Mark Huppin and Neil M. Malamuth
Although acts of sexual coercion have been reported throughout human history, recently this topic has garnered much public attention. For example, President Obama has expressed great concern and taken important steps to reduce such acts on college campuses and in the military (Calmes, 2014). There has even recently emerged a new area of public health concern labeled technology-based sexual coercion (Thompson & Morrison, 2013).
In this chapter, we discuss evolutionary psychological (EP) perspectives on sexual coercion, defined as acts that involve sexual behaviors whereby one of the individuals does not fully consent to the acts. These typically include some use of physical force, threat, deception, or some other form of coercion. Rape is an extreme form of sexual coercion.
EP perspectives seek to identify ultimate causes of behavior, complementing the focus on proximate causes characteristic of other psychological theorizing. In addressing ultimate causation, evolutionary psychologists have often asked whether the ability to inflict sexual coercion or to avoid it contributed to reproductive success in our species' ancestral history, possibly giving rise to dedicated psychological mechanisms pertaining to coercive sex. Although addressing such questions is standard in EP theorizing, some critics have raised concerns that this might imply that sexual coercion is “natural” in the sense of inevitable or morally neutral, an implication we clearly wish to avoid (i.e., the naturalistic fallacy).
In applying the EP paradigm, we begin by considering clues to motivational differences between men and women that may set the stage for the potential occurrence of sexual coercion. Differences in minimal parental investment (Trivers, 1972) contribute to a greater likelihood that a man will be motivated to have sex with certain women than vice versa and that, for men, sex may be more easily separated from emotions associated with long-term mating (Buss & Schmitt, 1993). Such differences create conflicts that can result in some men using coercion to overcome female reluctance and resistance (Gorelik, Shackelford, & Weekes-Shackelford, 2012). Consistent with the predictions derived from parental investment theory is the finding that across various societies and recorded human history, as well as across virtually all species where sexual coercion occurs, there are large sex differences in the use of sexual coercion. Males are typically the perpetrators and females are victims. If one examines criminal statistics, the sex differences are huge. The U.S. Bureau of Justice Statistics reported in the late 1990s that 99% of those imprisoned for rape were men (Greenfeld, 1997). Moreover, there are sex differences in the type of coercive methods used. For example, Hines and Saudino (2003) found that “unlike men who were sexually coercive, sexually coercive females did not use threats or force to make their partners have sex with them; they insisted on the acts instead [when their partners did not want to have sex]” (p. 214).
Although there are cultural differences in the frequency of sexual coercion, large sex differences are found even in the most egalitarian and low general violence nations. For example, Lottes and Weinberg (1996) reported that among Swedish college students, 41% of women and 22% of men reported being subjected to some form of nonphysical coercion to engage in sex by a member of the opposite sex. The rates for U.S. college students are much higher: 69% and 50%, respectively, as presented in the same research.
Much EP theorizing on sexual coercion has focused on models that implicate condition-dependent psychological mechanisms affecting an individual's propensity to coerce. Environmental experiences, particularly in critical early stages, are said to result in the calibration of mechanisms at relatively fixed values, which can lead to lifelong differences in thresholds for evoking sexual coercion. Whereas EP theorizing typically has not stressed direct links between genetic differences and sexual coercion, it has considered the possibility that genetic differences may underlie certain personality and other characteristics (e.g., general aggressive tendencies, responsiveness to socialization and peer influences) that affect the propensity to sexually coerce (e.g., Lalumière, Harris, Quinsey, & Rice, 2005; Waldman & Rhee, 2006; Westerlund, Santtila, Johansson, Jern, & Sandnabba, 2012). Lending support to the potential usefulness of also considering genetic factors is evidence of the ability to genetically breed mice that are either more or less sexually aggressive (Canastar & Maxson, 2003). Human twin studies also support the influence of genetic effects on sexual coercion, although researchers caution that this does not mean that there are genes affecting only sexual coercion (Johansson et al., 2008).
In this chapter, we focus on the male perpetrator's psychology but we also consider aspects of relevant female counteradaptation to the risk of male sexual coercion. In the past few years, this has been an area of emphasis of EP rape research. One reason may be that it is more likely that specialized mechanisms for avoiding sexual coercion evolved in women than that specialized mechanisms for engaging in sexual coercion evolved in men. This assumes that the reproductive costs to ancestral women of losing the ability to choose among mating partners due to sexual coercion would have been greater than the reproductive increase to men of, at times, using coercive sex.
Physical force, harassment, and other intimidation to obtain sex have been reported in many species. Based on a review of the literature on forced copulation among nonhumans, Lalumière et al. (2005) identified specific characteristics in those species that exhibit sexual coercion. Across all nonhuman species forced copulation is always perpetrated by males on females. Despite the tendency of females in some species to be assertive in the mating process, the authors could not find one instance of a female forcing sex on a male. Further, males are more likely to target fertile than infertile females for forced copulation. Relatedly, forced copulation does occasionally result in insemination, fertilization, and offspring. Also, males of most species tend not to engage solely in coercive sexual behaviors. In fact, most males that engage in forced copulation at other times court females. Finally, Lalumière et al. (2005) recognized the role of individual differences in sexual coercion. Certain males are more likely than others to engage in forced copulation. Some males are more successful at sexual coercion than others. Lalumière et al. conclude that sexual coercion (particularly in the form of forced copulation) “is a tactic used by some males under some conditions to increase reproduction” (p. 59).
A particularly interesting species is the orangutan, one of the few nonhuman primates for which sexual coercion is common. There is evidence for two distinct classes of orangutan males: large males and small males. Both types are sexually mature, though the onset of sexual maturity can be highly variable. Large males typically weigh over 80 kg in the wild, about twice the size of the small males (Knott, 2009; Knott & Kahlenberg, 2007). Although both types resort to forced copulations, they are more often perpetrated by small males, who force more than 80% of their total copulations at some orangutan sites, although only about half or fewer of their copulations are forced at other sites, suggesting the role of environmental contingencies such as population density and sex ratio (Knott, 2009; Knott & Kahlenberg, 2007).
The evidence from orangutans can be contrasted with other similar species in which forced copulation has not been reported, Bonobos and common chimpanzees (Stumpf, Emery Thompson, & Knott, 2008). This suggests the importance of factors such as the isolated social system unique to orangutans among the apes (see Smuts, 1995, and Smuts & Smuts, 1993, for analyses emphasizing the importance of female coalitions as a deterrent for male sexual aggression across various primate species and potential implications for humans). Chimpanzee males, however, do use less direct sexually coercive strategies such as harassing and intimidating females. These tactics can manipulate the future rather than the immediate behavior of the target. For example, long-term data from a study of wild chimpanzees showed that a female's willingness to initiate copulation with a male is positively correlated with how frequently the male has been aggressive toward her, suggesting that female mate preferences are constrained by sexual coercion (Muller, Emery Thompson, Kahlenberg, & Wrangham, 2011). A related study (Muller, Kahlenberg, Emery Thompson, & Wrangham, 2007) found that male chimpanzees achieved more matings with females toward whom they were more aggressive, and directed more aggression toward more fecund females.
An issue relevant to an evolutionary-based model of sexual coercion is its frequency in human history, because regularly occurring events are more likely to have a “logic embedded in the dynamics of natural selection for reproductive success” (Wrangham & Peterson, 1996, p. 138). Sexual coercion does appear to have occurred throughout human history (e.g., Chagnon, 1994), and cross-cultural surveys reveal that it occurs in most societies today (Basile, 2002; Broude & Greene, 1978; Fulu, Jewkes, Roselli, & Garcia-Moreno, 2013; Levinson, 1989; Monson & Langhinrichsen-Rohling, 2002). Moreover, even relatively rape-free societies described in such surveys (e.g., Sanday, 1981) have social rules intended to counter male sexual aggression, suggesting that there is universal risk for such behavior.
When fear of punishment is reduced, signaling conditions in which the costs of sexual coercion are low or the perpetrator has anonymity, many men do rape. This is evident in times of war (see Allen, 1996; Stiglmayer, 1994). At least one-third of men admit some likelihood of sexual coercion if they could be assured that they would not suffer negative consequences (e.g., Malamuth, 1989). In addition, sexually coercive fantasies are common among men (Greendlinger & Byrne, 1987, 54% of college men “fantasize about forcing a woman to have sex”; Crèpault & Couture, 1980, 33% of a community sample of men sometimes or frequently fantasize a scene “where you rape a woman”), and imagined sexual aggression is a key predictor of actual sexual aggression (e.g., Dean & Malamuth, 1997; Malamuth, 1981, 1988; Knight & Sims-Knight, 2003; Seto & Kuban, 1996). Aggressive sexual fantasies also covary with measures of high sexual preoccupation, high sexual compulsivity, and hypersexuality (Knight, 2010a). Imagined aggression may reveal important information about evolved mental mechanisms (B. Ellis & Symons, 1989; Kenrick & Sheets, 1993).
R. Thornhill and Palmer (2000) presented the most controversial evolutionary analysis of rape. They addressed whether sexual coercion is produced by adaptations or as a by-product of adaptations designed to solve other problems. Adaptations are naturally selected (i.e., they resulted in increased ancestral reproductive success). Criteria for establishing adaptation within evolutionary science include attributes of economy, efficiency, complexity, precision, reliability of development, and functionality in solving a specific problem (Buss, Haselton, Shackelford, Bleske, & Wakefield, 1998; see also Tooby & Cosmides, 1992). By-products are incidental characteristics that did not evolve because they solved adaptive problems. For example, male nipples, which have no design functionality, are by-products of the adaptive value of nipples in women (Symons, 1979).
Symons (1979) first discussed extensively whether rape is produced by adaptations or by-products of adaptations. He concluded that the available data are insufficient to conclude that rape is a facultative adaptation in humans. Rather, rape may be a by-product of male adaptations that produce sexual arousal and adaptations that motivate coercion to secure desired goods. Later evolutionary models of rape have extended Symons's proposal to include rape as a by-product of both sexual desire and a generalized possessiveness or desire to control others (L. Ellis, 1989). Still other evolutionary models conceive of rape as a manifestation of an alternative strategy, for example, psychopathy, whereby rape is a by-product of the use of coercion in other areas (Mealey, 1995).
The adaptation hypothesis suggests that in ancestral environments, being sexually coercive under some circumstances (and, particularly for women, having the capacity to avoid being sexual coerced) contributed to reproductive success sufficiently frequently to have resulted in some change in the evolved psychological architecture that would not have occurred without the recurring fitness consequences of sexual coercion. Therefore, this hypothesis posits specific psychological mechanisms pertaining to sexual coercion. Such specialized mechanisms might include reactions such as emotions or arousal patterns that, in the proximate environment, mediate between relevant environmental cues and behaviors.
From an EP perspective, the question is not whether sexual coercion is a better strategy for males than engaging in consensual sex but whether for some ancestral males, under some circumstances, it may have been reproductively effective to use sexual coercion as compared to not using it. In other words, did recurrent ancestral conditions exist under which for some men, some of the time, an overall fitness increase resulted from sexual coercion? Although the hypothesis that sexual coercion contributed to reproductive success has been criticized on grounds that rape rarely leads to conception, Gottschall and Gottschall (2003) estimated pregnancy rates resulting from penile-vaginal rape among women of reproductive age to be twice that of consensual per-incident rates (6.4% to 3.1%). Controlling for age, rape pregnancy rates per incident remained 2% higher than consensual rates (see also Holmes, Resnick, Kilpatrick, & Best, 1996, indicating a rape-related pregnancy rate of 5.0% per rape or 6.0% per victim in a national sample of reproductive age women; for indirect corroboration, see Beirne, Hall, Grills, & Moore, 2011, in a sample of 105 normally ovulating sexual assault victims, identifying “a trend and a distinct rise in the number of assaults when the victims were in the middle of their cycle” [p. 315], that is to say at peak fertility).
Also relevant to the potential fitness outcomes of sexual coercion is the fact that a substantial minority of women continues to have sex with the men who sexually assault them (Koss, 1988). This is particularly true of completed sexual assault, pointing to the use of sexual coercion as a tactic to secure subsequent copulations. From a comparative perspective, this is consistent with forms of sexual coercion in wild chimpanzees used to manipulate the female's future rather than her immediate behavior. Illustratively, Ellis, Widmayer, and Palmer (2009) identified more than two thousand North American undergraduate women who reported having been sexually assaulted, dividing victims into two groups: assault blocked (59.4%) and assault completed (40.6%; i.e., sexual intercourse occurred). A sizable number of women in both groups indicated future intercourse at least one time with the assaulter, with more women in the assault-completed group (27.2%) than in the assault-blocked group (19.4%) reporting this outcome. Overall, these results indicate that, “at least a minority of men may have evolved tendencies to use assaultive tactics to secure mating opportunities beyond those obtained by men who only employ ‘voluntary’ tactics” (p. 461) (see also Wilson & Durrenberger, 1982; 39% of rape victims had another date with their assaulters, compared to 12% of victims of attempted rape). Ellis, Widmayer, and Palmer hypothesized that completed assault may more readily secure subsequent copulations due to greater experienced female trauma or a felt need for support in the event of pregnancy.
Relatedly, in Holmes et al.'s (1996) study, 41.2% of rape-related pregnancy cases involved repeated assaults, one of which was assumed to result in the pregnancy. Although the data are unclear about what percentage of these women endured multiple assaults from a single perpetrator (indicating only that for these victims, rape-related pregnancy occurred in a context of ongoing abuse), it does point to the possibility that sexual coercion may have increased the likelihood of future copulations with the victim.
For their model, R. Thornhill and Palmer (2000) proposed various possible adaptive mechanisms. These included mechanisms designed: (a) to evaluate women's vulnerability to sexual coercion, theoretically functioning to direct rape toward cost–benefit scenarios most favorable to the prospective rapist; (b) to identify cues associated with fertility (e.g., age, ovulation status), so that men might preferentially target the most fertile women as rape victims; (c) to optimize sperm counts produced during rape; (d) to increase the probability of rape under conditions of sperm competition, when men would be vying most fervently with one another for valued insemination opportunities; (e) to potentiate rape in men who lack sexual access to females (the “mate deprivation” hypothesis); and (f) to produce arousal specific to opportunities of rape. These theories of adaptation as they relate to sexual coercion have been further elaborated and expanded elsewhere (see Camilleri, 2012; Camilleri & Stiver, 2014). In this chapter, we evaluate theory and data specially pertaining to sexual arousal specific to forced sex, showing how such an adaptive decision rule might be selectively constituted.
One hypothesized candidate for a specialized psychological mechanism motivating sexual coercion that has received focused attention is sexual arousal specific to forced sex, referred to here as sexual arousal to force (SAF). Such arousal may be a manifestation of a broader category of sexual arousal generated by controlling or dominating women, which can be accomplished by the use of force.
Using an adaptation model, R. Thornhill and Thornhill (1992) discussed SAF and argued that higher sexual arousal to coercive sex among men should be associated with greater success with coercive sexual tactics, thereby contributing to ancestral reproductive fitness under some circumstances. They noted that given the costs of forced mating in ancestral environments, including possible loss of status or life, men might be expected not to have evolved preferences for forced sex and, therefore, not to evidence SAF. If, however, under some recurrent ancestral environments, the reproductive benefits of forced mating outweighed the costs, psychological mechanisms enabling sexual arousal despite a woman's lack of consent may have evolved. Harris, Rice, Hilton, Lalumière, and Quinsey's (2007) selectionist hypothesis of psychopathy provides an example of a model suggesting that SAF could reflect a design feature of a rape adaptation. This hypothesis asks: “Do psychopaths respond more to sexual stimuli depicting violence, coercion, and rape simply because they are indifferent to the suffering of others, or does psychopathy entail a mechanism promoting coercive sex?” (p. 20). Harris et al. (2007) suggest that sexual coercion could be a fundamental feature of psychopathy.
Buss (1994/2003) suggests that the model pertaining to SAF and the data presented by Thornhill and Palmer (2000) do not enable differentiating among alternative hypotheses. In consideration of such criticisms, we elaborate both theoretically and empirically on the possibility that SAF might have evolved as a conditional specialized mechanism for sexual coercion in a manner that enables better testing of alternative explanations. Hagen (2004) argues that specialized mechanisms pertaining to rape would not be expected unless the problems involved in “successfully” committing such an act in ancestral environments were not the same problems as with the use of aggression in other contexts. The occurrence of sexual arousal in the context of coercive acts may be an important distinguishing characteristic. For most aggressive acts, sexual arousal would be irrelevant or even detrimental. Because the preferred sexual strategy for most men in most circumstances is to pursue consensual sex, the most common calibration of sexual arousal mechanisms should be to become sexually inhibited by indications of lack of sexual receptivity by women. However, if an individual is to effectively rape in ancestral environments, such aggression may require reversing of the default arousal pattern. This may be hypothesized as a unique adaptive problem associated with sexual coercion as contrasted with the use of coercion in nonsexual contexts.
In evaluating empirical data, we rely on studies that measure SAF (often by direct genital measures), and we contend that studies using related measures, such as reported dominance as a motive for sex (Nelson, 1979) and rape fantasies (Greendlinger & Byrne, 1987), assess closely related constructs that are also relevant to the present analysis.
Within some ancestral circumstances, the inhibition or activation of sexual arousal in response to cues associated with using force might have affected the likelihood of successfully dominating and exerting sexual control over an unwilling woman. Some emotions motivate avoidance of particular stimuli, whereas others motivate approaching or pursuing particular stimuli (for an overview see Elliot, 2008). Just as fear of spiders motivates avoidance of specific threats, sexual arousal cued to the use of force may motivate sexual coercion. This hypothesis is supported by the meta-analysis of Allen, D'Alessio, and Emmers-Sommer (2000), which documented that sexual arousal is associated with positive psychological affect, a precursor of approach or pursuit.
This hypothesis that sexual arousal cued to the use of force may serve as an approach emotion designed to increase the likelihood of engaging in sexually coercive behavior may be contrasted with nonevolutionary hypotheses of SAF. For example, Marshall and Fernandez (2000) hypothesized that SAF is not designed to facilitate sexual coercion, but instead that the causal connection is in the opposite direction. Marshall and Fernandez argue that SAF and other forms of “deviant” sexual arousal are the result of repeated sexual offending. This model suggests that because the offender lacks the requisite social skills and confidence to engage in consensual sex, he uses coercive tactics repeatedly, eventually resulting in the conditioning of SAF (but see, e.g., Ellis et al., 2009; men who commit sexual assault have sex with more women than do men, in general). Other hypotheses have also conceptualized such arousal as an abnormality that is likely to be evidenced by a small percentage of men (e.g., Abel, Barlow, Blanchard, & Guild, 1977). Representative of the abnormality hypothesis, the Diagnostic and Statistical Manual of Mental Disorders (5th ed.; DSM-5) Paraphilias subworkgroup recently proposed the introduction of a new psychiatric diagnosis, paraphilic coercive disorder, in which the diagnostic criteria featured sexual arousal from nonconsent, struggling, or resisting.
An evolutionary-based model uniquely suggests that, due to calibrating mechanisms grounded in the consequences in ancestral environments, a substantial percentage of “normal” men evidence the sexual arousal pattern that facilitates sexual coercion. How might such calibration occur? In keeping with the proposition that humans share a common evolved psychology that enables relevant developmental experiences to “set” mechanisms at different levels (Belsky, Steinberg, & Draper, 1991; Draper & Harpending, 1982; Trivers, 1972), the model we outline here (which we label the evolutionary functional [EF] model) emphasizes some relevant perceived negative experiences with women that may set the sexual arousal versus sexual inhibition to force mechanism more in one direction or the other. Although full testing of such a process would require a longitudinal study that would be difficult to conduct, it may be possible to prime similar processes to create a state condition related to the trait condition. Yates, Marshall, and Barbaree (1984) found that college men who were insulted by a woman were more sexually aroused by rape portrayals as compared to portrayals of consensual sex. Creating general arousal by physiological exercise instead of an insult by a woman did not result in a similar increase.
Other relevant findings pertain to the trait rather than the state of anger and hostility toward women. These studies indicate that men who are hostile to women, typically on measures that include items referring to perceived rejection from women (e.g., Check, Malamuth, Elias, & Barton, 1985), show relatively high SAF as contrasted with men who are relatively low on such measures of hostility toward women. For example, many studies focusing on the confluence model of sexual aggression (Malamuth & Hald, in press) have found a strong connection between measures of individual differences in men's hostility toward women and their SAF or similar constructs such as dominance as a motive for sex and rape fantasies. Other studies examining differences between behaviorally sexually nonaggressive men and sexual aggressors (some of whom are likely to have the relevant calibration of increased SAF) have found similar results (e.g., Murnen, Wright, & Kaluzny, 2002). Several priming studies have revealed that sexually aggressive men may be more prone to automatically associate women with hostility, sex, and power (Bargh, Raymond, Pryor, & Strack, 1995; Leibold & McConnell, 2004). Barbaree (1990) reported a study with a rapist who was asked to imagine raping women for whom he held different emotional feelings. He found that the greater the hostility to the woman, the greater the sexual arousal to rape cues. Forbes, Adams-Curtis, and White (2004) found that the key component linking various measures of male dominance ideology (e.g., attitudes supporting aggression or sexism) to aggression against women is hostility toward women. Baumeister, Catanese, and Wallace (2002) have summarized many studies indicating that experiencing rejection by women, particularly by men who are relatively narcissistic, contributes to sexually coercive behavior. Taken together, these findings provide some support for the hypothesis that perceived blocked access to desired women and associated hostility toward women may affect the calibration of men's sexual arousal patterns in ways that could affect the likelihood of committing sexually coercive acts.
How might a mechanism of SAF operate to affect the likelihood of committing sexually coercive acts? Consider a simple distinction between two types of men: one for whom the best prospects involve mating only with a consenting partner and the other a man whose prospects could be augmented by using sexual coercion. (Rather than a simple dichotomy, we prefer a more dimensional conceptualization but use a dichotomy to facilitate explication.) If we were to design a psychological mechanism that provided the best decision rule (for total ancestral fitness) for each of these men, what might be its properties? For the first man, there would be sensitivity to cues when a sexually desired female indicated disinterest, disgust, or other negative responses. This would be an effective mechanism for inhibiting approach tendencies where persisting in sex with an unwilling female would have high costs compared to pursuing consensual sex with alternative mating prospects. However, for the second type of individual, it could have been ancestrally adaptive to have this inhibiting mechanism disengaged. Potentially, for this latter type, there may even have been fitness benefits to increased SAF relative to consenting sex because engaging in coercion may require relatively greater persistence and energy to overcome the resistance of an unwilling partner. Consistent with this hypothesis, Bernat, Calhoun, and Adams (1999) found that the penile tumescence of self-identified sexually aggressive men who also held callous sexual beliefs (e.g., “Get a woman drunk, high, or hot and she'll let you do whatever you want,” “Prick teasers should be raped”) increased when force was introduced into a sexual scenario (see also Lawing, Frick, & Cruise, 2010, who found that adolescent sexual offenders high in callous/unemotional traits showed more sexualized aggression and had a greater number of victims than other adolescents with a sex offense).
Our analysis suggests that type 1 men should show inhibited SAF, whereas type 2 men should show at least equal sexual arousal to consensual and coercive sex (i.e., the shutting off of the inhibiting mechanism) or even greater arousal to some types of coercive sex (the activation of a mechanism creating greater sexual arousal). The distinction between two types of men bears some similarity to the distinction between large and small orangutans insofar as that distinction may serve as a useful illustration of how differently situated individuals may respond based on their unique developmental and current circumstances. In summary, if there were ancestral conditions in which, for some men, some of the time, there was an overall fitness increase resulting from sexual coercion, then for these men it may have been important not to be inhibited by cues of a woman's unwillingness and to potentially be sexually aroused by dominating and controlling the victim.
How might we select two groups of men for comparison purposes to correspond to the hypothesized two types described earlier? Previous researchers have compared convicted rapists to other men (e.g., N. Thornhill & Thornhill, 1992; R. Thornhill & Palmer, 2000). This is not the ideal comparison, however (e.g., Lalumière, Quinsey, Harris, Rice, & Trautrimas, 2003; Marshall & Kennedy, 2003). Convicted rapists include men who are “generalists” vis-à-vis antisocial behavior and men who are “rape specialists.” The latter group would have the mechanisms calibrated to increase the likelihood of sexual coercion. The former group may include individuals who have not had the relevant mechanisms calibrated but may use sexual coercion due to the workings of other mechanisms. These men engage in various antisocial acts because they differ from other men not necessarily on the specific mechanism of SAF (or other specialized mechanisms for sexual coercion) but on mechanisms underlying general antisocial behaviors (e.g., lack of inhibitory self-control, high impulsivity, low empathy, and/or callousness). They may be more likely to steal or to use coercion for obtaining any desired goal. Accordingly, convicted rapists are comparable to other types of violent criminals on most measures of antisocial traits and behaviors (Lalumière et al., 2005), most rapists have a history of nonsexual offenses (Kingston, Seto, Firestone, & Bradford, 2010), and the criminal records of rapists often resemble those of other offenders (Serin & Mailloux, 2003).
Using data from a large sample of prisoners released in 1994, Miethe, Olsen, and Mitchell (2006) found that rapists display less specialization (i.e., repetition of the same offense) than other offender types. Even within a subset of serial sex offenders “[o]nly a modicum of specialization was embedded in otherwise versatile criminal careers” (p. 221). A recent study of 170 rapists referred for civil commitment also highlights that, contrary to some social constructions, convicted rapists are versatile offenders (Harris, Mazerolle, & Knight, 2009; see also Harris, Smallbone, Dennison, & Knight, 2009). Using a commonly accepted definition that declared specialization if more than half of a rapist's offenses were sexual in nature, only 18 (11.8%) were rape specialists.
Whereas most convicted rapists may be criminal generalists, some rapists do appear to be rape specialists. The Massachusetts Treatment Center (MTC) Rapist Typology, which defines rapist subtypes structured in relation to three (previously four) motivational themes (see Knight, 2010b; Knight & Prentky, 1990; Knight & Sims-Knight, in press), includes two types, sadistic and sexual nonsadistic, more motivated than nonsexual subtypes by a paraphilic interest in rape and sexual gratification. The evidence for such a typology is not strong, however (see, e.g., Looman, Dickie, & Maillet, 2008, who find no between-group differences between sexual subtypes and rapists deemed nonsexual in responses to rape depictions; Kingston et al., 2010, find no evidence of sexual specialization in a sample of sadistic sex offenders; Healey, Lussier, & Beauregard, 2012, note a lack of consensus by the scientific community about what sexual sadism is and how it is defined). As Knight (2010b, p. 17-7) has acknowledged, “The validity data on the sexual types constitute one of the more problematic areas of the typology.” Beyond the nettlesome task of reliably grouping different types of sexually coercive men, difficulties with data interpretation add to the troubles (see Harris, Lalumière, Seto, Rice, & Chaplin, 2012, and Seto, Lalumière, Harris, & Chivers, 2012, suggesting a meaningful distinction between rapists and self-identified sexual sadists in the cues that elicit penile responses to rape scenarios; but cf. Knight, Sims-Knight, & Guay, 2013, who find more similarity than difference between these two groups after evaluating the same research).
Knight and colleagues, who developed the original MTC typology, in the latest data-driven revision of the typology, have suggested a structural reconceptualization that provides a more integrated approach. It orders individuals on a single continuum according to the gravity of the coercive sexual behaviors (e.g., Knight, 2010b). Severe forms of sadistic arousal, for example, occupy the high end of the continuum (Knight et al., 2013). The new structural model, which departs from earlier typological attempts to categorize rapists by interrelated but purportedly more discrete distinguishing characteristics, can “best be described as a modified, dimensional, circumplex model, replacing and restructuring the linear configuration proposed in its predecessor” (Knight & Sims-Knight, in press). Because the new model describes men as differing in the traits they possess in degree (rather than in kind) on a univariate dimension, it is consistent with an EP approach in which proximate factors (such as repeated rejections from desired women and a history of exploitative relationships) interact with universal psychological mechanisms to influence the expression of sexually coercive behaviors.
Classifying rapists according to the MTC Rapist Typology provides a potentially valuable means to identify men who may engage in sexual coercion partly because of the activation of specialized psychological mechanisms, but excludes most men from inclusion. The data indicate that it is among noncriminals, particularly those drawn from college populations, that specialization may be most evident. In general community samples, men who self-identify as having committed sexual coercion show more evidence for “specialization” than convicted rapists. Ronis, Knight, Prentky, and Kafka (2010) found that self-identified sexually coercive community men exceeded incarcerated rapists on diverse measures of sexual and paraphilic fantasies, including sadism, sexual preoccupation, and bondage. Self-identified sexual coercers among criminals who had not been convicted of sexual crimes also showed higher scores on such sexual and paraphilic fantasy than convicted rapists, suggesting that, even among criminals, self-identification might be a better way to identify specialists for sexual coercion than only considering the crime for which the person was convicted. Although there were no significant differences between the community and criminal self-identified sexual coercers on these sexual and paraphilic fantasy measures, the community sample evidenced the highest scores. Overall, these data support the conclusion that most of those currently identified by the judicial system and convicted of acts of sexual coercion display less evidence of specialized psychological mechanisms than other self-identified sexually coercive men. However, caution is necessary in interpreting the data provided by convicted rapists, who might seek to portray a positive image because of the belief that this will increase their likelihood of being paroled.
Researchers focusing on noncriminal samples generally have not addressed whether sexually aggressive men engage in other forms of antisocial behavior (Lalumière et al., 2005). We conducted analyses focusing on this issue in our longitudinal database of close to 150 men (Malamuth, Huppin, & Bryant, 2005). We administered several measures to the same men at about age 20 years (Time 1) and then again 10 years later (Time 2). Examining whether measures assessing SAF showed a pattern supporting specialization, we found support for such a specialized mechanism. Other findings provide corroboration for such a specialized mechanism (for a discussion see Malamuth et al., 2005). Malamuth and Impett (1999) conducted a series of mediational analyses to directly test the hypothesis that high SAF is a specific mediator of forced sex. They found evidence supporting SAF as a specific mediator of coercive sexual behavior.
Because rape is a traumatic event for women, and likely to have been a recurrent problem over evolutionary time, it may be more plausible that women have evolved counteradaptations designed to minimize their experience with male sexual aggression, than it is that men have evolved rape-specific adaptations. Negative fitness consequences of sexual assault for women include serious injury or death, partner abandonment, disruption of a woman's parental care, and circumvention of mate choice (Symons, 1979; R. Thornhill & Palmer, 2000). Even if male sexual coercion is a by-product of other adaptations these costs are no less traumatic, meaning that the argument for female counteradaptations is independent of causal explanations for men's behaviors.
Researchers have reported purportedly rape-avoidance behaviors that imply the existence of evolved mechanisms, although most arguments in favor of specificity presently constitute tentative hypotheses. One example is Wilson and Mesnick's (1997) “bodyguard hypothesis,” which offers that anti-rape adaptation may have produced women's mate preferences for physically and socially dominant men. Evidence suggests that women may be especially attracted to such men when the risk of sexual aggression from other men is higher (see also Mesnick, 1997). Smuts (1992) cites benefits of protection against potential rapists to help explain patterns of female social relationships, including strategies of forming long-term relationships with particular males, forming strong female–female bonds, and mustering support from relatives. Of course, one can imagine these behaviors having evolved due to the broader benefits they provided, including protection from nonsexual assault and predation.
Also in accord with the hypothesis of female anti-rape adaptation is research on the effects of ovulatory cycle status. This line of research assumes that rape is most costly when pregnancy is most likely, specifically during the ovulatory phase of the menstrual cycle. Bröder and Hohmann (2003), for example, found that, during the ovulatory phase, naturally cycling women reduced risky behaviors and increased nonrisky behaviors, whereas women using hormonal contraceptives that suppress ovulation did not show either effect. This study replicated earlier research that found reduced risk-taking during the fertile phase of the cycle (Chavanne & Gallup, 1998). Both of these studies relied on potentially unreliable self-report methods for identifying ovulatory cycle status, however. The results of both studies are also difficult to reconcile with the results of studies using luteinizing hormone tests to verify ovulation; those latter studies show that women are not risk averse during high fertility. For instance, when asked to illustrate what they would wear to a social event, undergraduate women indicate that they prefer more sexy and revealing clothing around high fertility as compared to low fertility. In the same set of experiments, women who had previously experienced sexual intercourse (but not those who had not) showed more skin and wore sexier outfits to a lab session at high fertility than at low fertility (Durante, Li, & Haselton, 2008; see also Haselton, Mortezaie, Pillsworth, Bleske, & Frederick, 2007). Relatedly, Gangestad, Thornhill, and Garver-Apgar (2010) found that partnered women were more likely when in the fertile phase to report greater willingness to pursue sexual opportunities, including a greater willingness to have sex with unfamiliar men.
An intriguing subset of ovulatory cycle studies relates to physical strength. Petralia and Gallup (2002) showed that, in response to a sexual assault prime, naturally cycling women demonstrate greater handgrip strength than baseline, but only in the fertile phase. By comparison, naturally cycling women in other phases and women using hormonal contraceptives showed no effect of imagined sexual assault on handgrip strength. Neither did a control group exposed to a neutral prime show effects that differed from baseline. Similarly, Prokop (2013) found that women at high conception risk score higher than women at low conception risk on a measure of perceived physical condition (e.g., “I am physically stronger than other people of the same age and sex”). These results support the possibility that specially designed mechanisms may mobilize resistance to rape when conception risk is higher.
Other research consistent with activation of dedicated perceptual mechanisms specially designed to limit the incidence of male sexual coercion includes a study that exposed women to taped short interviews of men. Fertile-phase women rated the men as more sexually coercive than did comparable nonfertile women, whereas fertility status did not affect ratings of traits hypothesized to be less clearly related to sexual coercion, such as kindness, commitment, and faithfulness (Garver-Apgar, Gangestad, & Simpson, 2007). Navarrete, Fessler, Fleischman, and Geyer (2009) found that White women demonstrated greater out-group bias against Black men when in the fertile phase, which the authors interpreted as consistent with a “coercion avoidance perspective” (p. 664). This interpretation assumes that group membership is a feature relevant to assessing risk of sexual coercion.
McKibbin, Shackelford, Miner, Bates, and Liddle (2011) tested the impact of a woman's relationship status, self-perceived physical attractiveness, and proximity to kin as they relate to anti-rape adaptation. Given that would-be rapists should prefer and target more attractive women (e.g., McKibbin, Shackelford, Goetz, & Starratt, 2008; R. Thornhill & Palmer, 2000), the authors predicted that a woman's attractiveness would correlate positively with frequency of rape-avoidance behaviors. Because mated women may experience greater losses from rape than do unmated women, including partner abandonment, they also predicted that mated women would perform more rape-avoidance behaviors. Finally, because a woman's relatives should be motivated to guard her from rape-related harm, the authors predicted that frequencies of rape-avoidance behaviors would increase with the number of family members living in close proximity.
Each of the predictor variables correlated positively with total scores on a rape-avoidance inventory. The authors also reported results according to the inventory's four subscales: (1) avoid strange men, (2) avoid appearing sexually receptive, (3) avoid being alone, (4) awareness of surroundings/defensive preparedness. Self-perceived attractiveness correlated with the third and fourth subscales, relationship status with the second and third, and total number of family members residing in close proximity with the second and fourth. However, in multiple regression analyses for the predictor variables on the total scale and its component parts, only a woman's relationship status consistently predicted her rape avoidance (see Snyder & Fessler, 2012, for purported failures to replicate several of these findings; see McKibbin & Shackelford, 2013, for criticisms of Snyder & Fessler).
Lastly, R. Thornhill and Palmer (2000) discussed evidence for anti-rape adaptations involving degrees and types of psychological pain experienced by rape victims. Unfortunately, as Ellsworth and Palmer (2011, p. 359) recently remarked,
instead of following Thornhill and Palmer's call for new and better research on this crucially important topic, the interest in [antirape adaptations] has been focused primarily on searching for flaws in the original data…and its interpretation.…We know of no recent evidence on psychological pain of rape victims related to the variables of age, marital status, and type of behavior, and we strongly encourage future research on these areas.
Further investigation would likewise be useful to reconcile studies showing increased mate seeking during high fertility (with results lending support to the “good-genes” hypothesis of sexual selection) with studies providing evidence consistent with anti-rape adaptation. In sum, although promising avenues of exploration into female anti-rape adaptation do exist, more research is needed.
EP theory and research seek to better understand the ultimate causes and the design of evolved psychological mechanisms underlying manifest behavior. In addressing sexual coercion, there has been considerable focus on whether there may have been, on average, fitness consequences in recurring ancestral environments of the ability to successfully avoid and/or inflict sexual coercion. The growing body of research in the past few years suggests that indeed there may be evolved specialized mechanisms in females designed to avoid being sexually coerced.
The discussion in this chapter focusing primarily on perpetrators suggests three competing models:
In seeking to identify potential candidates for specialized mechanisms, it is useful to reiterate that sexual coercion may be produced by differing motivations and antecedents. Rapists identified by the legal system are typically generalists who commit various types of antisocial behavior and often may not reveal the activation of specialized mechanisms motivating sexual coercion. In contrast, among sexual aggressors in the general population, a larger percentage of men are specialists who may be particularly useful for studying such mechanisms. We explored the possibility that SAF may be an evolved specialized mechanism for engaging in sexual coercion. The viability of the EF model for such arousal must be determined by its ability to generate testable predictions. It is important, therefore, to examine how this model has fared in its predictions in contrast to other models. The following is a summary of our conclusions in comparing the EF model to others focusing only on proximate causes, as well as to a by-product evolutionary model.
How many men in the general population would be expected to show relatively high SAF? In conceptualizing sexual coercion as either the result of sexual pathology or general antisocial characteristics, proximate models typically predict that only the few “sick” or “antisocial” would fail to inhibit SAF and/or show increased arousal by the inclusion of force. It is not clear whether a by-product model would make any predictions regarding the expected frequency of differing sexual arousal patterns.
The EF model suggests that some psychological mechanisms may have evolved that, when activated by environmental conditions (e.g., repeated rejection from desired women, early experiences with exploitative relationships, and environmental messages via the mass media that communicate favorable images of sexual violence), increase the likelihood of effectively implementing a coercive sexual act. Although the calibration of their arousal mechanism would not depend only on these experiences, and the relevant environmental conditions would need to be better described, within the general population a substantial minority of men would show lack of inhibition and/or increased SAF. The various sources of data described earlier appear to indicate that a substantial minority (e.g., as much as one-third of the population) reveal the type of arousal pattern that indicates the disengagement of the inhibitory mechanism and/or increased arousal to force and are, therefore, arguably most consistent with the EF model.
In addition to the expectations regarding frequency of sexual arousal patterns, various models may have differing expectations about which characteristics of men will be associated with the differing arousal patterns. Proximate models typically predict that men who show greater SAF will reveal general sexual deviance, a lower threshold for sexual arousal in general, and/or general antisocial characteristics. The data do not support these predictions: SAF is not associated with increased antisocial or deviant characteristics and behavior, and neither is it fully explainable by a lower threshold for becoming sexually aroused. A by-product model might not predict any systematic association between SAF and any characteristics or behaviors.
The EF model predicts that the degree of perceived blocked sexual access to desired women and resultant emotional responses (e.g., anger, hostility) will be relevant to the development of SAF. The data pertaining to correlates of SAF described earlier and elsewhere are consistent with the EF model by showing strong connections with hostility to women and perceived rejection.
Different predictions arise from the various models regarding the function of SAF and, therefore, its association with sexually coercive behaviors. Some nonevolutionary proximate models argue that responses such as fantasies of rape and SAF have no association to behavioral inclinations. Similarly, a by-product model would not make any clear predictions one way or the other about an association between SAF and actual coercion. The EF model suggests a direct role of SAF for energizing behavioral tendencies. Inhibited arousal would be expected to discourage sexual persistence in response to a woman's lack of sexual responsiveness; in contrast, the disengagement of such inhibition and increased arousal would be expected to facilitate sexually aggressive tendencies and, under some conditions, increased sexual aggression. The data showing that SAF is a key predictor of reported likelihood of raping and of actual sexual coercion, as well as the findings that such arousal is a critical mediator between hostile masculinity characteristics and sexual aggression, are consistent with the EF model.
Further theoretical development and empirical testing are needed to assess the viability of the EF model we have described. The difference in the type of questions raised by such a model is apparent when we contrast it to those raised by proximate models. For example, Barbaree and Marshall (1991) published a thorough attempt to compare differing models focusing on SAF. Although they describe the purpose of the models as designed to “account for men's sexual arousal to descriptions of rape” (p. 621), all six models provide descriptive analyses designed to identify the critical features distinguishing sexual aggressors from nonaggressors (e.g., the ability to suppress sexual arousal or the augmentation of sexual arousal by other emotional states such as hate). None of these models address why there might be certain design features that lead to observed recurring patterns of individual differences in SAF (e.g., Why it is that some men, but not others, who perceive rejection from desired women develop a pattern of SAF, whereas women who are similarly rejected by men do not show such a pattern?). Although we recognize that such questions should be asked with sensitivity to their potential misunderstanding and misuse within certain political contexts, we believe that they may provide useful insights into acts most people consider morally repugnant and consequently yield better preventative policies.