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Thornhill.1992.rape

Thornhill.1992.rape - BEHAVIORAL AND BRAIN SCIENCES(1992 15...

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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...
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