Sexual Conflict and Partner Rape

Transcription

1 C H A P T E R 15 Sexual Conflict and Partner Rape Joseph A. Camilleri and Vernon L. Quinsey Abstract The application of Darwinian concepts, particularly sexual selection, has gained currency in recent years (Goetz, Shackelford, & Camilleri, 2008). One facet of sexual selection, sexual conflict, is central to an understanding of the evolution of traits that arise in response to the mating strategies of the opposite sex. Sexual conflict theory may explain a variety of sexually coercive behaviors used by men against women, including rape, mate guarding, and infanticide. Here we review the literature on partner sexual coercion in the context of sexual conflict. Key Words: partner rape, marital rape, partner sexual coercion, sexual conflict Introduction Sexual Conflict Sexual conflict occurs where the reproductive interests of the sexes diverge. For sexual conflict to occur, the fitness benefit of a trait in one sex must result in a cost to the other (Parker, 1979, 2006), causing the evolution of sexually antagonistic counterstrategies, in some ways similar to host parasite coevolution. Sexual conflict theory was first popularized by Parker (1979) to explain how traits that benefit males, while imposing a cost on females, could have evolved. At the time, the idea of sexual selection was used only in the context of intermale competition, female choice, and parental investment. At present, sexual conflict is divided into two general categories. Intralocus sexual conflict involves traits that are expressed in both sexes (i.e., allele variation at the same locus in both sexes is in conflict but has different fitness consequences in each sex). A commonly cited example of intralocus sexual conflict is hip size the optimal size for women is larger for childbirth, but larger hips come at a cost to men in terms of reduced agility and mobility, hence the sexual dimorphism in hip size (Price & Hosken, 2007; Rice & Chippindale, 2001). Interlocus sexual conflict, on the other hand, occurs when male female interactions have different fitness consequences in the two sexes (i.e., different loci in each sex are in conflict). For example, interlocus sexual conflict occurs when locus A in males codes for attempt mating given a particular situation, and locus B in females codes for resist mating in the same situation (Parker, 2006). Interlocus sexual conflict is the subject of the present review. Not all sexually dimorphic traits, such as size in humans, result from or elicit sexual conflict (Parker, 2006). For sexual conflict to drive sexual selection, at least two criteria need to be satisfied. First, the harm experienced by one sex must be adaptive to the other sex (called adaptive harm or adaptive costs) in order to rule out harm as a by-product of the trait in question (called collateral harm or collateral cost; Lessels, 2006; Parker, 2006; Watson-Capps, 2009). Second, although there may be direct costs to either male or female fitness, indirect benefits must also be ruled out. The presence of male coercion, for example, does not rule out the possible indirect benefit of the coerced female having sexually _Shackelford-Ch15.indd 257 9/7/2011 7:04:52 PM

2 coercive sons (Mulder & Rauch, 2009). In this case, coercion does not result from sexual conflict. There are ways to assess if coercion is mutually beneficial. For example, Parker (2006) explained how shared genetic benefit is unlikely if the trait in males is recessive. What complicates the study of sexual conflict in humans is the difficulty in gathering data on coevolutionary trajectories current sources of conflict that may have evolved in response to previous sources of conflict (Mulder & Rauch, 2009). This does not mean novel hypotheses derived from sexual conflict theory cannot be tested in humans. Although many of the methodologies that might be employed to study sexual coercion, such as genetic experiments, cannot be used with humans, alternative approaches such as comparative studies, selfreported behaviors and cognitions, and physiological measures, can address some of the questions posed by sexual conflict theory (Arnqvist & Rowe, 2005; Goetz & Shackelford, 2009). Sexual conflict theory, while challenging the traditionally held belief that reproduction has been mutually beneficial and led to sexual cooperation (Chapman, Arnqvist, Bangham, & Rowe, 2003), has emerged as an important theoretical explanation for sex differences across species (Arnqvist & Rowe, 2005). More recently, sexual conflict has been used to understand sexually coercive behavior, with some attention being given to primates (Muller & Wrangham, 2009). We therefore define and review partner rape and describe how sexual conflict has been used to explain sexual coercion before turning to theoretical explanations of several types of partner sexual coercion using recent research on the causes of partner rape. We conclude by discussing directions for future research. Partner Rape and Sexual Coercion: Definition and Prevalence One of the most widely cited definitions of sexual coercion was provided by Smuts and Smuts (1993): use by a male of force, or threat of force, that functions to increase the chances that a female will mate with him at a time when she is likely to be fertile, and to decrease the chances that she will mate with other males, at some cost to the female (pp. 2 3). This definition provides a functional explanation for why coercion occurs, rather than an operational definition that identifies when the behavior is present or absent, thereby excluding sexually coercive acts that could result from a disorder or as a byproduct of another trait (Camilleri, 2010). What causes sexual coercion is an empirical question and should not be embedded in the definition of sexual coercion. We therefore define partner sexual coercion as forceful or manipulative tactics to obtain sex from a reluctant partner that may result in physical or emotional harm (Camilleri, Quinsey, & Tapscott, 2009). The possible fitness consequences of sexual coercion are a subject of empirical investigation rather than part of its definition. Sexually coercive acts vary in severity (see Partner Sexual Coercion section). Partner rape refers to forcing one s partner to copulate, which is the most extreme form of coercion in relationships. Depending on the sampling procedures, data collection procedures, and definition, prevalence rates of partner sexual coercion in humans vary from 7% to 34% (Basile, 2002; Costa, Braun, & Birbaumer, 2003; Finkelhor, Hotaling, & Yllo, 1988; Hanneke & Shields, 1985; Russell, 1990; Tjaden & Thoennes, 1998). Compared to rape victimization rates against women over the age of 14 years (13% 19%), some estimates of partner rape are relatively high (reviewed in Kolivas & Gross, 2007). In a recent search of the psychological literature, only about 2% of psychological papers focused exclusively on partner sexual coercion, and of those papers, 3% tested hypotheses regarding its causes (Camilleri & Quinsey, 2009a). Since that time there has been a steady increase in the number of empirical papers on partner rape. Not only is it important to study this topic, but also establishing a theoretical framework will help generate new hypotheses. Sexual Conflict, Sexual Coercion, and Evolutionary Psychology There have been three general evolutionary approaches to understanding sexually aggressive behaviors: sexual coercion, sexual conflict, and evolutionary psychology. In nonhuman animals, sexual coercion was approached by ethologists as a third mechanism of sexual selection, whereas sexual confl ict theory was used more broadly to explain sex differences and only rarely used to explain sexual coercion (Watson-Capps, 2009). Watson-Capps (2009) synthesized these ideas by suggesting that sexual conflict about when and with whom to mate results in sexual coercion, and that sexual coercion maintains sexual conflict. A recently edited book has summarized the literature on sexual conflict and coercion in primates (Muller & Wrangham, 2009). Although a section on humans was also included in this edited volume, there were substantial gaps 258 SEXUAL CONFLICT AND PARTNER RAPE 15_Shackelford-Ch15.indd 258 9/7/2011 7:04:53 PM

3 (Camilleri, 2010), probably due to the lack of research and theoretical organization of sexual conflict in humans. In humans, evolutionary psychology has been used to explain three manifestations of rape, including rape as an adaptation to gain sexual access to unreceptive females (e.g., Thornhill & Palmer, 2000), rape as a by-product of men s reproductive strategy (e.g., indiscriminate sex, mating effort; Lalumière, Harris, Quinsey, & Rice, 2005), and rape as a sexual disorder (e.g., pedophilia, coercive paraphilic disorder; Quinsey, 2010). The benefit of sexual conflict theory is that it provides an overarching process through which the various adaptive versions of sexual coercion (i.e., not just rape), could have evolved and been maintained in human populations. Muller, Kahlenberg, and Wrangham (2009) suggested that a taxonomy of coercion results from sexual conflict, including direct coercion, indirect coercion, and infanticide. Although these taxonomies have been applied to nonhuman animals, they may provide a more complete understanding of the various types of sexual coercion in humans. Direct coercion refers to forceful tactics used by males to overcome female resistance to copulate, including forced copulation, harassment, intimidation, and punishment. Indirect coercion refers to forceful tactics, sometimes referred to as coercive mate guarding, used by males to reduce the likelihood of partner extra-pair copulation. Indirect coercion includes herding, sequestration, and punishment. Lastly, infanticide is aggression directed toward a female s offspring sired by other males (Clarke, Pradhan, & van Schaik, 2009). We therefore focus on sexual conflict and partner sexual coercion by first addressing the possible sources of conflict, then by reviewing how sexual conflict might explain direct coercion, indirect coercion, and infanticide in the context of human pair bonds. Sexual Conflict and Partner Rape Sources of Conflict Sexual coercion is a mechanism that results from sexual conflict about when and with whom to mate. In humans, as with other species that form pair bonds, sexual conflicts may occur if divergent reproductive interests continue to exist even after acquiring a mate. We therefore discuss: (1) female resistance in pair bonds, because this resistance conflicts with men s interest in when to mate, and (2) cuckoldry risk, because this risk represents conflict over whom one s partner is mating with. Female resistance and mate choice. As originally noted by Bateman (1948), men can increase their fitness by obtaining more sexual partners whereas women cannot (Andersson & Iwasa, 1996; but see Cuckoldry Risk and Sperm Competition section). Alternatively, women are more successful at increasing their fitness by attaining higher quality mates, such as those who could provide the necessary resources for offspring to flourish (see Alcock, 2001; Buss & Schmitt, 1993). Thus, women s choosiness over whom to mate with conflicts with men s interest in increasing the number of mating opportunities. Results from Clark and Hatfield s (1989) study are often cited as an example of such divergent sexual interests men and women were equally as likely to go on a date with someone they just met, but 75% of men, and none of the women, agreed to sexual intercourse. Theory and data therefore raise the question of whether women continue to show more sexual disinterest than men, even with a committed sexual partner. It is well known that even in relationships, men are typically the sexual initiators and exhibit stronger sexual drive (Baumeister, Catanese, & Vohs, 2001; Impett & Peplau, 2003) and that marital dissatisfaction is related to sexual dissatisfaction in both men and women (reviewed in Litzinger & Gordon, 2005). For men, it is possible that marital satisfaction is more closely linked to sexual satisfaction because reduced intercourse in mating dyads may have been costly for ancestral males. Not only does increased mating with one s partner increase the total number of possible offspring, frequent mating also provides safeguards against cuckoldry (Birkhead, 2000). There is some research that indicates that women in relationships desire sexual intercourse with their partner less often than men do (e.g., Ard, 1977; Johannes & Avis, 1997; Trudel, 2002). This disinterest does not always lead to persistent refusal by women, however. Sexually disinterested women sometimes engage in intercourse to avoid the negative consequences of refusing or to fulfill a sense of obligation (Impett & Peplau, 2003; O Sullivan & Allgeier, 1998). In a sample of undergraduate students who were sexually active in dating relationships, O Sullivan and Allgeier (1998) found that a significantly larger proportion of women (50%) consented to unwanted sexual activity than men (26%) over a two-week period. Although the most commonly cited reason among women for relenting was to satisfy her partner s needs (42%), a substantial proportion cited wanting to avoid tension in the relationship (15%). This disinterest CAMILLERI, QUINSEY _Shackelford-Ch15.indd 259 9/7/2011 7:04:53 PM

4 could indicate other sources of conflict, such as infidelity or impending dissolution of the relationship. Sexual conflict can only arise from actual refusal, not just a disinterest, so to what degree do women refuse sexual advances by their partners? Data on sex differences in refusal rates are not readily available. In a small study on dating relationships, Byers and Lewis (1988) found that a large proportion of men (96%) initiated sexual activity using nonverbal cues, whereas a large proportion of females refused the sexual advances verbally (41.5%) or physically (42% moved or pushed man away; 48% blocked or did not perform sexual activity). Unfortunately, there are no initiation rates for women or refusal rates from men for a more appropriate comparison. O Sullivan and Byers (1996) found that significantly more women (89%) than men (56%) reported sexual reluctance with a partner. In dating relationships, men are more likely to attribute their virginity or lack of sexual experience to the unavailability of willing partners than are women (Driscoll & Davis, 1971; Sprecher & Regan, 1996). When collapsing across dating, common-law, and marital relationships, Camilleri (2008) found that a greater proportion of women (10.5%) than men (3.8%) reported having partners who never refused sexual intercourse. From the partner s perspective, in other words, more men than women reported having a partner who refused sex. Similar patterns were found when sampling from more committed relationships, though there have been surprisingly few thorough investigations of this topic. Several studies have measured sexual refusal but collapsed across items and did not report any sex differences (e.g., Bancroft, Loftus, & Long, 2003; Ellis, 1998). It is important to note the importance of certain controls, such as the number of sexual initiations made by men, because increased refusal rates may be due to increased initiations. Supporting this possibility, Byers and Heinlein (1989) found that, although women refused sex more often than men, there were no sex differences after controlling for the number of initiations made by men. Of course, refusal in relationships is substantially lower than refusal outside relationships. Being in a relationship assumes the existence of agreed-upon sexual activity and a degree of exclusivity, going some way in explaining the only moderate sex differences in refusal rates. In addition, as mentioned earlier, women sometimes relent for a variety of positive and negative reasons, including a feeling of obligation. A quarter of women in a sexual relationship from a national probability sample reported substantial distress about their sexual relationship (Bancroft et al., 2003). Interestingly, the source of this distress was not physical (e.g., inability to experience arousal, lubrication, or pain during intercourse) but was from the emotional relationship with the partner. A number of studies have shown that indicators of sexual and emotional problems in relationships (e.g., apathy toward a partner, relationship dissatisfaction, and emotional disengagement) are related to infidelity (e.g., Drigotas, Safstrom, & Gentilia, 1999; Fisher et al., 2009; Shackelford & Buss, 1997). More directly, Gangestad, Thornhill, and Garver (2002) found that women s interest in extramarital sex, not sex with one s partner, increased during ovulation. This appears to be particularly true when male partners are not viewed as attractive (Pillsworth & Haselton, 2006). Indeed, cuckoldable men, or men with less desirable characteristics, face greater pressure of sexual conflict than other men. These individual differences among men pose interesting questions. Cuckoldry risk and sperm competition. Despite forming pair bonds, many men and women are not entirely sexually monogamous. Estimates of female infidelity, for example, range from 20% to 40%, (reviewed in Buss, 2000, 2003; Shackelford, Pound, Goetz, & LaMunyon, 2005). Extra-pair copulations among women may have served several functions, including mating with males with better genes, gaining material benefits from an extra-pair partner, paternity confusion leading to more resources invested in offspring, status enhancement, diversifying genes, broadening social networks for employment, and replacing a low-quality mate (reviewed in Mulder & Rauch, 2009; Wilson & Daly, 1992). These fitness benefits come at a cost to males. An important fitness consequence is sperm competition, occurring when women have multiple mateships within one ovulatory cycle. In pair bonds, this leads to cuckoldry risk. Estimates of human offspring resulting from extra-pair copulations range from 1% to 30%, averaging around 10% of the population across cultures (Buss, 2003). Cuckoldry risk in humans is not as strong as cuckoldry risk in some other species that form pair bonds. Across bird species, for example, paternity certainty is related to mate guarding and copulation frequency, the most common male adaptation to sperm competition (Moller & Birkhead, 1993). Extra-pair matings in humans, although apparently less frequent than in some other species, provide important potential 260 SEXUAL CONFLICT AND PARTNER RAPE 15_Shackelford-Ch15.indd 260 9/7/2011 7:04:53 PM

5 reproductive benefits to women and fitness costs to men. The costs incurred by men vary with circumstance. For example, losing a partner to a rival becomes more costly depending on the availability of alternative mating opportunities. Nonexclusive mating by females has selected for a variety of adaptations and counteradaptations across species (Stockley, 1997), and we should expect the same in humans. Because cuckoldry presents substantial cost to men s reproductive fitness, sperm competition may account for certain sex differences in human psychology and morphology. Thus far, the influence of sperm competition in humans has been studied in terms of sexual interest (Pound, 2002), attraction and interest in a sexual partner (Shackelford et al., 2002), sexual behaviors (Shackelford, Pound, & Goetz, 2005), and penis morphology (Gallup et al., 2003). One of the most studied anticuckoldry adaptations is sexual jealousy (see Goetz et al., 2005). For example, men tend to be more suspicious than women of partner sexual infidelity. In two studies, Paul and colleagues (1993, 1994) found that 36% 37% of men and 14% 17% of women were unsure whether their partner had been unfaithful. They explained these differences by suggesting that having such an adaptive bias motivates men, more than women, to prevent infidelity. An adaptive reproductive strategy in one sex (extra-pair copulation) comes at a fitness cost to the other sex (raising unrelated offspring or losing a mating partner), meeting the criteria for sexual conflict. Partner sexual coercion is therefore understood as another possible adaptation to sperm competition (for a more comprehensive review of the human sperm competition literature, see Shackelford, Pound, Goetz, et al., 2005). Because the implications of sperm competition on sexual conflict are reviewed in another chapter in this volume, we will focus on its effects on sexually coercive behaviors. Partner Sexual Coercion Although the terms sexual coercion and rape are sometimes used synonymously, rape generally refers to the most extreme form of sexual coercion in terms of force and victim injury. A more refined vocabulary is needed because sexual coercion can take different forms. Muller et al. (2009) outlined a taxonomy of sexual coercion. The first set of behaviors are known as direct coercion, which includes the most severe form of forced copulation but also includes harassment in attempting to mate and punishment to increase the probability of future mating with that female (Clutton-Brock & Parker, 1995). The second set of behaviors is known as indirect coercion, which includes herding (shortterm aggression to separate females from other males), punishment (decrease likelihood of mating with other males), and sequestration (removal of female from social group). The last is infanticide, where aggression is directed not at the female but at her offspring. We will address each form of sexual coercion by describing how the trait could have evolved in response to sexual conflict. Direct Coercion in Humans Forced copulation. Earlier we described a number of traits that could have evolved in response to one manifestation of sexual conflict, cuckoldry. Cuckoldry risk is an important proximal determinant of when sexual coercion is likely to take place in relationships. Indeed, Lessells (1999) suggested that the most intense form of conflict is the risk of one parent raising unrelated offspring. The relationship between female infidelity and sexual coercion in relationships has been demonstrated in three studies. Goetz and Shackelford (2006) were the first to directly measure the relationship between infidelity and partner sexual coercion in humans. They found a correlation between self-reported likelihood of partner infidelity and the frequency of sexually coercive behaviors over one month. These correlations were found not just from the self-reports by men but were confirmed by women s reports as well. In a second study, Starratt, Goetz, Shackelford, McKibbin, and Stewart-Williams (2008) found that insults made by other men regarding partner infidelity were related to greater frequency of partner sexual coercion. These studies suggest that more coercion occurs when there are greater degrees of perceived or real conflict. Increasing copulation frequency is a well-known adaptation to sperm competition across many species (Birkhead, 2000). In humans, for example, there is a correspondence between copulation frequency and use of mateguarding tactics (Kaighobadi & Shackelford, 2008). Of course, sexual coercion frequency could vary with infidelity risk for reasons that are unrelated to sexual conflict perhaps women who are unfaithful do so in response to being victimized by sexually abusive men. If sexual coercion by men against their partners evolved as a response to cuckoldry risk, the relationship between coercion and risk should be specific. For example, Camilleri and Quinsey (2009b) tested CAMILLERI, QUINSEY _Shackelford-Ch15.indd 261 9/7/2011 7:04:53 PM

6 hypotheses regarding the temporal sensitivity to cuckoldry risk, the variability of coercion severity, and sex differences. First, because cuckoldry risk is a dynamic risk variable (i.e., risk fluctuates over time and circumstances), the probability of coercion should vary temporally with risk. That is, if partner sexual coercion functions by reducing the risk of cuckoldry through engaging in sperm competition, men should exhibit the greatest propensity for partner rape when the risk is present. Because of the costs associated with sexual coercion (e.g., physical injury, loss of partner, retribution from partner s family), it would be maladaptive for men to employ coercion when the risk is absent or after changing partners. Men should be willing to take such a risk when the fitness benefits of coercion outweigh the costs such as when cuckoldry risk is highest. Camilleri and Quinsey (2009b) asked men and women in relationships if they had experienced cues to partner infidelity and, if so, how recently. There were two measures relating to cuckoldry risk: total number of cuckoldry risk events, and average time since these events were experienced. The Sexual Coercion subscale (COERCE) of the Tactics to Obtain Sex Scale (Camilleri et al., 2009) was used to evaluate changes in partner rape propensity. On this measure, participants were asked how effective and how likely they would be to use a series of sexually coercive acts to obtain sex from a reluctant partner on that particular day. Higher COERCE scores indicate not only greater self-reported interest in using sexual coercion with a partner, but also greater self-reported probability of using a variety of tactics rather than only a few. They found that when the events took place recently, there was a significant positive relationship between the number of cuckoldry risk events and self-reported interest in partner rape. Weaker and nonsignificant relationships were found when cuckoldry risk events took place further in the past. Of course, self-reported interest is not the same as engaging in sexually coercive behaviors, but Camilleri and Quinsey (2009b), using police crime synopses, have found that 73% of convicted partner rapists experienced some degree of cuckoldry risk prior to committing the offense and experienced more such events than convicted partner batterers. An early study showed that partner rape is likely to occur when a partner leaves (Finkelhor & Yllo, 1985). Contested divorce is certainly a source of conflict because the outcome is not mutually beneficial (Lessels, 1999). In a married sample, 3% of women reported having a husband who ever used physical force or threat to try to have sex, whereas 25% of women who were separated or divorced answered this question in the affirmative. These data are consistent with the cuckoldry risk hypothesis, although it is possible these women left their partner because of the sexual violence. We also do not know when these acts took place it is possible they occurred early in the relationship, not after its dissolution. Camilleri and Quinsey (2009b), however, found that 9% of partner rapists had a partner who left or planned to leave, suggesting partner rape could be a last-ditch effort to copulate before losing that partner to a rival. Considering the costs associated with sexual coercion, Camilleri and Quinsey (2009b) also predicted that more severe tactics to obtain sex from a partner should occur when the risk of cuckoldry is obvious or high. Indeed, direct cues to cuckoldry risk (e.g., She told you that she had been sexually unfaithful to you ) were related to sexual coercion propensity, not sexual coaxing, whereas indirect cues to cuckoldry (i.e., greater proportion of time away from one s partner since last intercourse only when it s been a while since last intercourse) were related to propensity in sexual coaxing, not coercion. Lastly, if a psychological mechanism is sex specific, it should be present in one sex and absent in the other (many of the studies on human adaptations to sperm competition have tested for their effects among men but have not falsified the possibility of the same effects among women). If sexual coercion evolved among men in response to cuckoldry risk, support could be found by demonstrating a relationship between these variables among men, not among women. 1 Camilleri and Quinsey (2009b) asked women about their perceived infidelity risk and their propensity to engage in sexual coercion with their partner. In none of the analyses were measures of cuckoldry risk related to either sexual coercion or coaxing among women, providing further evidence that partner rape by men evolved in response to sexual conflict from female infidelity. Harassment. In many nonhuman species, males will persist in their pursuit of females until females relent or escape (Clutton-Brock & Parker, 1995). This persistence does not fall under forced copulation because physical aggression is not used. Because the degree of conflict depends on the degree of discordant interest in current and future mating within the pair (Lessels, 1999), responses may vary with the degree of conflict. We know that women show greater disinterest in intercourse than men in the context of relationships so do men harass their 262 SEXUAL CONFLICT AND PARTNER RAPE 15_Shackelford-Ch15.indd 262 9/7/2011 7:04:53 PM

7 partners more often in order to obtain sex when risks of cuckoldry are present? Shackelford, Goetz, McKibbin, and Starratt (2007) used an indirect proxy of sperm competition risk, proportion of time away from one s partner since last having intercourse, to study sexual interest. They found that men who spent a larger proportion of time apart from their partner persisted more at trying to obtain sex. Proportion of time apart since last sex is a useful proxy of sperm competition risk because extra-pair copulations take place when the male is absent, but one concern is that, for example, 50% time apart represents an elevated risk only if the time since last intercourse was further away. Some participants might have a 50% score, but if they had intercourse that morning, their risk is small compared with someone with a 50% score who had intercourse two weeks before. One way to address this issue is to statistically control for the time since last intercourse and to ensure participants had intercourse recently, as was done by Shackelford et al. (2007). Camilleri and Quinsey (2009b) took another approach by testing for the interaction between proportion and time since last intercourse and found that the strongest negative relationship between proportion of time away from one s partner and propensity toward sexual coaxing one s partner was when the time since last intercourse was far away. Also, Camilleri and Quinsey (2009b) found that this indirect measure of cuckoldry risk was only related to sexual coaxing behaviors, not sexually coercive behaviors, suggesting forced copulation is more closely related to direct high-risk situations. Lastly, this interaction was found only among men, not among women, supporting the view that harassment could also have evolved specifically among men in response to sexual conflict. Intimidation and punishment. Intimidation in the context of sexual coercion punishes females for refusing intercourse in order to increase the future probability of intercourse (Clutton-Brock & Parker, 1995). For example, male chimpanzees will attack a female during courtship until she cooperates. In humans, physical violence resulting from sexual jealousy is well known. What is not known is the ultimate function of physical violence in relationships. Sexual conflict theory suggests that one, or a combination of the following functions, is possible: (1) increasing the probability of mating with the partner (direct coercion), (2) decreasing the probability of extra-pair mating (indirect coercion), and (3) decreasing the probability of leaving the relationship (indirect coercion). Currently available data lend more support for intimidation and punishment as a form of indirect coercion, so we reserve our discussion on punishment for that section. Individual differences. Because forced copulation and harassment are in direct response to female refusals, some males facing such refusal may therefore be nonpreferred (Muller et al., 2009). In the context of pair bonds, the notion of being nonpreferred may apply if some males are more cuckoldable. Thus, any trait or lack of trait representing lowembodied capital (Lalumière et al., 2005) could pose higher risks of cuckoldry, leading to higher overall rates of sexually coercive behavior. Initial data do not provide support for this hypothesis. For example, partner rapists show normal IQ, and neurodevelopmental insults were unrelated to self-reported interest in partner sexual coercion (Camilleri & Quinsey, 2009a). It also appears as though coercive men are more successful in terms of partner number than noncoercive men (Lalumière, Chalmers, Quinsey, & Seto, 1996). Still, the relationship between characteristics of nonpreferred males and partner sexual coercion requires further investigation. Indirect Coercion in Humans Coercive tactics by males may not immediately result in copulation. Some coercive acts are also sexual if they function to minimize options of extra-pair mating or increase probability of future mating these acts are considered indirect coercion, sometimes also called coercive mate guarding. Sexual conflict arising from female sexual promiscuity may have led to preventative tactics among men, such as herding, punishment, and sequestration (Muller et al., 2009). Female promiscuity in socially monogamous species could also result in sexual conflict within pair bonds, where males use punishment in response to a refusal and herding/sequestration as a general preventative strategy. Herding/Sequestration. Herding and sequestering are related behaviors herding involves using aggression to immediately separate females from other males; sequestration involves aggressively separating a female from the social group to prevent extra-pair copulations (Muller et al., 2009). Mate retention behaviors appear to be common in humans. Buss (1988) documented 104 mate retention behaviors that clustered into 19 factors. A number of these factors are consistent with herding and sequestration, such as vigilance, concealing a mate, monopolizing partner s time, and threatening rivals. Research on mate guarding found that men spent more time with fecund (i.e., cycling) partners CAMILLERI, QUINSEY _Shackelford-Ch15.indd 263 9/7/2011 7:04:53 PM

8 than infecund partners (Flinn, 1988) and increased mate guarding during a partner s follicular phase (Gangestad, Thornhill, & Garver, 2002). More recently, Haselton and Gangestad (2006) showed that mate guarding by men increased when their partner was ovulating, especially when the woman was attractive and the man was not. Mate retention tactics appear to be used more frequently when female partners are young and attractive (Buss & Shackelford, 1997). Mate concealment and monopolizing a mate s time were used more frequently by men with younger partners. This effect, however, disappeared after controlling for men s age. Wilson, Daly, and Wright (1993) reported the use of violence during sequestration/herding. Using homicide as a conflict assay, women were at highest risk of being killed after separating from their husbands. Wilson et al. (1993) suggested that such killing is an extreme by-product of proprietary views of women by men that lead to sequestration. Others have proposed an alternative hypothesis for such data: killing an ex-partner could have functioned to exert fitness costs on rival males, for example (Buss, 2005). Buss (2005) also proposed that uxoricide could function directly to remedy a reputation of being a cuckold such a label could lower men s status by suggesting that he lacks the ability to keep a mate and fend off rivals. Unfortunately, there are no data available to test these alternative hypotheses yet, though the rarity of uxoricide relative to nonlethal abuse seems to suggest it might be a by-product. Another hypothesis related to herding and sequestration is that men who faced recurrent sperm competition risk should exhibit more mate retention tactics. Although several mate retention tactics correlated with recurrent risk of sperm competition, there was a negative relationship between risk and both concealment and derogation of the partner, and no relationship with vigilance (Goetz et al., 2005). The authors explained these results by saying men may not want to incite more conflict with their partner by using negative tactics. An alternative interpretation of these data is that a failure to use these negative tactics allowed for a recurring risk of sperm competition. Although experimental designs are optimal for determining causal effects, a longitudinal study would be useful in understanding the direction of these relationships. Punishment. Punishment, as it relates to sexual coercion, functions by decreasing the probability of extra-pair mating (Muller et al., 2009). In humans, physical violence resulting from sexual jealousy has been well established. Daly and Wilson (1988b) tested several hypotheses about the sources of conflict and the psychological mechanisms evolved in response to such conflict. Their data suggests that physical violence may function as a form of coercive control. In one study, they found that the use of autonomy-limiting behaviors by men (e.g., he insists on knowing who you are with and where you are at all times ) is related to more violent incidents against that spouse (Wilson, Johnson, & Daly, 1995). They suggest that uxoricide is a by-product of this proprietary adaptation. Second, if sexual conflict results from the possibility of extra-pair copulations, the greatest conflict should be observed when female partners have the highest reproductive value. Indeed, Wilson et al. (1995) found that uxoricide rates and nonlethal assaults are committed against younger women. These results have been confirmed in another sample, and these effects are not likely due to an overrepresentation of young male perpetrators the highest uxoricide rates were found when the age discrepancy between partners was also high (Shackelford, 2001; Wilson et al., 1993, 1995). Third, punishment may also occur in response to a partner s attempted or actual separation. This scenario poses a high degree of sexual conflict because of the impending loss of future reproductive opportunities. Again, using homicide rates to assay conflict, Wilson and Daly (1993) found that in samples from three different regions, uxoricide rates were consistently higher when partners were separated than if they were coresiding. It remains unclear if the violence functions to herd, sequester, or punish regardless, higher rates of violence occur when the partner has left. Camilleri and Quinsey (2009b) hypothesized that if physical violence in relationships function as coercive control to prevent female infidelity, then sexual violence should occur when the infidelity took place (or was thought to have taken place). There are some data that address this hypothesis. Shields and Hanneke (1983) found that 47% of married women who were raped and beaten by their husband had sex with another man, whereas only 23% of those beaten and 10% who were not victimized reported having sex with another man. In another study using a sample of partner rapists and nonsexual partner assaulters, Camilleri and Quinsey (2009b) looked at the number of cuckoldry risk events that occurred prior to the offense (as mentioned in the crime report). Although the numbers of partner rapists (73%) and partner 264 SEXUAL CONFLICT AND PARTNER RAPE 15_Shackelford-Ch15.indd 264 9/7/2011 7:04:54 PM

9 assaulters (62%) who experienced a cuckoldry risk event were not significantly different, partner rapists, on average, experienced more such events than the nonsexual assaulters. A more accurate test of this hypothesis would involve comparing these groups on whether infidelity was known, suspected, or probable Camilleri and Quinsey (2009b) suspected that more partner rapes occur when risk is certain or high whereas partner assaults occur when risk is suspected or probable. With a smaller data set, they did not have enough variability in these specific types of cuckoldry risk events. Infanticide in Humans Clarke et al. (2009) reviewed how infanticide could be sexually selected from the condition where males encounter females with offspring sired by another male. Evidence supporting this hypothesis in other species includes observing infanticide against unrelated offspring that results in the female returning to regular ovulatory cycle and sexual receptivity. Humans are unique, at least among primates, because they rarely commit infanticide (Muller et al., 2009). Looking at 2004 infanticide rates in Canada (killing a child less than one year of age), mothers accounted for nearly as many cases as did fathers (Brzozowski, 2004) in other primate species, with the exception of chimpanzees, infanticides are mostly committed by males (e.g., Hiraiwa- Hasegawa & Hasegawa, 1994). Thus, infanticide by humans, rather than having evolved as a response to sexual conflict, may require an alternative explanation. One possibility proposed by Daly and Wilson (1988a) is that infanticide is a by-product of what they called discriminative parental solicitude, commonly known as the Cinderella effect. They found, for example, that the presence of a stepparent substantially increased the risk of infanticide. These data do not rule out infanticide by men resulting from sexual conflict, only that currently available data provide little support. One study provides insight into the possibility of sexual conflict influencing infanticide in humans. Burch and Gallup (2004) found that men are more likely to physically abuse pregnant partners, but these men also had higher rates of sexual jealousy than men who abused partners who were not pregnant. Infanticide as a type of sexual coercion in humans has yet to be demonstrated. Summary and Conclusions A Darwinian perspective encourages a broad and inclusive conceptualization of rape. Rape is seen as one aspect of sexual conflict, a conflict that involves sexually dimorphic strategies and counterstrategies comprised of psychological, behavioral, and physiological tactics. The employment of particular tactics is context specific, varying according to whether those potentially using them are in a relationship and, if so, according to the probability of extra-pair copulations. The probability of extra-pair copulations, in turn, is affected by a variety of factors, including residential propinquity of the spouses, the relative age and attractiveness of the partners, the genetic compatibility of the partners, the presence of alternative partners, and so forth. The social relationship between sexual aggressor and victim is conceptualized as a particular environmental determinant of the strength and form of sexual conflict between the parties. Similarly, a Darwinian orientation causes one to identify the similar functions of a wide variety of apparently disparate male behaviors such as sequestration, mate guarding, harassment, and sexual coercion. Lastly, a Darwinian view leads us to expect that other species that experience similar environmental causes of sexual conflict may exhibit some of the same responses as humans. In sum, sexual conflict theory of sexual coercion starts with the identification of putative ultimate (evolutionary) causes of sexually dimorphic strategies and proceeds with the study of proximal environmental variations that affect the deployment of these strategies. The relationship of the parties involved in sexual behaviors is a principal determinant of the form and strength of sexual conflict and, therefore, the occurrence of sexual coercion. Future Directions The explanatory power of sexual conflict as it relates to partner rape depends on its ability to provide a consilient view on the causes and manifestations of sexual coercion in human pair bonds. To this end, several avenues of research should be pursued. Our first set of recommendations deals with understanding the extent to which sexual conflict is present in humans. More research is needed on divergent sexual interests in relationships in order to understand the extent to which they diverge and to identify the causes of such differences. For example, there may be specific genes relating to sexually dimorphic courtship behaviors, or individual difference characteristics of cuckoldable men, suggesting there may be static risk factors of partner sexual coercion. Also, a primary assumption of sexual CAMILLERI, QUINSEY _Shackelford-Ch15.indd 265 9/7/2011 7:04:54 PM

10 conflict that requires a clear demonstration in the context of partner rape is that a fitness benefit to one sex comes at a cost to the other sex. And in the interest of cohesion, further integration of the sexual coercion and sexual conflict literatures are needed. Second, sexual conflict theory has the potential to explain many different types of aggressive behaviors in relationships. For example, more research on the differences between forced copulation and physical aggression in relationships is needed. There is some indication that forced copulation functions to reduce cuckoldry risk after infidelity has occurred (i.e., direct coercion) whereas physical aggression functions to prevent infidelity from occurring in the first place (i.e., indirect coercion). These data are preliminary and require a more thorough investigation. One topic that has not been mentioned yet but could be subsumed under sexual conflict theory is stalking, possibly as a specific type of harassment. It should be noted that not all manifestations of sexual coercion seen in other species necessarily apply to humans, though tests are still needed. For example, at the moment there is little support for the hypothesis that infanticide functions as a form of sexual coercion in relationships. Anthropological data on child sacrifice and infant exposure would be illuminating. Third, to guide this research, certain methodological considerations should be addressed. For example, to understand causation, we need to determine if female interest in extra-pair mating preceded or resulted from physical violence in forensic samples. Researchers also need to include appropriate controls, such as: men s age when testing for the relationship between women s age and victimization; copulation attempts by one sex when studying copulation refusal in the other; and collecting data from both sexes when testing for sexspecific adaptations. Lastly, researchers should apply findings on the causes and consequences of sexual conflict in relationships toward reducing or preventing sexually coercive behaviors. A way to achieve this goal is to focus on contexts that minimize conflict by studying couples who are satisfied with their relationships. Acknowledgments We would like to thank Martin Lalumière, and the editors, Aaron Goetz and Todd Shackelford, for their helpful comments and recommendations. Notes 1. It is possible, however, that a relationship among women could result from factors that are unrelated to cuckoldry risk. References Alcock, J. (2001). Animal behavior: An evolutionary approach (7th ed.). Sunderland, MA: Sinauer Associates, Inc. Andersson, M., & Iwasa, Y. (1996). Sexual selection. Trends in Ecology and Evolution, 11, Ard, B. N. (1977). Sex in lasting marriages: A longitudinal study. The Journal of Sex Research, 13, Arnqvist, G., & Rowe, L. (2005). Sexual confl ict. Princeton, NJ: Princeton University Press. 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