothesis, men’s use of sexual coercion will not vary with differences in genetic conditions but with environmental conditions (see also Shields & Shields 1983; Thornhill 8r Thornhill 1983). The hypothetical rape-specific adaptation should regulate men's use of force in a facultative way by process- ing environmental information that was associated with engaging in rape during human evolution (Crawford 8: Anderson 1989; Tooby 6: Cosmides, 1990, provide de- tailed discussions of why psychological adaptation will have negligible heritability). That rape adaptation is species-typical does not exclude the possibility of minor geographical variation in the design of its underlying psychological traits. For example, selective differences between human populations could have led to differences in how easily the rape adaptation is activated by environmental cues. Even for minor differ- ences between populations to evolve by selection, how- ever, it is necessary that there have been a long period of low or no migration between populations and cumulative directional selections that differed between populations. 4. Coercion and men’s mating strategy We dissect the overall human male reproductive strategy to point out where we feel rape fits into it (see Thornhill 6r Thornhill, 1983, for detailed discussion). The reproduc- tive strategies of all organisms, including men and women, have the following two components: mating effort (the reproductive effort associated with conceiving offspring) and nepotistic cfibrt (the reproductive eEort associated with aiding ofispdng and other relatives). Men’s mating effort consists of obtaining matings, keep- ing mates, and enhancing paternity confidence with mates (see also Buss 1988; Daly 6: Wilson 1983; Flinn 1988). [See also Hartung: “Matrilineal Inheritance” BBS 8(4) 1985.] The effort associated with obtaining a mate can be called men’s mating strategy. This can be conceptually partitioned into three tactics (Shields 6: Shields 1983): (1) honest advertisement and courtship, (2) deceptive adver- tisement and courtship, and (3) coercion. Coerced mat- BEHAVIORAL AND BRAIN SCIENCES (1992) 15:2 365 Thomhill a: Thomhill: Evolution of sexual coercion ings or rapes are achieved by physical force or by explicit or implicit threat of physical harm or negative social consequences. In humans, there is a large sexual asymmetry in the minimal reproductive effort required for the production of offspring. The minimum for a man is a few minutes of time and an energetically cheap ejaculate; the minimum for a woman is nine months of pregnancy and a long period of lactation. Because of this sexual asymmetry, during human evolution males who could gain sexual access to multiple females out-reproduced males who could not and thus were favored by sexual selection. Because women’s sexuality was not molded by the same selection as male sexuality, three complementary human sex differences are observed: Compared to women, men are less discriminating about sexual partners, more moti- vated to seek copulation with many partners, and more eager to include copulation as part of an interaction with the opposite sex. Sexual selection on females in human evolutionary history favored individuals who could gain access to males whose resources and genetic endowment could promote the survival of offspring. Selection pres— sure on females to compete for multiple mates was weak relative to that on males, because sexual access to multi- ple males has little eEect on the number of offspring a female can produce, whereas access to multiple females can have a great effect on the number of offspring a male can produce. Women are more selective of mates than men because in human evolutionary history females made a larger minimal investment in ofispfing (thus losing more reproductive potential than males from a _ poor mate choice), which resulted in stronger natural selection on females than on males for mate choice. Furthermore, females are the objects of more sexual competition than males and thus have a greater oppor- tunity to choose effectively (see Alexander 1979a; Buss 1987; Daly 6: Wilson 1983; Smith 1984; Symons 1979; 1987b; Symons 6: Ellis 1989; Townsend 1987; 1989). Because of the different ways that selection acted on the sexes during human evolutionary history, evolution- ary psychologists believe human sexual psychology is dimorphic, that is, the respective adaptations differ in men and women (Symons 1979; 1987b). The evolved sex difference in mating strategy leads to differences in how men and women feel about whether, when, and how often it is in their interest to mate. Because women are more selective about mates and more interested in evaluating them and delaying copulation, men, to get sexual access, must often break through feminine barriers of hesitation, equivocation, and re- sistance (see Kirkendall, 1961, for a review of human heterosexual sexual interactions). Men get women to copulate by using all three tactics mentioned above. As we show later, these tactics can be used either singly or in combination and they often blend into one another. 5. Selection for rape According to the rape-adaptation hypothesis, during hu- man evolutionary history there was enough directional selection on males in favor of traits that solved the prob- lem of forcing sex on a reluctant partner to produce psychological inclinations specifically toward rape. Here are some reasons why it seems reasonable to assume that ancestral human males sometimes increased their re- productive success by rape.5 1. There is strong evidence that the following mas- culine traits are evolved ones: Compared to women, men are more aroused by visual and fantasized sexual stimuli; men are more indiscriminate about mating partners; and men are more likely to infer the presence of sexual interest in a potential mate when there is no such interest (Abbey 1982; Abbey & Melby 1986; Kinsey et al. 1948; 1953; Saal et al. 1989; Symons 1979; Townsend 1987). 2. There is strong evidence that women’s selectivity reflects adaptation to the circumstances of mate choice per se (see Buss 1987; 1989a; Symons 1979; T0wnsend 1987; 1989). Women’s traits and the male traits men- tioned in (1) evolved, indeed coevolved, in the same evolutionary environment. Thus, sexually attractive females who were sexually uninterested and resisted the sexual advances of males are likely to have been a consis- tent feature of the human evolutionary environment; this produced directional selection favoring males whose sex- ual arousal did not depend on the sexual interest or arousal of a potential mate. 3. There are also costs to males associated with trying to mate by force that, all else equal, would have made rape maladaptive in the human evolutionary environ- ment. As discussed by Shields and Shields (1983) and Thomhill and Thomhill (1987), potential injury to the male and to his family members, loss of status or re- sources, and other factors inevitably contribute to the costs of rape. Given the cost of forcin g matings, one might have expected selection to counteract (1) above and in- stead favor males who were sexually interested only in sexually interested females. This is not the inclination that has evolved in men, however; as we show below men use sexual coercion and show high levels of sexual arousal in response to both coercive and mutually consensual sexual materials in laboratory settings. Hence there seems to be no specific psychological trait that is spe- cialized to prevent men from using force to mate. Thus, despite the opportunity for selection to act against males who were sexually coercive'and despite the costs that should have made any such selection strong and effective, men are commonly sexually coercive, and their arousal to depictions of sexual acts in the laboratory is unaffected by whether or not force is used. In evaluating the potential advantage of forced sexual intercourse during human evolution, it is important to consider the survival prospects of the offspring from such matings. Psychological adaptation for parental care in both men and women confirms that offspring required a large amount of parental care during human evolution (see Daly 8: Wilson 1988). Alexander and Noonan (1979) have suggested one way rape may have contributed to successful ofispring produc- tion in human evolutionary history: A female's sexual unwillingness could be evidence that she was pair— bonded, and thus that the rapist's offspring would be unwittingly reared by the cuckold. Raped females would be reluctant to reveal to their mates that they had been raped because this would make their mates cooperate less in the care of existing oEspring (for an analysis of paternity and cuckoldry in humans, see Alexander 1979b; Buss 1988; Daly & Wilson 1988; Daly et al. 1982; Dickemann -n-qr . v .quwaqmwam mow-mu“ long-term mates. Sexual uninterest or resistance may reflect interests outside the pair-bond; if so, men's moti- vation to use forced copulation within mateships may be an evolved strategy for ejaculate competition with the sexual rival. It is important to consider the alternative possibility that men’s ready capacity for sexual arousal with both willing and unwilling (see sect. 7 and 8) sexual partners evolved as a result of selective factors other than success- ful reproduction through rape. Sexual coercion might be a side effect, not adaptive in its own right (perhaps even maladaptive), of two more general adaptations: (a) the psychological mechanisms underlying men’s general de- sire for sexual intercourse, and (b) the mechanisms under- lying our species' general tendency to use force to attain any reward that was correlated with successful survival and reproduction in our evolutionary history. According to this side-effect hypothesis, there are no psychological mechanisms specifically designed for processing informa— tion about rape. 6. Predictions In this section, we outline several predictions that follow from the rape-specific hypothesis. The list is not exhaus- tive; it focuses on what can be tested against the existing literature on human sexuality. Prediction 1. Men will exhibit high levels of sexual motivation and performance in both coercive and non- coercive mating situations. If men have a rape-specific adaptation, then their sexual arousal and ability to copu- late and ejaculate should not be aEected by whether the woman has given her consent. In contrast, the rape- adaptation hypothesis would be falsified by evidence that men are sexually aroused and competent only or pri- marily when they perceive that a woman is interested or not resisting coitus. Data supporting the first prediction would suggest an adaptation for rape, but demonstrating it would require direct evidence of rape-specific design so as to eliminate the rival side-effect hypothesis. In evolutionary theory, individual “interests” are based on evolved reproductive interests (Alexander 1987; Daly 8: Wilson 1988). This suggests that men will pursue copulation by force when it serves their evolved interests; a woman’s interests should be considered by a man only to the extent that they coincide with his own. Interests may coincide to varying degrees. The smaller the overlap of interests with the partner, the greater the probability of sexual coercion by men. Predictions 2 to 6 are based on the conflicting reproduction interests of the sexes.. Prediction 2. Gaining physical control over an unwill- ing sexual partner by force should be sexually arousing to men because it facilitates forced copulation. As long as the victim of sexual assault is capable of resistance, there are a risk of injury to the assailant and a potential for thwarting of the assault. Rape-specific selection would have gener- ated mechanisms for assessing such risks and regulating male motivation and behavior accordingly. In the context of coercion, male sexual arousal has a cost; it might Thomhill 6t Thomhill: Evolution of sexual coercion interfere with his ability to detect the presence of danger- ous conspecifics or predators. In contrast, the side’effect hypothesis does not predict that gaining control of an unwilling partner will be sexually arousing. Prediction 3. A man’s age should afi'ect his willingness to use sexual coercion. Because in our evolutionary past young men (mid-teens to early 205) were trying to enter the breeding population for the first time, they engaged in the most intense mate competition, and therefore took the highest risks and experienced the highest mortality (Alexander 1979b; Thomhill 6t Thomhill 1983; Trivers 1985, Chapter 12; Wilson 8: Daly 1985). Young males’ difficulty in entering the breeding population probably arose because older males controlled the resources, sta- tus, and politics, and thus monopolized the females. The opportunity cost of engaging in risky behavior is lower for younger men and therefore mortality is greater for young men than for older men (see also Thomhill & Thomhill 1983). The side-efiect hypothesis predicts the same age- - dependence of sexual coercion as the rape—adaptation hypothesis. Many types of male sexual activity show a major peak in the mid-teens and early 205 (Cagnon 1977; Kinsey et al. 1948); young men are more quickly and easily aroused by explicit sexual stimuli in laboratory settings (e.g., Langevin 1983). Young men are also most prone to exhibit violence in general (Alexander 1979b; Daly & Wilson 1988; Wilson & Daly 1985). Hence greater sexual coerciveness in young men could be interpreted as a side-effect of their greater libido and their greater tendency to use violence to get what they want. Prediction 4. Men's willingness to use sexual coercion should be related to their social status. Women-prefer as mates men of high social and economic status (Buss 1987; 1989a; Symons 1979; Townsend 1989). Sexual access to preferred mates (young and attractive) is thus positively correlated with the status, resource holdings, and pres- tige of a man. This correlation has been demonstrated repeatedly in industrial societies (Buss 1987; 1989a), traditional societies (Betzig 1985; Betzig et al. 1988), and in the historical record (Betzig 1985; Voland & Engel 1990). We have predicted that men of low socioeconomic status will be more inclined to rape (Thomhill & Thomhill 1983) because they have less access to preferred mates, but because the poor are also more prone to crime and other forms of violence, a positive outcome would like- wise be consistent with the side-eifect hypothesis. Prediction 5. Sexual coerciveness will be very sensitive to the probability of detection and negative social conse- quences or punishment. Men’s willingness to use force will be constrained by the countervailing desire of dis- playing sexual “morality.” Lack of concern for social image will promote sexual coercion regardless of age and social status (also Shields & Shields 1983; Thomhill &Tllomhill 1983). Prediction 5, like Predictions 1, 3, and 4, follow from both the rape-specific and the side-effect hypoth- eses. Evidence supporting Prediction 5 could be inter- preted as a side-efiect of men’s adaptation for copulation and species-typical adaptation for regulating the personal use of force in general. Predictions 2 to 5 are based on the conflicting re~ productive interests of the sexes. For example, we expect men who are young and of low socioeconomic status to be more coercive sexually than older men of high socioeco- BEHAVlOFiAL AND BRAIN SCIENCES (1992) 15:2 367 Thomhill & Thomhill: Evolution of sexual coercion nomic status because the interests of the former conflict more with those of women, who prefer mates with re- sources and status. Also, a man’s reputation is determined by how others perceive him, which is in turn influenced by what they know of his social exploitativeness. Hence a man’s interest in enhancing his social reputation may sometimes be served by not coercing sexually unin- terested women. Now consider congruence and noncongruence in the reproductive interests of pair-bonded mates. Two impor- tant factors that promote similarity of interests between men and women in mateships are shared genetic ofispring and the woman's sexual fidelity; these two factors are interrelated. Men must typically provide resources and other benefits, because women’s sexual interest is tied to men’s status and resources (e.g., Buss 1987; 1989a; Sy- mons 1979; Townsend 1989). When a man provides abundant resources to a mate and her offspring her reproductive interests are served. Under such conditions conflicts of interest (thus coercion of one party by the other) should be minimal. Infidelity by a pair-bonded woman leads to uncertainty about paternity in her mate; their reproductive interests consequently diverge. When women perceive that their reproductive interests are not being served by a long—term mate, they may show him less emotional commitment and sexual interest and seek other mates. Men may find women’s sexual arousal excit- in g because it is a sign of exclusive sexual access and hence a high probability of siring that mate’s offspring. Pair- bonded men may have accordingly evolved to view a lack of sexual interest from a mate as a sign that she may have another mate. This leads to the last prediction in this section. Prediction 6. A pair-bonded man will be most likely to .' use sexual coercion with an unwilling pair—bonded mate when he suspects or discover infidelity. Again, if this prediction is met, rape adaptation is not yet demonstrated because the outcome could be a side-effect of adaptation to enhance paternity reliability and to use force to attain desired goals. In sections 7 to 11 we examine the available data on the six predictions just outlined. These predictions put the rape adaptation hypothesis in jeopardy; although the failure of even one of them would falsify the hypothesis, only Prediction 2 can falsify its rival, the side-effect hypothesis, providing direct evidence of adaptation to rape. The final section of the target article outlines some additional predictions that, if met, could eliminate the side-effect hypothesis and provide decisive evidence of rape-specific adaptation. 7. Coercive sexual behavior by men In this section we analyze the natural history of coercive sexual behavior. We show that coercion is a significant component of men's sexual activity in general. Men frequently pursue sexual access by using a combination of coercion and noncoercion as indicated by the following: 1. Homosexual rape is a common occurrence in men's prisons and is not usually perpetrated by men with a strong homosexual orientation; it is usually the act of heterosexually oriented men who lack access to preferred 368 BEHAVIORAL AND BRAIN SCIENCES (1992) 15:2 sexual partners (for reviews, see Loc able to assun‘ cacco 1975). \creased i" 2. Rapists who become visible to pth- at?! nd up in crime statistics are frequently found to have pair-bonded with one woman and raped another woman, a nonpair- bond mate. Among convicted rapists 19% to 43% are married at the time of the rape or have been previously married, and a much larger percentage have been in- volved in (presumably less coercive) pair-bond mateships (see Thomhill 8: Thomhill 1983). (The majority of rapes that become visible to police involve strangers; McDer- mott 1979; Russell 1984). 3. Many men use both noncoercive and coercive means to copulate with pair-bond mates (on rape by husbands and boyfriends, see Finkelhor & Yllo 1983; 1985; Russell 1982; Shields 8: Hanneke 1983). 4. The self-reported sexual histories of young men and the literature on date and acquaintance rape in general indicate an abundance of sexual coercion (Kirkendall 1961; also reviews by Malamuth 19813; 1984; Pirog-Cood 52 Stets 1989; Russell 1984). There is no question that many men’s sexual reper- toires include a mixture of noncoercive and coercive tactics including physical force. In addition, men often pursue an individual copulation with a mixture of tactics. Thus, it is wrong to dichotomize copulations as involving honest versus deceptive courtship or as unforced versus forced. Courtship and the interactions associated with maintaining pair-bonds include explicit and implicit promises about commitment. In part these promises may be broken because there was no intention to keep them (see Kirkendall 1961). The forced versus unforced dichot- omy may apply only to a small subset of human copula- tions: An example is the action of a man, without any sexual negotiation or honest or even deceptive courtship, using physical force or the threat of physical harm to capture and copulate with a woman against her will. The legal system in modern societies is concerned with whether or not an allegedly forced copulation was legally forced, because the answer determines whether the crime of rape was committed. The difiiculty that the legal system has in distinguishing forced from unforced sexual intercourse illustrates the conceptual problem arising from the fact that men tend to use mixtures of the three mating techniques (described previously in sect. 4) in a single attempted mating. The common use of mixed coercive and noncoercive approaches is also revealed by the recent literature on acquaintance rape, including sexual coercion in dating (Kirkendall 1961; see also reviews by Belknap 1989; Ellis 1959; Malamuth a: Briere 1966; Firog-Good or Stets 1969; Russell 1984). Young women have a significant risk of sexual victimization by men with whom they are ac- quainted, including dates. Men tend to date with the hope of mutually consensual sex; they use both honest and deceptive courtship to get dates. Women tend to date because of their interest in courtship (for a review of the human courtship literature, see Crammer 1989). Sexual coercion is apparently a common occurrence on dates, though it typically is used only when noncoercion fails (e.g., Kirkendall 1961; see, Ellis 1989, for a review). The sexual coercion with dates and acquaintances can include clearly physical force. Alternatively, the woman may protest that she does not want to be sexually intimate, but the man persists nonviolently until he achieves sexual intimacy. Compliance in the latter case is gained by coercion in the form of social pressure or intimidation that leads the woman to feel that if she does not comply, there will be unpleasant consequences. As in the literature on acquaintance rape, the data on rape by husbands and boyfriends and on rape of children (Finkelhor 1984; Thomhill, in press) demonstrate that men often combine noncoercive and coercive tactics, including physical force. 7.1. Sexual coercion/noncoerclon: A contlnuum. Not only are the three mating tactics (honest advertisement, deceptive advertisement, and coercion - see sect. 4) used in combination to obtain single copulations, making the dichotomies of honest versus deceptive courtship and forced versus unforced copulation simplistic, the three tactics often grade into each other, with only arbitrary boundaries between them. Others have also pointed out that the forced versus unforced dichotomy is inappropri- ate because they are on a continuum (e. g. , Clark 8: Lewis 1977; Medea & Thompson 1974; Russell 1982). We sug- gest that it is the continuity between forced and unforced copulations, even more than the combination of sexually coercive and noncoercive tactics in the pursuit of single copulations, that creates the great difficulty in all en- deavors to deal with the concept of rape, whether in the legal context or in everyday life. Many sexual interactions between men and women probably grade into coercion. As discussed earlier, the difference in the action of selection on males and females in human evolutionary history has resulted in women’s being attracted sexually to partners with high status and resource-providing potential (Buss 1987; 1989a; Symons 1979; Townsend 1989). The association between women’s erotic responsiveness and the resources provided by mates means that in general men must purchase consen- sual sexual access to women (Symons 1979) and that much of intersexual conflict and bargaining will end with male coercion because men’s ability or willingness to pay will often be inconsistent with women's needs or expectations. In humans, courtship and the interactions between pair-bonded mates accompanying copulation may include male violence toward the mate or her ofl'spring or explicit or implicit threats of violence that grade into displays and vows of emotional commitment. Most commonly, how- ever, explicit or implicit threats of unpleasant nonviolent consequences (e.g., a man's withdrawal of financial sup— port or emotional involvement) rather than actual or threatened male violence grade into noncoercive behavior. Buss's (1989b) findings on sexual conflict in mateships bear on the coercion/noncoercion continuum. BUSS stud- ied undergraduates who had been involved in a recent heterosexual relationship and newlywed couples who had been married one year or less. He found highly significant sex differences in the incidence of anger about sexual rejection for both the undergraduate and newlywed sam- ples: Men were more upset and angry about women withholding sex than vice versa. We suggest that the greater anger and distress shown by men are often per- ceived as threats of violence or of the withdrawal of Luvs unuu vs -..v..--_--_. _ ., resources or emotional commitment. Conflict about sex~ ual access during dating is to be expected, but its occur- rence between newlyweds when sexual conflict is rela- tively limited (see, Buss 1989b, for empirical support and discussion) is surprising and intriguing. If men’s emo- tional reactions toward mates do reflect threats, even mateships involving relatively high congruence in the interests of mates may often include sexual coercion. Buss found that male anger and distress occurred at about the same rates among undergraduates and newlyweds. The literature could have falsified the rape-specific hypothesis if it had shown that sexual coercion is used infrequently or only by a small percentage of men. In— stead, consistent with the specificity hypothesis, men tend to persist in seeking sex and can become sexually aroused regardless of whether a woman is interested in or resists their advances. 8. Viewing coercive and noncoercive stimuli in the laboratory 8.1. Rapists. To analyze further the environmental cues to which men are sexually responsive we examined the literature on men's arousal in response to audio and video stimuli in the laboratory showing coercive and noncoer- cive sex. Because penile erection is the only physiological re— sponse in men that occurs almost exclusively during sexual arousal, researchers generally agree that measures of penile tumescence best reflect men’s sexual arousal and preference for various sexual stimuli (Zuckerman 1971; also Abel et al. 1977; Earls & Marshall 1983; Langevin 1983; Quinsey & Chaplin 1982). In addition to penile tumescence, self-reported sexual arousal is assessed in some research. Although phallometric data increase ob- jectivity, their drawback is that volunteers for such re- search sometimes diEer significantly from nonvolunteers in their sexual behavior and personality. In contrast, no differences have been found between the general popula- tion and volunteers for studies that use only self-report of sexual arousal (see Farkas et al. 1978; Malamuth & Check 1983, for review and discussion). Malamuth (1981a) provides a useful review of the . earlier laboratory studies on sexual arousal in men. All five studies by G. Abel and associates that were reviewed by Malamuth involved incarcerated, heterosexual, and mostly mentally ill rapists; these showed equally high levels of penile tumescence when they listened to au- diotape depictions of rape and mutually consenting inter- course. This finding was replicated by Barbaree et a1. (1979) and Quinsey et al. (1981).6 To assess the generality of incarcerated rapists’ similar responses to rape and noncoercive sex it is necessary to test the general male population as well. 8.2. Nonraplsts. Malamuth’s (1981a) review lists only two studies comparing the sexual arousal of incarcerated rap- ists and nonrapists to portrayals of coercive and noncoer- cive sex. Both studies can be criticized on scientific grounds. Abel et al. (1977) and Barbaree et a1. (1979) found that, unlike incarcerated rapists, incarcerated non- rapists showed little sexual arousal in response to depic- BEHAVIORAL AND BRAIN SCIENCES (1992) 15:2 369 Thornhill & Thomhill: Evolution of sexual coercion tions of rape compared to consensual sex. Abel et al.’s study used very small samples, however, comparing only 13 rapists and seven nonrapists. In addition, all their subjects were patients in the same mental institution, but otherwise the nonrapists were not an appropriate com- parison group because the majority of the rapists were heterosexual whereas four of the nonrapists were known to be aroused by homosexual stimuli, and about the same number were pedophiliacs. Barbaree et al.’s study was also not controlled adequately; although they compared the sexual responses of rapists with those of heterosexual male college students, the responses were measured under what were probably very inhibitory conditions for male college students: the treatment and incarceration areas of a psychiatric center. Malamuth (1981a), Langevin (1983), and others have also criticized Abel et al. and Barbaree et al. on methodological grounds. Four other studies reviewed by Malamuth (1981a) show that for undergraduate university men simulations of rape can be just as stimulating as simulations of consen- sual sex (Farkas 1979; Malamuth 1981b; Malamuth & Check 1980a; Schmidt 1975). The sexual responses of men to rape in these studies varied with the victim’s sexual response in the depictions. If the victim was portrayed as eventually becoming “involuntarily sexually aroused” by the sexual assault the subject became just as aroused (both in terms of self-reports measures and penile tumescence) as by consensual sex (see also Malamuth & Check 1980a; 1980b; Malamuth et al. 1980a). On the other hand, when the victim behaved continuously as if the sexual assault were abhorrent to her, men showed significantly less arousal (Malamuth et al. 1980b; Mal- amuth 6r Check 1980a; 1980b). Malamuth (1981a) also, pointed out that the two studies (Abel et al. 1977; Barba; ree et al. 1979) that had found nonrapists to be more aroused by consensual sex than rape used tapes that emphasized the victim’s abhorrence during the sexual assault. A more recent study of college undergraduates by Malamuth et al. (1986) supports the finding that when the rape victim shows signs of sexual arousal, men find it as stimulating as noncoercive sex. In this study each subject read two sexually explicit stories. One portrayed a hetero- sexual rape. In describing the rape scene the investigators said, “We sought to make it representative of rape depic- tions typically found in pornography. Consequently, there was some suggestion that despite her nonconsent and pain, the victim showed some sexual arousal." The other story was a sexually explicit account of a man and woman having mutually consenting intercourse. The sub- jects’ reports of their sexual arousal to the two stories were significantly different: Mutually consenting sex was more exciting than rape; for penile tumescence, however, the difference between the two stories was small and not statistically significant. Much of the research by Malamuth and his colleagues (see reviews in Malamuth 19813; 1984; also Tieger 1981) has been focused on identifying variation among under— graduate college men in “propensity to rape." He found considerable variation in each of a number of large sam- ples derived from different colleges in North America. Malamuth’s subjects were asked to rate themselves on a five—point scale of likelihood of committing heterosexual rape if they could be assured of not being caught or 370 BEHAVIORAL AND BRAIN SCIENCES (1992) 15:2 punished. The scale ranged from “not at all likely to rape” (scored as 1) to “very likely to rape” (scored as 5). Thirty- five to 44% of the sample, depending on the study, scored 2 or higher and were accordingly interpreted by Ma]- amuth as likely to rape. It has been shown repeatedly that high-likelihood-of-rape subjects show greater sexual arousal (in both self-report and penile tumescence data) to rape depictions than low-likelihood-of-rape subjects, but that the two groups typically exhibit similar sexual arousal to depictions of mutually consenting intercourse. It has also been repeatedly shown that the self-reported likeli- hood of rape across the range of ratings 1—5 is positively correlated with sexual arousal to rape depictions, but not with arousal to consenting depictions (Malamuth 1984, for review of studies). Malamuth and Check (1983) conducted an experiment using sexually explicit audiotapes that varied according to whether or not the woman consented and whether the outcome of the sexual interaction for her was arousal or disgust. Both self-report and penile erection revealed that when the woman was portrayed as experiencing disgust, both low- and high-likelihood-of-rape subjects (as measured by the 5—point scale, ab0ve) were less aroused sexually by the nonconsensual than the consensual inter- actions. If the woman was depicted as eventually showing some sexual arousal, however, low-likelihood-of-rape subjects (56—65% of the various samples) found both kinds of interactions highly arousing (as measured by self- report and penile tumescence). These findings suggest that sexual arousal in the victim in simulated rape disin- hibits the sexual arousal of nonrapist men. We now turn to studies that indicate that not even sexual arousal in the victim is required to produce high arousal in nonrapist men hearing or viewing rape simulations. Part of Barbaree et al.’s (1979) study suggested that the woman’s sexual arousal or consent was not critical in nonrapist men’s reactions to erotica. They studied the penile reactions of male university students to sexually explicit audiotapes that varied in how they portrayed female consent and arousal. In the first tape the woman initiated sexual foreplay and was enthusiastic throughout; in the second, the woman was passive, neither rejecting nor accepting the man’s sexual advances; in the third, the woman was resistant at first, but was finally seduced. The authors concluded that neither the woman’s sexual con- sent nor pleasure was a critical variable for the subjects in the study, who responded similarly to all three stimuli. Briddell et al. (1978) studied the efiect of actual alcohol consumption and beliefs about it on the sexual responses of male undergraduate social drinkers. Half were given an alcoholic beverage, the other half a nonalcoholic one; half of each group was told they had been given alcohol, half that their drink was nonalcoholic. All were then pre- sented with taped narratives portraying (l) mutually en- joyable coitus, (2) rape, and (3) sadistic aggression. The rape tape contained no indication of an enjoyable sexual response of the victim. In the sadistic aggression, the aggressor was a man portrayed as using deadly force, and the victim was a woman, but there was no reference to sexual involvement. Briddell et al. found that the average penile tumes- cence to consensual sex and the rape were the same for the subjects who believed they had consumed alcohol, regardless of whether they actually had. The subjects who believed they had consumed a nonalcoholic beverage, on the other hand, showed significantly less arousal to the rape than the consensual intercourse even when they had actually consumed alcohol. All four groups were signifi- cantly less aroused by the sadistic aggression than by the other two tapes, suggesting that men are not sexually aroused by violence per 56 (even when it occurs between a man and a woman; see also sect. 8.4). A study by Quinsey et al. (1981) examined the effect of the belief that sexual responsiveness to unusual themes was expected in the testing situation. The study included four groups of men. Two groups were composed of insti- tutionalized mentally ill subjects: (1) rapists and (2) nonsex-offenders. The other two groups were composed of men from the local community, most of them unem— ployed. One group of community men was given “regu- lar" instructions: They were asked to listen closely and imagine that they were the man in the tape. The other group of community men was given these instructions plus three sentences of “permissive” instructions that stated that sexual arousal to unusual sexual stimuli is normal. All men listened to the following kinds of au- diotapes: (1) neutral tapes describing nonsexual and non- aggressive interactions between a man and woman, (2) tapes explicitly describing foreplay and heterosexual coitus with a willing partner, (3) tapes explicitly describ- ing situations in which significant'force was used by a man in achieving sexual intercourse with a female stranger who remained unwilling throughout the interaction, and (4) tapes involving no sexual activity but describing a woman being beaten and injured by'a man. There were no significant difierences among the four groups of men in penile responses to tapes depicting the neutral heterosexual interactions, nonsexual physical abuse, or mutually consenting intercourse. All four groups showed significantly more arousal to rape and mutually consenting intercourse narratives than to physi- cal abuse and neutral narratives. The rapists responded significantly more to rape narratives than the nonsexual- ofi'ender patients and the community men with regular instructions, but the community men who had received permissive instructions showed a significantly greater response to rape narratives than the community men with regular instructions, and they did not difier significantly from the rapists in their response to the rape narratives. Quinsey et al.’s study suggests that the social inhibitions of normal men afiecting their sexual arousal to depictions of rape (without any sexual arousal in the victim) can be easily modified by brief instructions to the effect that sexual arousal to unusual sexual stimuli is normal. Malamuth’s (1981b) study of the sexual responses of undergraduates examined further the effect of the con- tent of portrayals of heterosexual rape on men’s sexual response. In the first part of the study, the subjects viewed 16 slides accompanied with a narrative by a male Voice. The slides and text were similar to a recent issue of an erotic magazine. There were two versions of the series. The first 11 slides and narrative were identical, but the last five went in the direction of either consensual sex or rape. The study also varied the subjects’ instructions: Half received permissive instructions and half neutral ones, as above. All men showed high levels of sexual response. Most important, there was no significant difference in penile tumescence or self-reported arousal between men Thomhill 8: Thomhill: Evolution of sexual coercion viewing the rape and the consensual sex. Furthermore, in this study, in contrast to Quinsey et al.’s (1981) study discussed above, permissive versus neutral instructions did not have a significant effect on men’s arousal in response to rape, which was equally high in both cases. Blader and Marshall (1984) have also shown in a study of male university students that groups given “normative” versus “nonnormative” (i.e., permissive and neutral) in- struction do not differ significantly in their response to depictions of rape. In the discussion of the results from the first part of his study, Malamuth emphasizes that “the data suggest that within the context of explicit sexual stimuli, the manipulation of the woman’s consent alone does not significantly affect nondeviants’ arousal” (p. 41). In the second part of Malamuth's (1981b) study, the men were presented with an audio narrative (no pictures) describing an armed rapist violently raping a woman who clearly abhors the assault during the entire experience. Malamuth emphasizes that the rape narrative was “vir- tually identical” to the one used by Abel et al. (1977) and Barbaree et al. (1979) except that in his study it was read by a woman instead of a man. As we discussed earlier, the studies of Abel et al. (1977) and Barbaree et al. (1979) suggested that nonrapists exhibit less sexual arousal to rape depictions than rapists do. In the second part of Malamuth’s study, the men responded with the same high arousal to the audio-only rape narrative read by a woman as to the audiovisual consensual and rape series in the first part of the study. Hence, even a rape depiction that emphasizes the victim’s abhorrence can stimulate high sexual arousal in normal men when read by a woman V rather than by a man (see also Farkas 1979). Blader and Marshall (1984) found that reporting arousal while being exposed to erotic stimuli can reduce arousal. Penile tumescence was measured in male undergradu- ates in response to audiotape stimuli. Half the subjects, in addition to having penile tumescence measured con- tinuously, reported their subjective arousal throughout the period of exposure to the stimuli by pushing a lever. There were four audiotape stimuli: consensual sex, rape with restraint, rape with gratuitous physical violence, and assault against a woman with no ostensible sexual content. All stimuli were read by a male voice. The rapes were not portrayed as producing sexual arousal in the victim. After the experimental stimuli, a consensual sexually explicit videotape was presented to all subjects to elicit maximum penile tumescence, for comparison with their responses to each audiotape stimulus. In addition, for each subject an index of relative arousal to rape was computed by dividing the response to rape by the response to consen- sual sex. The subjects showed a higher relative response to the restrained rape (index = 66%) than the violent rape (45%). This pattern was then broken down in terms of whether or not they were asked to report their sexual arousal during the measurement. There was a significant difi'erence in the rape index between the report (34%) and no-report (58%) groups for violent rape but not for re- strained rape (65% and 68%, respectively). Self-report of arousal during the period of stimulation lowered penile response with violent rape only, not with restrained rape, consensual sex, or nonsexual assault. Blader and Marshall (1984, p. 629) say: The critical element here seems to be the addition of BEHAVIORAL AND BRAIN SCIENCES (1992) 15:2 371 Thomhill 6r Thornhill: Evolution of sexual coercion violence. It appears that cognitive factors involved in the self-report task can influence physiological re- sponses, but only at a point beyond a threshold of inappropriateness, i.e. , the introduction of clearly gra- tuitous violence. Male sexual arousal to coercive stim- uli may be a more robust eHect than has been pre- viously believed. 8.3. Overview of laboratory studies. It appears from the laboratory studies discussed above that not only incarcer- ated rapists but many other men (the studies collectively implicate young men in general) are sexually aroused to similar degrees by stimuli explicitly portraying consen- sual sex and rape. The inhibition of sexual response to depictions of rape experienced by many young men can be eliminated by any of the following factors: (1) signs of “involuntary” sexual arousal in the rape victim, (2) the belief that one has consumed alcohol, (3) instructions that sexual response to unusual stimuli is normal, (4) the narration of a rape by a woman rather than a man, and (5) not requiring men to report their sexual arousal while it is being measured by phallometry. Note that sexual arousal in the women is only one of several variables that can disinhibit men's sexual arousal in response to rape depic- tions, and not a necessary one. Both the ease with which men’s arousal to forced sexual interactions can be disinhibited and the similarity of their responses to consensual sex and rape under these labora- tory conditions are consistent with the hypothesis of a specific adaptation to rape: Ancestral men were not sex- ually aroused only by sexually willing women; consent was not a prerequisite for arousal. What is the evidence that men’s sexual responses in the laboratory, however, are related to their actual sexual behavior, including sexual coercion? First, research by Malamuth and his colleagues shows that the laboratory measures of arousal in response to depictions of rape are positively correlated with: (a) self-reports of personal use of force against women in sexual relations and (b) self-reports of the likelihood of using sexual coercion in the future (for review see Malamuth 1984; also Malamuth 1986). Sec- ond, measuring penile tumescence in response to audio and video stimuli is felt to be the best method of identify- ing the sexual preferences of men who seek psychiatric assistance for homosexuality, bisexuality, or pedophilia (Greer & Stuart 1983; Langevin 1983). Third, the sexual arousal of men with homosexual, bisexual, or pedophilic preferences to laboratory depictions likewise seems to be correlated with their history of actual sexual behavior (Langevin 1983). Finally, the typical mate preference of heterosexual men is young women (Buss 1987; 1989a; F elson & Krohn 1990; Symons 1979; 1987b; Thornhill & Thornhill 1983; Townsend 1987; 1989), and laboratory studies of sexual arousal confirm that heterosexual men show the greatest penile response to women between the ages of 18 and 25 and significantly less to pubescent or prepubescent girls (reviewed by Langevin 1983). It was predicted earlier in the target article that men would be sexually responsive and capable in both coer- cive and noncoercive sexual interactions. The laboratory studies discussed above examine only penile erection, not the actual sexual motivation involved in initiating and performing sexual coercion. There are no reliable data on the comparative sexual competence of men in actual 372 BEHAVIORAL AND BRAIN SCIENCES (1992) 15:2 coerced and noncoerced sexual settings. Thornhill and Thornhill (1983; see also Palmer 1989) have critically evaluated the literature on “sexual dysfunction” ofrapistS; the most relevant point is that many men do achieve sexual access by coercion, including physical force, ac- cording to the literature on acquaintance rape, rape by boyfriends and husbands, rape of children, and homosex- ual rape in prisons (see references above). Indeed, we argued above that male coercion in one form 0r another is involved in some (perhaps large) proportiOn of human copulations. This widespread use of force suggests that men are quite capable of sexual performance in the context of coercion. 8.4. The role of violence. Rape contains both sexual and violent stimuli. Are men aroused only by the sexual stimuli and not by the violence in rape, and does this account for the data on their similar reactions to coerced and consensual sex in the laboratory? The study of under- graduate men by Blader and Marshall (1984) discussed above specifically addressed this question. First, the subjects showed low arousal to depictions of nonsexual violence by a man to a woman compared to their arousal to sexual stimuli. Second, the men were less aroused by depictions of gratuitous violence accompanying rape than by depictions of “restrained rape,” in which the level of violence was no higher than what was “necessary to accomplish rape.” Men’s sexual arousal to violence per se has been exam- ined in other studies of nondeviants as well as incarcer- ated rapists. Quinsey et al. (1981) reported that most rapists do not show much sexual arousal to depictions of nonsexual violence toward women, not diHering from ' normal men in that respect (also, Briddell et al. 1978; see sect. 8.2). Overall, men’s penile responses to nonsexual violence are about the same as their responses to neutral stimuli (see review in Langevin 1983). Quinsey et al. (1984), in contrast, reported that incar- cerated rapists showed substantially more sexual arousal (penile tumescence) than nonrapists to depictions of non- sexual violence involving a woman victim and man per- petrator. (The authors suggested that the discrepancy between their 1981 and 1984 findings might have stemmed from the chance inclusion of more sadistic individuals in the rapist group, but they could not test this possibility.) In the 1984 study, however, the rapists were not responding to violence per se, because their arousal to nonsexual violence involving men only was no diflerent from their responses to neutral stimuli (men and women in nonsexual, nonaggressive heterosocial settings). In Quinsey et al. (1984) as well as in Abel et al. 1977, the degree of arousal of individual rapists to nonsexual male- on-female violence was positively correlated with their arousal to rape depictions but not with their arousal to nonsexual violence involving men only. As Quinsey et al. emphasized, their results indicate that the rapists were sexually responding to violence against women, not to violence itself. 9. Aggressive control of the sexual partner We suggest that violence per se will not be sexually stimulating for most men, but that, as predicted earlier, men will tend to be sexually aroused by achieving physi- cal control of an unwilling sexual partner through force because it represents an evolved cue that mating can now be successfully obtained or completed. That men find aggressive control of a sexual partner during rape sexually arousing is suggested by aspects of Quinsey et al.’s (1984) study. They recorded the penile responses of rapists and nonrapists to audiotaped stories about heterosexual bondage and spanking but without sexual content. Two categories of bondage and spanking stories are relevant here: (1) consenting bondage and spanking with a female partner (woman tied up and spanked), and (2) nonconsenting bondage and spanking with a female victim. The subjects in the study inter- preted these depictions as implicitly sexual: The rapists and nonrapists were significantly more aroused sexually by both bondage and spanking stories than by neutral stimuli. Rapists and nonrapists responded identically to both consensual and noncoercive stories. The prediction that men will find aggressive, even violent, control of unwilling women stimulating is also supported by the common inclusion in hardcore erotica (Dietz & Evans 1982) of sexual violence with women as victims and men as perpetrators. According to the re- search report of the President’s Commission on Obscenity and Pornography (1970), the pornographic magazine and movie business caters to the “average” man and not just to men with “anomalous” sexual preferences. This implies that many men are sexually motivated by vicariously assuming physical control over their sexual partners while fantasizing with pornographic material. Serial murders may be relevant to the idea that physical control of sexually unwilling women is sexually stimulat- ing to men. Serial murderers kill many victims often over several years; they are almost always men and the victims are almost always women, especially young women. ‘Se- rial murderers seem to be motivated to murder, in part, to achieve sexual arousal from the aggression accompany- ing the murder (Langevin 1983; Leyton 1986). Serial murders may be a pathological expression of the hypothe- sized psychological adaptation that makes physical con- trol of unwilling women facilitate men’s sexual arousal. Many incarcerated rapists report that physically dominat- ing their victims is sexually stimulating and motivates rape (Croth’s, 1979, "sadistic" and “power” rapists; Lan- gevin 1983, Chapter 12). A relevant note pertains to pedophiles, men with sexual preferences for children. It seems that the sexual arousal of pedophiles is frequently and perhaps primarily facilitated by the vulnerability, lack of dominance, and helplessness of their child sexual partners (studies re- viewed in Howells 1981; Langevin 1983, Chapter 8; Quinsey 1977). Pedophiles have a low general sense of eflicacy and self-esteem in their social relationships with both women and men. In the literature on pedophilia, it is argued that relating to children sexually gives pedophiles a feeling of power and control that is otherwise absent in their lives. Finkelhor (1984) is critical of this argument as an explanation for the sexual interest in children shown by pedophiles. He points out that those who propose the argument apparently feel that sexual arousal automat- ically follows from the pedophile’s dominance and control of a child. Finkelhor agrees. that the argument could account for the nonsexual motivation surrounding ped- ophilic behavior, but not a sexual preference for children. Thomhill & Thomhill: Evolution of sexual coercion But if selection has designed the men to find physical control of unwilling mates sexually facilitating, the ped- ophile's ability to dominate children would be a salient feature in their sexual arousal. In summary, there is some evidence that aggressively dominating unwilling mates facilitates the sexual arousal of men. Additional studies might provide direct evidence of psychological adaptation specifically for rape itself. It would be difficult to -expla.in men’s sexual arousal in response to the physical domination of an unwilling mate as a side-effect of a specific noncoercive sexual adaptation, along with a general adaptation that is designed to secure species-specific goals by force yet is not regulated by goal- specific cues. The results from the study of bondage and spanking, discussed above, are especially intriguing. Apparently, men find the theme of nonconsenting bondage and spank- ing (women tied up and spanked) sexually arousing de- spite the absence of sexual content. Why should such themes be interpreted as sexual at all, unless there is an adaptation to rape? Any future studies of sexual arousal to depictions of bondage should include women’s reactions to tied-up men. The rape-adaptation hypothesis would predict that women will not find the achievement of physical control of a man sexually arousing. 10. Coercive sexuality of men: Additional influences It was predicted that sexual coercion would be related to age and social status in men: Young men and socioeco- nomically deprived men are indeed the most likely to use force in connection with sex (reviewed in McDermott 1979; Russell 1984; Thomhill 8t Thomhill 1983), but behavior could just reflect difl'erential opportunity. Bet- ter evidence would come from studies that examined the influence of men’s age or social status on sexual arousal to viewing rape and consensual sex. Such studies have not yet been done, but they could provide more direct evidence of adaptation to rape. If researchers found that young and middle-aged men were similarly aroused by viewing consensual sex but young men were more highly and quickly aroused by rape, this result would be dficult for the side—elfect hypothesis to explain. A similar effect with low- and high-socioeconomic-status men would also favor the rape-specific hypothesis. As L. Cosmides has pointed out to us, these efi'ects would be even more decisive if the groups of men compared were engaging in similar amounts of sexual intercourse. It was predicted that regardless of social status and age, men will pursue sex coerciver when it will not harm their social reputation. That men are sensitive to the efi'ect of rape on social reputation is indicated by the data from the studies mentioned above showing that rape is considered personally acceptable by many men when there is no chance of being caught or punished. Moreover, the fre- quency of rape of strangers in war (Brownmiller 1975; Shields 8: Shields 1983), and even in peacetime in West- ern societies, supports this prediction. War and Western societies provide a degree of anonymity that allows men to try to get sex by force with no negative eEects on their reputation for moral sexuality. Shields and Shields's (1983) analysis of rape by U.S. and Vietnamese soldiers BEHAVIORAL AND BEAN SCIENCES (1992) 15:2 373 during the Vietnam War indicates that anonymity and other social factors that can affect the probability of punishment for rape (e.g., if rape is condoned or pro- moted by a warring government) are important determi- nants of men 's motivation to use force. Even acquaintance rape may often be motivated by a sense of anonymity and impunity in large societies. Rape of nonstrangers is more common than rape of strangers (e.g., Russell 1984), yet the literature on sexual harassment in the workplace (e.g., Report of the U.S. Merit Systems Protection Board 1981; see Studd & Cattiker, in press, for excellent review) reveals that men rarely rape women whom they interact with regularly in situations in which detection of rape or rumor of rape could damage men socially. As discussed earlier men’s arousal to rape depictions is disinhibited by (a) instructions to the effect that arousal to unusual stimuli is normal in the context of the experiment and by (b) measuring sexual arousal only by phallometry rather than by requiring concurrent self-report. Thus men’s motivation to appear moral appears to serve as a restraint on their tendency toward sexual coercion. It is still conceivable that this arises as a side-effect of psycho- logical adaptation for regulating coercion together with an adaptation for consensual sex. 11. Sexual conflict within mateships Here we briefly summarize the evidence that, as pre- dicted by the rape-specific hypothesis, conflict of re productive interests resulting from women’s sexual in- fidelity is an important cause of rape within pair-bonded mateships. There is considerable evidence that men equate sexual unwillingness and resistance in their long- term mates with infidelity, and that the rape of mates is motivated by sexual jealousy. First, the rape of a long‘ term mate is particularly likely to occur during or after the breakup of a mateship (F inkelhor 8r Yllo 1985; Russell 1982, in which concern about infidelity is directly impli- cated). Second, the strong relationship between wife battering and sexual jealousy of husbands has been dis- cussed by several authors (see especially Daly & Wilson 1988). There also appears to be a strong relationship between battering and raping the wife. “Sexual matters” were the major source of marital conflict in F inkelhor and Yllo’s (1985) study of marital rape victims. Russell’s (1982) study reveals that sexual matters, including sexual jeal- ousy on the part of the husband, played some role in 53% of the wife beatings reported by victims of wife rape. Frieze (1980) studied 137 chronically battered wives, 34% of whom reported rape as well as battering. She found that the wives of battering husbands who also raped perceived their husbands as more sexually jealous than did wives of husbands who battered but did not rape. This study also showed that the degree and frequency of violence toward wives by husbands who had both bat- tered and raped their wives was significantly greater as was their likelihood of beating their wives when they were pregnant and the lengths to which they would go to control the behavior of their wives, even to the point of extreme claustration. If future research were to reveal that coercive sex in mateships is induced by cues that indicate the woman’s 374 BEHAVIORAL AND BRAIN SCIENCES (1992) 15:2 sexual infidelity, there are still two possible interpreta- tions: (a) the rape-specific hypothesis and (b) the side- effect hypothesis — an adaptation to enhance paternity reliability and a general adaptation to pursue goals by force. Either way, further research on this issue would undoubtedly yield important information about the de- sign of men’s sexual psychology. There is evidence that men in pair-bonded mateships adjust ejaculate size and possibly ejaculate composition as well, to conditions of ejaculate competition (Baker & Bellis 1989b). Thus if female sexual unwillingness was indicative of infidelity in pair-bonded females in human evolutionary history (see sect. 5), it might be found that ejaculates produced during rape have features designed for eHectiveness in ejaculate competition. This finding would not necessarily demonstrate adaptation to rape, however, because the ejaculate properties would reflect selection in the context of ejaculate competition, and ejaculate competition is not specific to rape. 12. Conclusions and suggestions for future research The hypothesis that men have psychological traits that are designed for the specific purpose of rape has survived the following tests: It is consistent with (a) what is known of the natural history of men’s sexual coerciveness; (b) the results of laboratory studies of arousal to depictions of sexual coercion; (c) the results of laboratory studies and other evidence indicating that men are sexually aroused by physical control of an unwilling mate through force; ((1) data suggesting that men’s desire to give the appearance of having a moral sexuality is an important condition regulating men’s use of sexual coercion, and (e) informa- tion on rape in mateships. In addition, there is reason to infer that our male evolutionary ancestors sometimes enhanced their reproductive success through rape. There is still insufiicient evidence, however, to demonstrate phenotypic design specifically for rape. Thus, it must be concluded that all current data on the sexuality of men (with the possible exception of [0]) can also be explained by the hypothesis that rape is a side-effect of more general adaptation. The data discussed in the target article also clarify the rival hypothesis because certain side-effect explanations for rape are falsified by the findings. In the literature, one explanation of coercive sex is that it stems from a patho- logical psychological condition (for references and discus- sion, see Russell 1984; Shields & Shields 1983; Thomhill & Thomhill 1987), a hypertrophied side-effect of a more general adaptation either to consensual sex or to attaining goals by force or both. Coercive sex seems too widespread to be explained as a pathological side-effect, however. Another side~eflect theory is that heterosexual men in general are sexually aroused by almost any context involv- ing a woman, and this unregulated libido, coupled with men's violent nature, leads to rape (see Russell 1984). According to evidence in this paper, however, men’s libido is not unregulated in the presence of women as stimuli. Most men do not find depictions of neutral or gratuitously violent man-on-woman interactions without explicit sexual context sexually arousing. Also, it seems that both men's arousal to laboratory depictions of coerced sex and their actual use of sexual coercion are influenced by concern about social reputation. Men's sexual arousal and behavior in the context of coercive sex are clearly regulated, but is the regulation specific to sexual coercion? The answer to this question is crucial to distinguishing between the two rival hypotheses. The evidence in this paper does not contradict the hypothesis that rape is a side-eEect of the interaction of two types of adaptations, neither of which evolved in the context of sexual coercion: (1) men’s adaptation for copula- tion and (2) a species—typical adaptation to pursue by force goals (such as comfort, sexual satisfaction, a full stomach, status, etc.) that consistently promoted high reproductive performance during human evolutionary history. Accord- ing to this view, men’s pursuit of these goals by coercive means is not regulated by goal-specific psychological mechanisms and environmental cues, only by very gen— eral goal-independent ones: That is, the same environ- mental cues and the same psychological mechanisms influence men’s use of force to obtain any goal, whether sexual or otherwise. Laboratory experiments like those discussed in this paper can control confounding variables and help deter- mine the actual cues that men are adapted to process. Although slides and videotapes of sexually explicit activity typically evoke greater arousal in men than audiotapes (Laws 8: Osborn 1983), audio narratives read by research assistants have the advantage that presumptive causal variables can be easily and effectively-manipulated. Other approaches, such as self-reports of arousal to sexually explicit stimuli and naturalistic studies of sexual conflict between pair-bonded men and women (see Buss 1989b) could also yield important information. Variables that may influence the risks and reward of rape could also be manipulated in laboratory experiments; rape should be most likely under low risk/ high reward conditions. We discussed earlier how physical domination of a woman influences the costs of rape for the rapist and suggested further research on whether it might be a cue directly affecting men's sexual arousal. Other risk factors to vary may include (1) the social power of the rape victim’s mate and family, (2) the probability of detection or punish- ment, as in peacetime versus wartime; (see also Shields 8: Shields 1983), and (3) the age of the victim (because of its correlation with fertility). lfmen showed arousal to depictions of rape primarily or only when rape-specific risks were minimal and rewards maximal but were less sensitive to the same conditions when they accompanied depictions of consensual sex, this would favor the rape—specific hypothesis. For example, men might show equal arousal to consensual sexual sce- narios regardless of whether the woman was portrayed as having a powerful family; or they might even show max- imum arousal when the consenting partner had a power- ful family (which is expected reversal of the kin-group variable: positive for “honest” matings, negative for “dis- honest” ones). Coercive sex might also be more stimulat- ing when portrayed as occurring in wartime than in peacetime, with no corresponding differences for consen- sual sex. The rape-specific hypothesis predicts that the more the evolved interests of a man and woman diverge the greater Thomhill 8t Thomhill: Evolution of sexual coercion the likelihood of sexual coercion. This suggests laboratory experiments in which the age of the woman is held constant and the familiarity and expectations of future interactions between the partners are varied. Regardless of whether rape turns out to be a manifesta— tion of a rape-specific adaptation or a side-effect of adapta- tion to other circumstances research on the evolved design of men’s sexual psychology could lead to a much better understanding of rape. It could identify the en- ) vironmental cues that men s brains are designed to pro- cess and therefore the circumstances that determine their use of sexual coercion. This is not just a matter of theoreti- cal importance; it is practically important, too, because it can help identify the specific kinds of contexts that will discourage rape. ACKNOWLEDGMENTS We acknowledge the support of the H. F. Guggenheim Founda- tion. R. T. also acknowledges the support of P. Risser, vice president for research at the University of New Mexico. We thank G. Barlow, R. Bixler, D. Buss, L. Cosmides, C. Crawford, M. Daly, J. Dupré, M. Chiselin, B. Cladue, S. Hamad, V. Quinsey, W. Shields, N. Simon, D. Symons, D. Thiessen, I. Tooby, R. Trivers, M. Wilson, and several anony- mous readers for their useful criticisms on the manuscript. For useful discussion or correspondence or both about the ideas presented, we thank L. Betzig, D. Buss, L. Ellis, A. Kodric- Brown, L. Cosmides, M. Daly, N. Malamuth, P. Stacey, D. Symons, J. Tooby, I. Townsend, and P. Thomhill. As always, A., M., and P. Thomhill were inspirational. S. Andrew’s help with library work, and A. Rice's help with word processing and .dseful suggestions on the manuscript are greatly appreciated. N O T E S I. The terms sexual coercion and rape are used as synonyms in this paper. 2. Adaptations promoted fitness in our evolutionary past; we are not assuming that they continue to be adaptive in our current environment; see section 3.2. 3. Polygyny here means greater sexual selection on males than on females; there is no implication that male-female pair- bonds are absent. 4. Note that the term heritability does not apply to the traits of an individual. The dichotomy “genetic versus environmental” cannot be legitimately applied to the features of an individual. 5. We thank D. Symons for help with these three points. 6. 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