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Selected to Kill in His Name: Evolutionary Perspectives on Religiously Motivated Violence

Abstract and Keywords

Religion motivates, exacerbates, and even justifies violence. This chapter argues that religious beliefs regarding violence—particularly those of monotheistic, Abrahamic faiths—are shaped by evolved psychological mechanisms. Further, it argues that religiously motivated violence is most likely to occur in evolutionarily relevant contexts. Guided by sexual selection theory and parental-investment theory, it first provides an overview of human sexual selection from an evolutionary perspective. It discusses how and why an evolutionary perspective—and principles of sexual selection and parental investment in particular—may provide a richer understanding of religiously motivated violence. Next follows an overview of research addressing several types of religiously motivated violence such as mate guarding and controlling behaviors, wife beating and uxoricide, honor killing, child abuse and filicide, male and female genital mutilation, war, and terrorism. Finally, it highlights the parallels between religiously motivated violence and evolved psychological mechanisms for violence, concluding with suggestions for future research.

Keywords: evolutionary psychology, religion, violence, sexual selection, parental investment

An evolutionary perspective has informed explanations of violence throughout the animal kingdom (Newton-Fisher & Thompson, 2012). Violence is the behavioral output of evolved psychological mechanisms designed to solve particular adaptive problems, and it reliably occurs in specific contexts (Buss & Shackelford, 1997a). Evolutionary psychological research has revealed several functions that violence may have served over human evolutionary history, and the contexts in which violent behavior is most likely to occur (Buss, 2005; Buss & Shackelford, 1997a; Kaighobadi, Shackelford, & Goetz, 2009; Liddle, Shackelford, & Weekes-Shackelford, 2012; Shackelford & Weekes-Shackelford, 2012). The general contribution of the extant research is the understanding that violence is the behavioral output of evolved psychological mechanisms designed to solve particular adaptive problems and, thus, violence reliably occurs in specific contexts (Buss & Shackelford, 1997a).

Evolutionary psychological research typically has explored the contexts—and predictors—of violence within the theoretical framework of sexual selection (Buss & Shackelford, 1997a). It is reasonable to hypothesize that other human psychological factors may exacerbate violence within already violence-prone contexts. In this chapter, we implicate religious beliefs as a psychological factor that may motivate and exacerbate violence. We argue that religious beliefs are shaped by preexisting, conditionally violence-promoting psychological mechanisms, and that religiously motivated violence is likely to occur in contexts that elicit violent responses. Essentially, we argue that religion makes violence worse—religious beliefs, practices, and norms lower the threshold for violence, and explicitly promote and justify violent actions in evolutionarily relevant contexts. To solidify this argument, we provide an overview of human violence from an evolutionary psychological perspective and we discuss how the theory of sexual selection can be drawn on to understand various forms of religiously motivated violence. We then conclude with a discussion of directions for future research that could potentially provide evidence to test the argument that religion motivates and exacerbates violence.

Evolutionary Psychological Perspectives on Violence

Human violence is not a novel phenomenon. Evidence indicates that violence has occurred throughout human evolutionary history (Pinker, 2011). Archeological evidence indicates violent deaths among early humans, including indicators of clubbing (Ferguson, 1997), arrowheads and barbs (Smith, 2007), and mass killings via blows to the head (Keeley, 1996). Further evidence of humans’ violent history comes from examining specialized tools whose function was not just hunting game, but also inflicting violence on other humans (Smith, 2007). The historical writings of many societies worldwide describe interpersonal violence (Keeley, 1996), and upon transitioning from nomadicity to permanent settlements, humans erected costly fortification structures to defend against out-groups (Smith, 2007).

Given humans’ deep evolutionary history of violence, it is reasonable to suspect evolutionary functions underpinning such psychology and behavior. A substantial amount of violence occurs within species throughout the animal kingdom—including humans. An evolutionary perspective can provide profound insight as to why, when, and where violence is most likely to occur. Violence, and the contexts in which it occurs, can be primarily explained in light of two evolutionary theories: sexual selection theory and parental investment theory (for a review, see Liddle et al., 2012).

Sexual Selection Theory

Sexual selection theory refers to “the advantages that certain individuals have over others of the same sex and species, in exclusive relation to reproduction” (Darwin, 1871, p. 256). Sexual selection is directly related to the reproductive success of individuals within a species, arising from intrasexual and intersexual competition for access to mates. Sexual selection gives rise to traits that facilitate reproduction, but these same traits are sometimes costly to an organism’s survival. For example, traits such as the antlers of stags, the horns of antelopes, and the tail of peacocks are energetically costly to develop and maintain, but are essential for attracting and acquiring mates. Because these traits facilitate reproductive success—at the cost of reduced survival prospects—such elaborate traits and adornments can only be maintained by sexual selection.

Competition for mates can occur via intersexual and intrasexual selection. Intrasexual selection refers to competition between members of the same sex for sexual access to members of the other sex. Intrasexual selection has resulted in the evolution of animal weaponry such as antlers, horns, and claws (Emlen, 2014), and in humans, increased male body size and substantially greater male upper body strength (e.g., Lassek & Gaulin, 2009). Male-male competition in humans—notably driven by men’s motivation to secure sexual access to females—has resulted in the evolution of predictable patterns of aggression (Daly & Wilson, 1988), facilitated by jealousy (Buss et al., 1992, Easton & Shackelford, 2009) and many other psychological traits (see Schmitt et al., 2003).

Intersexual selection refers to the process by which the mate preferences of one sex influence the selection of traits in the opposite sex—a “co-evolutionary tango” between the sexes (Schilthuizen, 2014). In most sexually reproducing species, males compete for sexual access to females (Bateman, 1948). Because females typically invest substantially greater time and energy into rearing offspring (Trivers, 1972; see “Parental Investment Theory” section), females are more selective when choosing mates and, consequently, males evolve traits that females find attractive. This asymmetry results in males evolving elaborate coloring, adornments, and other traits that attract females, whereas the corresponding females of a given species typically do not evolve the same traits. Intersexual selection has resulted in the evolution of male traits such as the peacock’s elaborate tail, vibrant coloration in birds and fish, and many kinds of bird vocalizations (Andersson, 1994) and courting behaviors (Coleman, Patricelli, & Borgia, 2004). Intrasexual selection and intersexual competition afford a more comprehensive understanding of violence within species, and specifically—as we argue here—provides a rich, theoretical framework for investigating religiously motivated violence in humans.

Parental Investment Theory

In most sexually reproducing species, males are more likely to engage in violent intrasexual competition than are females. Parental investment theory (Trivers, 1972) can explain the robust sex difference in violent behavior, the underlying psychology that produces these behaviors, and psychological mechanisms designed to solve adaptive problems of violence. Parental investment theory refers to the allocation of resources to offspring at the expense of other potential resource allocations (e.g., survival, mating effort, additional offspring). Because of the differences in size and required energy costs for developing sperm and ova, males and females often differ substantially with regard to the minimum obligatory investment necessary for offspring survival and reproduction. In humans, the minimum obligatory parental investment for men can end with a single copulation and ejaculate. For women, however, parental investment requires at least nine months of pregnancy, and often several years of nursing.

Sex differences in minimum obligatory parental investment result in differing optimum reproductive strategies for men and women (Parker, 2006). The reproductive success of a man is limited by the number of matings he secures with fertile women. For women, however, ova production is limited and, thus, reproductive success is limited by the time and energy required to rear offspring (Bateman, 1948; Trivers, 1972). Women are therefore more selective when choosing a mate due to the greater energetic costs associated with selecting a poor quality mate—such as investing valuable resources into offspring with a lower probability of survival and/or future reproductive success. The sex that provides greater obligatory investment—typically female—tends to avoid violent confrontations because the costs associated with injury and death negatively impact their reproductive success (e.g., a woman being unable to care for current or future offspring). However, the less investing sex—typically male—tends to pursue competitive, sometimes violent, behavioral strategies because men can reap greater benefits from successful competition with same-sex rivals for sexual access to women, thus potentially siring numerous offspring.

Parental investment theory’s predictive power is particularly evident when examining species in which the discrepancy in minimum obligatory investment is reversed between the sexes. Examples of these so-called sex-role-reversed species include Australian cassowaries (Ghiglieri, 1999), Mormon crickets, pipefish seahorses, and Panamanian poison arrow frogs (Trivers, 1985). In such species, males invest more than females in their offspring and, accordingly, males are more selective when choosing mates, whereas females compete with each other—often violently—for sexual access to males.

Explaining Male Violence Against Other Men

Liddle et al. (2012) review and apply sexual selection theory and parental investment theory to violence, presenting evidence to suggest that violence among nonhuman animals is not arbitrary or pointless. Rather, violence appears to be determined by unconscious cost-benefit calculations. Cost-benefit calculations underpin the majority of human violence. Human and nonhuman animals possess evolved psychological mechanisms for violence, and these mechanisms motivate violent behavior in response to specific environmental inputs in which, on average (i.e., over evolutionary history), the benefits of violent interactions outweigh the potential costs. Liddle et al. (2012) describe several environmental inputs that these mechanisms may be sensitive to, and how these inputs affect the likelihood of engaging in violent behavior.

Differences in male and female reproductive strategies, resulting from sex differences in reproductive biology, have profound, downstream consequences for interpersonal psychological and behavioral processes. In accordance with parental investment theory, males of most animal species, particularly mammals, are overwhelmingly more prone to violence than are females (Ghiglieri, 1999). Because females are, on average, more selective than males when choosing a mate, female mate choice limits males’ sexual access to females (Bateman, 1948; Trivers, 1972). In humans, risky competition often involves violence, predominantly between men (Daly & Wilson, 1988). The robust sex difference in violent behavior is likely because violence can be a more effective means for men, relative to other forms of nonviolent competition, to overcome sexual rivals and acquire resources that women prefer in their mates (Buss & Shackelford, 1997a). Cross-culturally, men are more likely to be the perpetrators, and targets, of violence (Archer, 2004, 2009; Burbank, 1992; Buss, 2005; Daly & Wilson, 1988, Ghiglieri, 1999; Hyde, 1986; Lester, 1991).

Sexual dimorphism in humans—men are heavier (Ghiglieri, 1999) and taller (Holden & Mace, 1999) than women—suggests a history of effective polygyny in which the variance of reproductive success has been greater in men than women (Buss & Shackelford, 1997b). Lower paternal investment, and increased reproductive variability in men (compared to women), further facilitate aggressive and violent male intrasexual competition for mates (Archer, 2009; Campbell, 2005).

Principles of intrasexual competition and intersexual selection further clarify the relationship between violence and mating success (Archer, 2009; Daly & Wilson, 1988; Liddle et al. 2012). Male intrasexual competition is intimately related to status and reputation. Women prefer mates of high status, which serves as an honest signal of a man’s potential ability to provide for a woman and her offspring. Historically, men of high status have had greater access to food, superior territory, and stronger social support (Buss, 2005). Perpetration of violence against other men is one way in which men can navigate status hierarchies and achieve high status (Buss & Shackelford, 1997a). Consequently, high-status men have greater sexual access to women, a phenomenon that has been documented among many tribal societies (Chagnon, 1988, 1992; Daly & Wilson, 1988) and in the United States (Buss, 2011; Campbell, 1993; Ghiglieri, 1999; Palmer & Tilly, 1995).

Explaining Male Violence Against Women

Evolutionary theories, using principles of intersexual selection and differential obligatory parental investment, afford insight to the circumstances in which men perpetrate violence against women (Goetz, Shackelford, Starratt, & McKibbin, 2008). In humans, fertilization and gestation occur within females, consequently resulting in paternity uncertainty for males. That is, men are never fully certain that their offspring are, in fact, genetically their own. Because men often invest heavily in their putative offspring (Chrastil, Getz, Euler, & Starks, 2006; Trivers, 1972), the costs of cuckoldry—the unwitting investment into genetically unrelated offspring—can be substantial (Leivers & Simmons, 2014).

Cuckoldry has posed an adaptive problem for men throughout human evolutionary history (Pham & Shackelford, 2014). A cuckolded man benefits his rival at a serious cost to his own fitness due to the diversion of his finite resources away from his current or future genetic offspring. Contemporary, worldwide, empirical evidence documents nonzero paternity rates (i.e., cuckoldry), ranging from 1% to 30% (Goetz, Shackelford, Platek, Starratt, & McKibbin, 2007; Voracek, Haubner, & Fisher, 2008). Cross-cultural, historical, and behavioral evidence indicate that over evolutionary history, paternity uncertainty was an adaptive problem faced by ancestral men (see, Anderson, 2006; Buss, 2000; Daly, Wilson, & Weghorst, 1982; Goetz & Shackelford, 2006, 2009; Thornhill & Gangestad, 2008; Shackelford, 2003; Shackelford & Goetz, 2007; Euler & Weitzel, 1996; Platek, Keenan, & Mohamed, 2005; Voracek et al., 2008).

Because cuckoldry was a recurrent feature of human evolutionary history, strong selection pressures have been imposed on men to guard their paternity. Male psychological mechanisms—such as male sexual jealousy (Buss, 2000)—have evolved to combat problems associated with detecting and preventing cuckoldry. Because of the substantial costs associated with cuckoldry, men may resort to violent and coercive behavior directed toward women to guard their paternity, and to maintain, regain, or secure exclusive sexual access to their in-pair partner.

Religion Exploits Evolved Psychological Mechanisms

Recent theoretical arguments posit that religions, or religious beliefs and practices, are an exploitation of evolved psychological mechanisms designed to detect status, particularly among men. For example, Garcia (2015) argues that qualities of dominant “alpha” males were imported onto Gods—specifically, Gods of the Abrahamic religions (i.e., Christianity, Judaism, and Islam). Put differently, God was created in the image of man, rather than God creating man in his image.

Across the animal kingdom—including human and nonhuman primates—dominant males judiciously deploy violence as a means to intimidate subordinates, acquire territory, control female sexuality, and maintain rank and status within the group. As previously discussed, violence has often served as an effective means to these goals (Buss & Shackelford, 1997a), and dominance and status are intimately intertwined with violence. It has been argued that when humans transitioned to nation-state societies, religion—specifically, monotheistic religion (i.e., devotion to one god)—was implicated as a means to regulate great masses of individuals and to regulate interactions between nation-states. During this transition in human history, in particular, monotheistic religions expanded and flourished (Johnson & Earle, 2000; Wright, 2010).

Human and nonhuman primates (particularly, great apes) live in hierarchical societies. The dominant individual of the group—typically, a male—is responsible for regulating and controlling group behavior. As human groups grew exponentially in size, the power of a single dominant God subsumed the role of the traditional alpha male, including the associated dominant and violent traits (Garcia, 2015). Thus, there is a good case to be made that the dominant God of the Abrahamic religions is merely an extreme, all-powerful, elevation of a mortal alpha male.

Participation in monotheistic religions, rather than polytheistic religions (i.e., worshiping several gods), entails submission to a single god, which parallels behaviors enacted toward alpha males in human and nonhuman primates particularly well. In the Abrahamic religions, God serves as the protector of the in-group, regulates female sexuality, justifies—and promotes—violence toward out-groups, and requires submission to him that is maintained through his dominance (i.e., force or threat of force) and prestige (i.e., freely conferred deference) (Garcia, 2015). The exorbitant influence of religion is maintained arguably because of our deep evolutionary history of living in hierarchical societies. Sexual selection, in particular, has given rise to evolved psychological mechanisms designed to detect and navigate status (and dominance), and regulate in-group/out-group interactions.

Sexually Selected Underpinnings of Religiously Motivated Violence

Sexual selection theory and parental investment theory afford an evolutionary explanation of the underlying psychology of violent human acts, as well as the contexts in which violent acts are most likely to occur. It is worth noting that, as with any evolutionary explanation for particular psychological processes and behaviors, it is not imperative that individuals be consciously aware of the evolutionary roots underlying their psychology and behavior. Psychological mechanisms that promote violent behavior in response to particular environmental inputs (e.g., threats to one’s status, indications of partner infidelity) are selected for over evolutionary history if, on average, these mechanisms result in greater replicative success of the genes that built them—regardless of individuals’ conscious awareness or unawareness of the ultimate evolutionary reasons for their psychology and resulting behavior. With this in mind, we turn to an examination of specific religious beliefs and how these beliefs may exploit preexisting psychological mechanisms designed to promote context-dependent violence.

Partner-Directed Violence

Men are more prone to violent behavior than women due to lesser obligatory parental investment and, thus, greater reproductive variability. Male violence is not reserved only for male rivals—it is also directed at women, and especially at romantic partners. Accordingly, it is expected that men’s religiously motivated violence toward a romantic partner will occur in contexts where a man’s exclusive sexual access to his romantic partner is threatened. It is also expected that the religious justifications for partner-directed violence will be explicit, and that these justifications will be rooted, ultimately, in evolution by sexual selection.

Mate retention behaviors function to thwart a romantic partners’ infidelity, and can range from vigilance of a romantic partner to perpetrating physical violence against the partner (Buss, 1988; Buss & Shackelford, 1997b; Sela, 2016). Although both men and women perform mate retention behaviors, men often use more violent behaviors than women when guarding their partner (Buss, 2005; Daly &Wilson, 1988). In 2001, 20% of reported incidents of nonfatal violence against women 12 years or older were perpetrated by an intimate partner (Bureau of Justice Statistics, 2003). If partner-directed violence does not prevent or correct a partners’ infidelity or defection, a man may resort to even more extreme measures, such as killing his partner, to effectively eliminate rival males’ sexual access to her (Buss, 2000, 2005).

Between 1976 and 2005, 30% of female homicide (“femicide”) victims were killed by an intimate partner, making it the largest class of victim-offender relationship (Bureau of Justice Statistics, 2007). Previous research has identified contexts in which men are more likely to be violent, such as the perceived risk of partner infidelity (Goetz, Shackelford, Romero, Kaighobadi, & Miner, 2008). Mate killing can also repair a man’s reputation. In many cultures (e.g., cultures of honor; Nisbett & Cohen, 1996), cuckolded men are viewed as emasculated and, thus, killing an unfaithful partner is a means by which a man can repair his reputation (Buss, 2005; Daly and Wilson, 1988). Men direct violence at their partner, ultimately, to avoid the devastating costs associated with cuckoldry. Male sexual jealousy is often the proximate mechanism motivating men’s partner-directed violence; it is among the most frequently cited causes of men’s partner-directed violence, both physical and sexual (e.g., Buss, 2000; Daly & Wilson, 1988; Daly et al., 1982; Dobash & Dobash, 1979; Dutton, 1998; Frieze, 1983; Gage & Hutchinson, 2006; Russell, 1982; Walker, 1979). Accordingly, it is expected that men’s perpetration of partner-directed violence, motivated by male sexual jealousy, should be sanctioned by religion—especially in the contexts of sexual conflict (e.g., real or perceived infidelity, offspring from previous relationships, a wife’s refusal to have sex with her husband). Religious texts promote, and justify, men’s perpetration of partner-directed violence in sexual contexts:

  • Leviticus 20:10–121: “And the man that committeth adultery with another man’s wife, even he that committeth adultery with his neighbour’s wife, the adulterer and the adulteress shall surely be put to death. And the man that lieth with his father’s wife hath uncovered his father’s nakedness: both of them shall surely be put to death; their blood shall be upon them. And if a man lie with his daughter in law, both of them shall surely be put to death: they have wrought confusion; their blood shall be upon them. Both parties in adultery shall be executed.”

  • Deuteronomy 22:22: “If a man be found lying with a woman married to a husband, then they shall both of them die.”

  • Matthew 5:31–32: “It hath been said, Whosoever shall put away his wife, let him give her a writing of divorcement. But I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery: and whosoever shall marry her that is divorced committeth adultery.”

  • Quran 4:152: “[T]hose of your women who commit illegal sexual intercourse, take the evidence of four witnesses from amongst you against them; and if they testify, confine them [i.e., women] to houses until death comes to them or Allah ordains for them some [other] way.”

  • Quran 4:34: “[T]he righteous women are devoutly obedient [to Allah and to their husbands], and guard in the husband’s absence what Allah orders them to guard [e.g., their chastity, their husband’s property]. As to those women on whose part you see ill-conduct, admonish them [first], [next], refuse to share their beds, [and last] beat them [lightly, if it is useful].”

Religious texts such as the Bible and Quran instruct men to stone, burn, torture, and poison women who are suspected of extramarital sex. Men are instructed to punish wives that commit infidelity (actual or imagined), and exhibit—vaguely defined—promiscuous behavior (e.g., “play the harlot” or “commit whoredome,” Ezekiel 23:1–49; If they do not “lower their gaze [from looking at forbidden things], and protect their private parts [from illegal sexual acts] … draw their veils all over Juyubihinna [i.e., their bodies, faces, necks and bosoms]”; Quran 24:31). Both the conviction of the crime and the execution of the punishment are at men’s discretion (e.g., Quran 24:6). Some current manifestations of these practices are brutal beating and killing of wives for showing skin (e.g., a husband recurrently beat his wife, and finally killed her with a knife and cut her into pieces after she refused to cover her face with a veil outside the house, saying this was the best way to “punish [her] for rebelling against Allah’s orders”; Jafri, 2013b), or leaving the house without permission (e.g., a husband axed his wife after she “insulted him” by staying out overnight. He asked her to loudly recite the Kalimas—Islamic texts—before killing her, and told police he killed his wife to make her “a lesson for other women who do not obey their spouses”; Anonymous, 2013).

Religious codes of conduct are also enforced on women by designated groups of men (convenient for the husbands while they are away from their wives). For example, every year “chastity squads” of the morality police unit in Iran forcefully arrest and fine thousands of citizens, especially woman and adolescents, for not following the Islamic dress code (e.g., Cohen, 2011). Further, extramarital sex is a public offense in Iran, punishable by stoning to death (Razavi, 2006; public stoning videos of alleged adulteresses are available online3). Another example is the Jewish Modesty patrol in ultra-Orthodox neighborhoods in Israel. The modesty patrol beat, spit, and hurl stones at women and girls who wear clothing deemed provocative, or who have allegedly consorted with men other than their husbands (Associated Press, 2008).

The Abrahamic religious texts deem it socially acceptable, expected, and even justified for men to be overt in their violent mate guarding behaviors. In this way, some men gain an advantage in intersexual competition (e.g., by more effectively controlling their partner and preventing her infidelity). Men may also gain an advantage in intrasexual competition by retaliating more harshly—and with fewer reputational costs—against male rivals than would otherwise be permissible (e.g., “Both parties in adultery shall be executed”; Leviticus 20:10–14). By endorsing and promoting partner-directed violence, religious texts offer men a socially acceptable, justified solution to the adaptive problem of cuckoldry (i.e., increasing paternity certainty). Men’s religiously motivated, partner-directed violence also reduces male-male competition over women because religiously prescribed violence severely limits women’s autonomous behavior. A husband’s right to confine, beat, torture, and murder his wife for her infidelity is just one set of examples by which religiously motivated violence exploits an unfortunate suite of men’s evolved psychology.

Genital Mutilation

Male and female genital mutilation is any permanent modification of the external genitalia that involves the ablation of tissue (WHO, 2017). Male genital mutilation includes superincision (longitudinal bisection of the dorsal skin), circumcision (removal of the entire foreskin), and castration (removal or crushing of one or both testes or the penis). The most common male genital mutilation today is male circumcision, which is mandated in the Bible (e.g., Genesis 17:10–14, Exodus, 12:48; Josh 5:2) and instructed by Islamic hadiths4 and fatwas5 (e.g., Al-Munajjid, n.d.a, n.d.b), and is therefore practiced by Jews, Muslims, and Christians.

Female genital mutilation includes hoodectomy (removing the clitoral hood), vaginal infibulation (narrowing of the vaginal opening by cutting and repositioning the inner or outer labia), excision (partial or total removal of the clitoris and the labia minora, with or without excision of the labia majora), and clitoridectomy (partial or total removal of the clitoris) (WHO, 2017). Female genital mutilation is prevalent worldwide, especially in Africa. Female genital mutilation is permissible, and some say encouraged, in the following hadiths: “Circumcision is Sunnah for men and an honorable thing for women” (Musnad Ahmad); “Cut off only the foreskin [outer fold of skin over the clitoris; the prepuce] but do not cut off deeply [i.e., the clitoris itself], for this is brighter for the face [of the girl] and more favorable with the husband” (Mu`jam al-Tabarânî al-Awsat6); “A woman used to perform circumcision in Medina. The Prophet (peace be upon him) said to her: Do not cut severely as that is better for a woman and more desirable for a husband” (Book 41, Number 52517). Religious authorities in countries where female circumcision is common encourage the practice by conveying to their—often illiterate—female audience that female circumcision is a religious requirement (Slack, 1988).

Despite the variability in the execution of genital mutilation, each type presents health risks (e.g., bleeding and infections) and reproductive costs (e.g., infertility). It is unlikely that damaging or removing mechanically, neurally, and endocrinologically specialized, healthy sexual tissue is neutral with respect to its evolved function (Wilson, 2008). For men, if genital mutilation impairs success in sperm competition (i.e., when sperm from two or more males simultaneously occupy a female’s reproductive tract and compete to fertilize ova) by affecting sperm delivery (see Wilson, 2008, c.f. Morris & Krieger, 2013), we expect an associated fitness benefit somewhere else. In certain circumstances, it might advantage men to reduce the functionality of their own sperm competition adaptations (e.g., penis anatomy, ejaculate adjustment by way of arousal, and even reduced female partner satisfaction) if these men secured some other benefit that would aid them later, and without which they would be worse off.

Wilson (2008) argues that there is an underlying and common function to genital mutilation: providing hard-to-fake signals of compliance with the social assignment of reproduction. Conflict over paternity may favor men who invest preferentially in spouses with female genital mutilation and cooperate preferentially with peers who submit to male genital mutilation (Wilson, 2008). Wilson generated and tested several predictions from this functional, sexual conflict hypothesis of male and female genital mutilation. He found that male genital mutilation was associated with polygynous societies at high risk for extramarital sex, but that male genital mutilation appeared to reduce this risk. He also found that male genital mutilation is performed by a nonrelative in public view of other men, and that a genitally mutilated man gains access to social and sexual privileges that may outweigh the costs associated with genital mutilation. Wilson’s findings suggest that genital mutilations may impair the evolved capacity for extra-pair fertilizations, thereby decreasing paternity uncertainty and reproductive conflict. Benefits of trust and social investment from powerful, married men outweigh the costs of genital mutilation.

Honor-Killing

Honor-killing is the murder of a family member (typically, a woman) by one or more other members (typically, men) because the individual has brought shame to the family by “straying from the righteous path of God.” Women have been killed for refusing to enter a pre-arranged marriage (e.g., Jafri, 2013a; Mirza, 2008), committing adultery (actual or alleged), being in a relationship that displeased their relative (e.g., Spolar, 2005), or being raped (often by another family member; e.g., Mirza, 2008). The observed male-bias in killers is consistent with men’s general tendency to be more violent than women, and with fathers being more prone to violence than mothers due to paternity uncertainty. The methods of honor-killing often reflect “overkill” (e.g., using excessive methods, more than would be reasonably necessary to kill) and often include bludgeoning, mutilation, burning, and dozens of stabbings per victim (Chesler, 2009, 2010; Mirza, 2008).

Intense torture and overkill involved in honor-killing suggest that there may be a unique psychology to honor-killing, requiring a special justification (i.e., religious motivation8). An especially strong emotional motivation may be needed to commit an honor-killing because there are powerful, evolved psychological mechanisms designed to care for, and protect, offspring or other kin in most circumstances. Because of these strong kin-focused psychological mechanisms, honor-killing appears deeply counterintuitive: Parents, siblings, cousins, and uncles killing their kin is an evolutionary paradox. However, as is the case with genital mutilation, there may be associated long-term reproductive benefits—to the killers. Overkill methods suggest that there may also be an important communicative element to honor-killing, such as upholding the honor, and therefore the status, of men within their family and community. If a man’s reproductive success depends on his (and his family’s) status and reputation, and these are jeopardized by another family member (e.g., his daughter is accused of having sex before marriage, besmirching the father’s and family’s reputation; Quran 24:2, 17:32), then it is expected that a man will attempt to repair his own and his family’s reputation, sometimes by any means necessary. If honor-killing is an acceptable practice in the community, honor-killing is a socially sanctioned way of restoring a man’s social reputation and status. Several passages in the Bible and the Quran justify, and even require, honor-killings:

  • Deuteronomy 22:13–21: If a man decides that he “hates” his wife, he can claim she wasn’t a virgin when they were married. If her father can’t produce the “tokens of her virginity” (bloody sheets), “then they shall bring out the damsel to the door of her father’s house, and the men of her city shall stone her with stones that she die.”

  • Deuteronomy 22:23–24: “If a damsel that is a virgin be betrothed unto an husband, and a man find her in the city, and lie with her [t]hen ye shall bring them both out unto the gate of that city, and ye shall stone them with stones that they die.” In other words, if an engaged virgin is raped in the city and does not cry out loud enough, then the men of the city must stone her to death.

  • Ezekiel 23:1–49: Two sisters who were guilty of committing “whoredoms in their youth” by pressing their breasts and bruising “the teats of their virginity.” One of the sisters “played the harlot” as she “doted her lovers …. Thus she committed her whoredoms with them, with all them that were the chosen men of Assyria and with all on whom she doted. With all their idols she defiled herself. Neither left she her whoredoms brought from Egypt; for in her youth they lay with her, and they bruised the breasts of her virginity and poured their whoredom upon her,” etc. As a punishment (they “executed judgment upon her”), one sister was stripped, her children were taken from her, and she was killed with a sword. The other sister was tortured by cutting her nose and ears off, and she was made to “pluck off” her own breasts, then she was raped and mutilated, and finally, stoned to death.

  • Leviticus 21:9—“And the daughter of any priest, if she profane herself by playing the whore, she profaneth her father: she shall be burnt with fire.”

  • Quran 4:15, 34 (see examples in the section “Wife Beating, Killing, and Raping”).

  • Quran 24:2 “The fornicatress and the fornicator, flog each of them with a hundred stripes. Let not pity withhold you in their case … if you believe in Allah … [a]nd let a party of the believers witness their punishment. (This punishment is for unmarried persons guilty of the above crime, but if married persons commit it [illegal sex], the punishment is to stone them to death, according to Allah’s Law).”

  • Mirza (2008) reviews several relevant sahih hadiths9 that include statements such as: “No one commits adultery while still remaining a believer, for faith is more precious unto Allah than such an evil act!,” “A woman came to the prophet and asked for purification by seeking punishment … [She] admitted she was pregnant … When the day arrived for the child to take solid food, Muhammad handed the child over to the community … [H]e had given command over her and she was put in a hole up to her breast, he ordered the people to stone her. Khalid b. al-Walid came forward with a stone which he threw at her head, and when the blood spurted on her face he cursed her.”

According to these religious texts, adulterers and fornicators lose their rights and human value, bring great shame on their families, and should be punished with flogging, mutilation, and stoning to death. The victims’ children suffer as well, eliminating any reproductive success that forbidden copulation might otherwise offer. Further, the punishment for such actions is public and difficult to fake. Honor-killers will often announce their actions and goals publicly to the community. For example, after the 16-year-old Jordanian girl Kifaya Husayn was raped by her 21-year-old brother, her uncles persuaded another brother that she must die because she had disgraced their family by being raped. This 32-year-old brother bound her to a chair, told her to recite an Islamic prayer, and then slashed her throat. He then ran out into the street, waving the bloody knife, crying: “I have killed my sister to cleanse my honor” (Choo, 1998).

It is important to note that the goal of honor-killings, unlike intimate partner violence for example, is to save or repair the honor and reputation of the victim’s family, rather than for animosity or wealth (Mirza, 2008). Honor-killers often love the girl as their own (daughter, sister, niece, etc.), but commit the killing because they view it to be their moral obligation to save their family honor, erase damaging stigmas, and restore their religious piety (Mirza, 2008). Following the honor-killing, family members usually mourn and cry for the victim (even the killers themselves), but feel their actions are justified and necessary for all involved parties (Mirza, 2008).

Child Abuse and Filicide

Children are at the greatest risk of physical abuse and murder (filicide) if they live with a stepparent, even after controlling for potential confounds such as socioeconomic status (Daly & Wilson, 1985, 1988, 1998; Wilson, Daly, & Weghorst, 1980). This increased risk of abuse and filicide by stepparents has been documented across diverse cultures (Bjorklund & Pellegrini, 2002; Daly & Wilson, 1988, 1998). The risk of abuse and filicide is greater for children living with a stepfather than a stepmother (e.g., Daly & Wilson, 1994; Weekes-Shackelford & Shackelford, 2004). Accordingly, it is expected that among cases of religiously motivated child abuse and filicide, a higher proportion of perpetrators will be stepparents rather than genetic parents, and that fathers will be more violent than mothers. Because stepparents are already motivated to harm—or care less for—genetically unrelated children (stepchildren), stepparents might exhibit violent behavior more intensely if they have religious justification. Religious texts offer (violent) guidelines about parenting:

  • Exodus, 21:15: “And he that smiteth his father, or his mother, shall be surely put to death.”

  • Exodus, 21:17: “And he that curseth his father, or his mother, shall surely be put to death.”

  • 2 Kings 2:23–24: “[A]nd as he was going up by the way, there came forth little children out of the city, and mocked him, and said unto him, Go up, thou bald head; go up, thou bald head. And he turned back, and looked on them, and cursed them in the name of the LORD. And there came forth two she bears out of the wood, and tare forty and two children of them.”

  • Proverbs, 13:24: “He that spareth his rod hateth his son: but he that loveth him chasteneth him betimes.”

  • Proverbs, 19:18: “Chasten thy son while there is hope, and let not thy soul spare for his crying.”

  • Proverbs, 22:15: “Foolishness is bound in the heart of a child; but the rod of correction shall drive it far from him.”

  • Proverbs, 23:13–14: “Withhold not correction from the child: for if thou beatest him with the rod, he shall not die. Thou shalt beat him with the rod, and shalt deliver his soul from hell.”

  • Proverbs, 29:15: “The rod and reproof give wisdom: but a child left to himself bringeth his mother to shame.”

  • Hebrews, 11:17: “By faith Abraham, when he was tried, offered up Isaac: and he that had received the promises offered up his only begotten son.”

The Bible instructs parents to beat their children with rods, and even kill them, as a sign of parental love, discipline, salvation, and devotion to God. It may also be reasonable to expect higher rates of child-directed violence in religious (vs. nonreligious) contexts. Further, high-risk situations such as cohabiting with a stepparent may be even riskier in religious (vs. nonreligious) contexts.

Religiously motivated child-directed violence is underreported (e.g., there is no national statistic of this phenomenon), perhaps because it is more acceptable to use physical punishments in religious communities (Heimlich, 2011). For example, Ellison and Bradshaw (2009) found that conservative Protestants spank their children more frequently than do other Christian believers, and that religious beliefs (in a hierarchical God and in Hell) are associated with differences in punishment. Religiously motivated child-directed maltreatment, including physical abuse, emotional abuse, sexual abuse, and medical neglect, is an urgent yet understudied social issue (for a review, see Heimlich, 2011).

Another context-specific risk for child-directed violence is maternal infanticide: a biological mother killing her infant. High-risk factors include a mother’s young age, low paternal investment prospects (e.g., a mother is unpartnered or with a man other than the infant’s father), a mother’s perception of the infant’s low-quality (e.g., illness, deformities), and her assessment of unfavorable child-rearing circumstances (e.g., multiple births, economic hardships) (Daly & Wilson, 1988). Young, poor, unmarried mothers are most likely to kill their newborns (e.g., d’Orban, 1979; Putkonen, Weizmann-Henelius, Collander, Santtila, & Eronen, 2007)—a pattern that reflects unconscious, evolved decision-rules in women regarding resource allocation strategies (Daly & Wilson, 1988). However, even in societies where infanticide is less condemned, killing an infant can be extremely upsetting for the mother (Daly & Wilson, 1988). Although evolved psychological mechanisms influence infanticide, having a proximate justification (i.e., religious justification) may facilitate the mother’s behavioral decision and emotional consequences. It is expected that religiously motivated infanticide might occur in similar conditions (i.e., when she is young, poor, and unpartnered), and perhaps at a higher frequency, due to increased (religious) social acceptance and support for the mother after the fact. It would also be expected that some infanticidal mothers provide explicitly religious explanations for the killings.

Masturbation and Birth Control

Other examples of religious doctrines exacerbating violence in contexts relevant to sexual selection are masturbation and birth control. Regarding masturbation (e.g., as a means of avoiding impregnation), in the book of Genesis (38:8–10), Judah tells Onan to “go in unto thy brother’s wife” after the brother is killed. But, “Onan knew that the seed should not be his; and … when he went in unto his brother’s wife … he spilled it on the ground…. And the thing which he did displeased the Lord; wherefore he slew him also.” In other words, both masturbation and birth control are punishable by death, according to this passage. First, masturbation has been hypothesized to increase ones’ sperm-competition ability (e.g., Baker & Bellis, 1995). It therefore may be in the interest of men to condemn other men for masturbating, but practice masturbation themselves. Second, if men secure their exclusive sexual access to their partner (as religious rules explicitly instruct), then we expect them to object to any form of contraception because the likelihood of cuckoldry is low and, therefore, their investment in their partner’s offspring is more likely to provide replicative benefits rather than costs.

War and Terrorism

Religion promotes and justifies violence toward out-groups. The following religious passages provide guidelines for behavior during war. Religious texts instruct the soldiers to kill all the men, children, and non-virgin women. Young, virgin women, on the other hand, should be raped and acquired as sex slaves. Men possess evolved psychological mechanisms that motivate the elimination of their competitors, young and old (especially out-group members) and to copulate with fertile women. The following religious passages provide justification for such behavior:

  • Numbers 31:1–54: Under God’s direction, Moses’s army defeats the Midianites. They kill all the adult males, but take the women and children captive. When Moses learns that they left some alive, he angrily says, “Have you saved all the women alive? Kill every male among the little ones, and kill every woman that hath known man by lying with him. But all the women children, that have not known a man by lying with him, keep alive for yourselves.”

  • Leviticus 20:13–14: In the cities that God “delivers into thine hands, thou shalt smite every male thereof with the edge of the sword: But the women, and the little ones … shalt thou take unto thyself.”

  • Zechariah 14:1–2: “God will make ‘all nations’ fight against Jerusalem. The women will be ‘ravished’ and half its people enslaved.”

  • Judges 21:11–23: “Ye shall utterly destroy every male, and every woman that hath lain by man. And they found among the inhabitants of Jabeshgilead four hundred young virgins, that had known no man by lying with any male: and they brought them unto the camp…. [A]nd they [the Israelites] gave them [the Benjamites] wives which they had saved alive … and yet so they sufficed them [the Benjamites] not.” To complete the number of virgins (one per Benjamite), they were instructed to ambush “the daughters of Shiloh [when they] come out to dance in dances, then come ye out of the vineyards, and catch you every man his wife of the daughters of Shiloh. And the children of Benjamin did so, and took them wives, according to their number, of them that danced, whom they caught.”

The last passage, in particular, is explicit about the conditions under which kidnapping and raping women of an out-group is required—when there is a shortage of reproductively valuable women. Sex-ratio is an important indicator of increased sperm competition risk (e.g., greater number of potential rival males), and men are expected to be especially violent when sexual access to reproductive-aged women is limited.

If men are capable of severe violence when attempting to secure mating opportunities, we should not be surprised at the extremities of violence when promised the ultimate, supernatural reward: dozens of beautiful virgins at their disposal. The Quran promises the ultimate reward for participating in terrorism, specifically (including suicide-terrorism; see Sela & Shackelford, 2014):

  • 3:151–157: “We shall cast terror into the hearts of those who disbelieve…. O you who believe! Be not like those who disbelieve (hypocrites) and who say to their brethren when they travel through the earth or go out to fight: ‘If they had stayed with us, they would not have died or been killed,’ … It is Allah that gives life and causes death. And if you are killed or die in the Way of Allah, forgiveness and mercy from Allah are far better than all that they amass (of worldly wealths).”

  • 4:74: “Let those (believers) who sell the life of this world for the Hereafter fight in the Cause of Allah, and whoso fights in the Cause of Allah, and is killed or gets victory, We shall bestow on him a great reward.”

  • 4:91: “You will find others that wish to have security from you and security from their people. Every time they are sent back to temptation, they yield thereto. If they withdraw not from you, nor offer you peace, nor restrain their hands, take (hold of) them and kill them wherever you find them. In their case, We have provided you with a clear warrant against them.”

  • 59:2: “Allah’s (Torment) reached them [disbelievers] from a place whereof they expected it not, and He cast terror into their hearts so that they destroyed their own dwellings with their own hands and the hands of the believers.”

  • 4:95: “Allah has preferred in grades those who strive hard and fight with their wealth and their lives above those who sit (at home). Unto each, Allah has promised good (Paradise), but Allah has preferred those who strive hard and fight, above those who sit (at home) by a huge reward.”

  • 37:40–48: “the chosen slaves of Allah [i.e., the true believers of Islamic Monotheism)] For them there will be a known provision [in Paradise], … In the Gardens of delight [Paradise] … [B]eside them will be Qdsirdt-at-Tarf [chaste females (wives), restraining their glances (desiring none except their husbands)], with wide and beautiful eyes.”

  • 44:51–54: The pious (faithful Muslims) “will be in place of Security (Paradise)…. And we shall marry them to Hur (very fair females created by Allah as such, not from the offspring of Adam, with intense black irises of their eyes and intense white sclera) with wide, lovely eyes.”

The descriptive details of young, beautiful, virgin women with healthy-looking eyes that grant exclusive sexual access to the devoted Muslim who fights with his life for Allah are consistent with men’s evolved mating preferences for women (e.g., Conroy-Beam, Buss, Pham, & Shackelford, 2015)—cues indicating youth and fertility.

Future Directions

An evolutionary psychological perspective has been fruitful in identifying the evolved psychological mechanisms—and their relevant contextual inputs—that facilitate violent behavior. A scientific appreciation of the proximate role of religious beliefs as a moderating factor of violence is crucial to understanding and, ultimately, minimizing violence. We have provided preliminary theoretical groundwork of how religion motivates, exacerbates, and justifies violent behavior. Such an endeavor, however, is only useful if it guides new avenues of empirical study. What follow are some examples of how an evolutionary psychological perspective, in general, and a sexual selection framework, specifically, can be profitably applied to future research investigating religiously motivated violence.

Several examples discussed in this chapter relate to reputation and honor of men, in particular. It has previously been suggested that “cultures of honor” contribute to the differences in regional violence (e.g., in the United States; Nisbett & Cohen, 1996). Nisbett and colleagues have speculated that the psychological mechanisms underpinning cultures of honor may have been theft of property. Using a sexual selection framework, however, offers a different explanation: Placing cultures of honor in a mating context, Shackelford (2005) suggested that the adaptive problem may have been theft of wives, more so than property. Psychological mechanisms of mate retention may be exacerbated in displays of culture of honor behaviors. If manifest behavioral indicators of a culture of honor are the output of evolved psychological mate retention mechanisms, then the extent to which rates of female infidelity remain higher in the southern United States than elsewhere in the nation may account for the persistence of honor culture in the present day (Shackelford, 2005). In addition to the social mechanisms identified in recent research—for example, collective representations that condone violence, such as laws (Cohen, 1996) and media representations (Cohen & Nisbett, 1997), and institutional nonstigmatization of violence (Cohen & Nisbett, 1997)—regional differences in female infidelity rates might also account for the persistence of a culture of honor in the present-day southern United States. There are also known regional differences in religiosity (Newport, 2013; Putnam & Campbell, 2010), whereby religion may facilitate the persistence of cultures of honor.

The argument presented in this chapter posits that religion motivates and exacerbates violence by exploiting preexisting psychological mechanisms. Correlational and experimental studies could provide evidence for this argument. For example, individuals of different monotheistic religions could be compared to atheists in regard to endorsements of violence, particularly male violence against women, such as the use of sexual coercion and physical violence in romantic relationships. Experimental research could compare an individual’s acceptance of, and justification for, male-perpetrated violence against women following experimental priming of religious beliefs or religious artifacts (vs. neutral controls). Researchers studying group conflict may also benefit from the inclusion of religiosity in their investigations. For example, one could investigate whether in-group prosociality and the perpetration of out-group violence increases as a consequence of religious priming by employing aggression paradigms—such as noise blasts toward another participant—in the research lab.

We primarily focused on men’s religiously motivated violent behaviors, but women also perpetrate violence. Although female aggression is more often indirect and social, rather than direct and physical (e.g., Archer, 2004; Björkqvist 1994; Campbell, 1999), in some contexts, such as infanticide, women are the primary perpetrators of violence. When examining the influence of religion on female-perpetrated violence, it is important to consider the adaptive problems women have faced over evolutionary history with reference to sexual selection and parental investment, and the possible (violent) solutions to these problems. Religious women might be motivated to execute violence under similar conditions as nonreligious women but, perhaps, to a greater degree. Do religious communities excuse female violence under these circumstances? Do religious texts promote female social aggression, such as gossip and derogation of other women, for not being religious or virtuous enough, as would be predicted by intrasexual competition?

Conclusion

Human violence, in general, is rarely committed arbitrarily and without reason. Violent behaviors often have an evolutionary, functional explanation (violent behaviors stemming from psychological abnormalities notwithstanding). Violent behavior is most likely to occur in contexts in which, over deep evolutionary history, the potential fitness benefits of violence outweigh the potential costs. Similarly, violence committed in the name of religion is rarely arbitrary; rather, religious texts condone, or even command, violence in evolutionarily relevant contexts—including cases of infidelity, threats to one’s status, reproductive success, or fitness in general. These parallels between evolutionarily relevant contexts of violence and contexts in which religious texts instruct violence is unlikely to be coincidental, but rather a reflection of the evolved psychological mechanisms that detect and navigate status hierarchies in social species (Garcia, 2015).

The status of religious texts as a moral guide to life provides justification for particular forms of violence, which may serve as additional input to the suite of evolved psychological mechanisms that ultimately guide behavior. Evolved psychological mechanisms that facilitate acts of violence—given specific environmental inputs—do not function in a vacuum. Humans have evolved psychological mechanisms for cooperation as well, and we should expect conflict between mechanisms that facilitate violence and those that facilitate cooperation (Kurzban, 2010). Although the complex interactions between evolved psychological mechanisms are only beginning to be understood, it is probable that mechanisms promoting violence must sometimes “compete” with mechanisms that deter violence. If religious texts and beliefs serve as inputs for decision-making, these inputs may increase the likelihood of violence—for example, toward women and out-group members—in the relevant contexts. There remains a great deal to be understood regarding the interaction of religious beliefs and evolved psychological mechanisms. What is clear, however, is that we cannot afford to ignore the role that religion plays in exacerbating, motivating, and justifying violence. An evolutionary psychological perspective can provide a foundational framework to better understand how and why religiously motivated violence occurs.

Acknowledgment

Portions of this chapter are based on works by Sela, Shackelford, and Liddle (2015, 2016).

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                                                                                                                                                                                                                            Notes:

                                                                                                                                                                                                                            (1) All biblical quotations are from the King James Version.

                                                                                                                                                                                                                            (2) All quotes from the Quran are taken from the Hilali-Khan translation, available online from http://muttaqun.com/files/PDF/The_Holy_Quran_English_Arabic.pdf.

                                                                                                                                                                                                                            (3) Stoning to death of a couple in Afghanistan—http://www.youtube.com/watch?v=jJkYJ3cbxh0; Afghan woman executed for adultery—http://www.youtube.com/watch?v=9ed0TZN2Egk.

                                                                                                                                                                                                                            (4) The Hadith is a record of traditions or sayings of the Prophet Muhammad and his companions. This collective body of Islamic traditions is revered and received as a major source of religious law and moral guidance, second only to the Quran. (Hadith. 2013. In Britanica.com. Retrieved September 1, 2013, from http://www.britannica.com/EBchecked/topic/251132/Hadith.)

                                                                                                                                                                                                                            (5) A fatwa is a formal legal opinion given by an Islamic legal authority (Mufti) in answer to an inquiry by a private individual or judge (Fatwa, 2013).

                                                                                                                                                                                                                            (6) Al-Tabarani, quoted in Al-Albani (1983; see also Keller, 1997).

                                                                                                                                                                                                                            (8) A literature search yielded not even one case of an honor killing that was not committed by a religious adherent and for a nonreligious reason.

                                                                                                                                                                                                                            (9) Used in classification of the hadiths, it is the highest level of authenticity given to a narration. (Sahih, 2013).