Abstract
Academic debates persist about the psychology of suicide terrorists, with one view being that they are psychologically healthy individuals who primarily engage in altruistic self-sacrifice to serve their family, organization, or cause. Some proponents of this view now argue that suicide attackers are actually responding to their evolved sacrificial tendencies. However, the present review questions this hypothesis. For one thing, it appears inconsistent with the evidence on which individuals become suicide bombers and why. More broadly, research from the animal kingdom suggests that there is an important limit to “selfless” or “altruistic” behavior among non-human mammals, which appear to have been naturally selected to save themselves rather than deliberately give up their lives to protect offspring from predation, infanticide, or starvation. Furthermore, kin selection theory suggests that intentional self-sacrifice would be maladaptive for virtually all mammals, including human beings, and that this behavioral tendency would not have been naturally selected after all.
Decades after the emergence of modern suicide terrorism, heated academic debates persist about the psychology of suicide attackers. The primary point of contention is whether those who detonate themselves or intentionally crash airplanes into buildings should be categorized as committing “self-sacrifice” or “suicide.” According to the former perspective, suicide terrorists are psychologically normal and stable individuals who are so fully committed to a cause that they are willing to give up anything, including their lives, to serve it (Pape, 2005; Brym, 2007; Post, Ali, Henderson, Shanfield, Victoroff, & Weine, 2009; Atran, 2010). According to the latter perspective, suicide terrorists are often struggling with personal crises, mental health problems, or suicidality, or they are pressured and coerced into giving up their lives, although they may claim to have noble or heroic intentions (Lester, Yang, & Lindsay, 2004; Merari, 2010; Lankford, 2013, 2014).
The present review will not attempt to resolve this broader debate; rather, it will address a specific contention about suicide attackers that warrants much greater scrutiny. Among those who argue that suicide terrorists commit genuine self-sacrifice, one of the interesting new developments is the hypothesis that this behavior is the product of the attackers' naturally selected sacrificial tendencies.
Kin Selection Benefits as a Possible Explanation for Suicide Terrorism
The possibility that kin selection theory could explain sacrificial behavior by suicide terrorists has been proposed by a number of scholars (Liddle, Bush, & Shackelford, 2011; Orbell & Morikawa, 2011; Gallup & Weedon, 2013; Fink & Trivers, 2014; Gray & Dickins, 2014), but is perhaps best summarized by Qirko (2014):
In fact, there are several models of altruism through which inclinations to sacrifice can be understood as part of normal human evolved psychology. One is kin selection theory, which explains how genes associated with behaviors that reduce individual fitness can nevertheless spread if the behaviors result in fitness benefits for genetic relatives (Hamilton, 1964; Park, 2007). Hence, although associated mechanisms of kin dispersal and recognition will vary, dispositions to sacrifice for (some) others are likely normal in many species (Emlen, 1995, 1997), including humans (Qirko, 2014, p. 375).
By this view, suicide terrorism could potentially be attributed to kin selection, because in each case the terrorist may be dying for a cause that could help his or her family. In other words, the evolutionary cost of the individual's tendency to intentionally die (“reduced individual fitness”) could be offset by the “fitness benefits for genetic relatives” (Qirko, 2014, p. 375).
For instance, when suicide bombers from Al Qaeda, ISIS, or Hamas blow themselves up in an attempt to defeat enemy threats, they could be improving the safety or welfare of closely related kin, which could increase the likelihood that their shared genes get passed on to future generations (Darwin, 1871; Liddle, et al., 2011; Orbell & Morikawa, 2011; Gallup & Weedon, 2013; Fink & Trivers, 2014; Gray & Dickins, 2014). As Liddle, et al. (2011) concluded, because some families of suicide bombers receive financial payments or increased social status after their loved one's deaths, this provides support “for kin selection as a driving force behind Palestinian suicide terrorism” and indicates “that a similar driving force may exist in other regions” (p. 189).
In addition, some researchers suggest that kin selection also explains why suicide bombers would sacrifice their lives to serve terrorist organizations, even if their own families would not benefit. According to this view, a human tendency to sacrifice oneself for kin may be extended to others who are perceived as “fictive kin,” such as the members of one's group or organization (Atran, 2010; Liddle, et al., 2011; Orbell & Morikawa, 2011; Qirko, 2014). This hypothesis could fit with past studies that have shown that when a speaker uses kinship terms, such as “brother” or “sister,” to metaphorically refer to another person, that person's agreeableness and support for the speaker increases significantly (Johnson, 1987; Salmon, 1998). Therefore, when a suicide bomber becomes so powerfully committed to members of a terrorist organization that they feel like a surrogate “family” or “brotherhood,” a tendency to sacrifice one's life for family members could be extended to the protection of non-relatives and the terrorist cause.
Are Suicide Terrorists Primarily Attempting to Obtain Direct Benefits for Kin?
The aforementioned hypotheses are creative and worth considering, and in general there is no doubt that humans can become very attached to both their genetic and fictive kin. However, previous research on terrorism suggests a number of reasons to question claims that suicide attackers are primarily attempting to obtain direct benefits for kin. (1) In many cases, suicide terrorists leave home secretly, and their families never even receive confirmation that they intended to die, let alone increased social status or financial compensation after their deaths (Peraino, 2008; Ballen, 2011). (2) Suicide terrorists often come from families that are relatively well off, and are not in desperate need of money to survive (Hassan, 2001; Kreuger, 2003; Pape, 2005; Caplan, 2006). (3) Far from encouraging their children to sacrifice themselves, parents often forbid them from doing so, and are devastated if one of them disobeys and becomes a suicide bomber (Atran, 2010; Lankford, 2013). (4) Suicide terrorists themselves rarely cite payments for their family as their primary motive (Caplan, 2006; Lankford, 2014). (5) Suicide terrorists do not appear less likely to blow themselves up when financial payments are reduced or become less certain, as would be expected if these payments were critical to their decision-making (Krueger, 2003). Ultimately, as Caplan (2006) summarizes, it seems that in the rare cases “when death benefits work, they do so by making volunteers feel less guilty about abandoning their family; they do not think they are doing their family a favor” (p. 96).
Furthermore, the hypothesized application of kin selection theory appears inconsistent with which individuals actually become suicide bombers. As de Catanzaro (1991) proposed, individuals should theoretically be most likely to sacrifice themselves when their (a) remaining reproductive potential and (b) ability to contribute to their family are at their lowest or negative levels. This primarily would predict suicide or selfsacrifice by the elderly (de Catanzaro, 1991; Gallup & Weedon, 2013). However, when it comes to suicide terrorism the family members who are no longer fertile, who have the fewest remaining years to contribute to their families, and who are most likely to be costly medical and financial burdens—such as elderly aunts, uncles, and grandparents—almost never commit suicide attacks (Pedahzur, 2005; Lankford, 2013).
In fact, most suicide terrorists are young men and women. Reports indicate that the average 9/11 suicide terrorist was age 24, while the average Palestinian suicide bomber is age 21 (Dickey, 2009). They are in their prime reproductive years. Contrary to the proposed application of kin selection theory, these individuals are forfeiting many years of future earnings, support, and assistance for their families, along with all future reproductive opportunities (Pedahzur, Perliger, & Weinberg, 2003; Lankford, 2014). For example, five of the 9/11 hijackers were already married, at least one additional hijacker was engaged, and several others were actively seeking wives—and yet only two had reportedly sired a child (National Commission on Terrorist Attacks upon the United States, 2004; McDermott, 2005). If direct benefits for kin were indeed the primary explanation for their behavior, why would they choose to intentionally die before creating families of their own? This is the opposite of what the theory would seem to predict. Similarly, the young women who become “black widow” suicide bombers in Chechnya and other regions kill themselves after their loved ones have already died (Speckhard, 2012). At one point, they may have hoped to build healthy families of their own, but instead of starting over with a new spouse they choose to blow themselves up, leaving no one behind. This would not make much sense as a sacrificial strategy to benefit genetic kin.
The notion that suicide attackers are sacrificing themselves for “fictive-kin” also seems inconsistent with the evidence. Most volunteer suicide terrorists make the decision to blow themselves up before they enter the terrorist organization, not afterwards (Pedahzur, 2005; Lankford, 2014). In fact, they rarely spend much time in the organization, and their suicide bombing is typically the first and only terrorist attack that they ever participate in. This strongly suggests that they are not primarily motivated by social bonds with a “family” or “brotherhood” of fellow terrorists, because those fellow terrorists were essentially strangers to them when they made their decision to intentionally die (Lankford, 2014).
Do Mammals Have a Tendency to Commit Intentional Self-sacrifice?
Beyond this evidence from previous research on terrorism, there is a much broader question about the evolutionary psychology of mammals. Do mammals have a behavioral tendency to commit intentional self-sacrifice? For a number of important reasons that are consistent with kin selection theory, the present review suggests that they do not.
It can be helpful to think of a behavioral tendency as equivalent to what Dawkins (1976) and other scholars have referred to as an “evolutionarily stable strategy” (69). These strategies are “programmed” by millions of years of natural selection, whereby individuals with failing strategies disappear from the gene pool, and those with winning strategies that maximize their inclusive fitness flourish. As Dawkins (1976) explains further,
A “strategy” is a pre-programmed behavioral policy. An example of a strategy is: “Attack opponent; if he flees pursue him; if he retaliates run away.” It is important to realize that we are not thinking of the strategy as being consciously worked out by the individual. Remember that we are picturing the animal as a robot survival machine with a pre-programmed computer controlling the muscles (Dawkins, 1976, p. 69).
As the present review will demonstrate, research from the animal kingdom suggests that an evolutionarily stable strategy for mammals is to draw a hard limit on “selfless” or “altruistic” behavior. The winning strategy appears to be: “Help your kin—but if doing so will result in your serious injury or death, save yourself instead.” This directly contradicts any tendency for intentional self-sacrifice. Those who argue that mammals have a tendency to deliberately sacrifice their lives for kin are ignoring the importance of this hard limit on altruism. Furthermore, as will be explained in more depth, the programming of this self-preservation strategy is completely consistent with what kin selection theory predicts, and appears to be the best way for mammals to increase their inclusive fitness.
Of course, much like humans, many other mammals clearly do risk injury or death to defend their kin. There are also many antipredator adaptations, such as alarm signal calls and distraction displays that may increase the mortality risk for the actor while reducing the risks for others nearby. Acts of maternal aggression in response to threats to offspring are well documented in the biological record as well (Dawkins, 1976; Parmigiani, 1989). However, none of these behaviors qualify as intentional self-sacrifice, because these animals are not deliberately forfeiting their lives—they are attempting to achieve their goal (e.g., protect kin) and survive as well.
If mammals actually had an innate tendency to practice intentional self-sacrifice, what would it look like in nature? Under this scenario, we would expect it to serve nature's ultimate “cause”: the successful propagation of one's genes. However, it appears that mammals do not deliberately give up their lives to protect offspring from predation, infanticide, or starvation.
Predator-Prey Interactions
What Behaviors Would Suggest a Tendency for Intentional Self-sacrifice?
If dying to protect one's kin actually provided an adaptive advantage for mammals, then we should expect them to often exhibit this behavior in nature. By this logic, it should be rare for a parental animal to survive at the scene, while its young are killed and consumed by a predator. Parents would have a hardwired “over my dead body”-type protective tendency, whereby the predator would often be forced to kill the parent in order to gain access to its young. Even if the parent's own death were certain, it might be expected to sacrifice its life in an all-out attack for the chance of wounding or temporarily distracting the predator, and thus buy its offspring more time to escape.
What Does the Evidence Show?
However, it appears that the opposite behaviors are more common in nature. In many cases, a parental animal survives an attack, while its young are killed and consumed. Although in some cases young prey are killed because their parent is not around to defend them, it also appears that some predators can flush or chase parental animals away from their own young, and then enjoy the meal without further challenge. In these cases, one key factor may be the large power differential: when the predator has overwhelming size and strength advantages over the parent of its prey, defensive attacks from the parent would be rendered virtual suicide missions (Hespenheide, 1973; Aljetlawi, Sparrevik & Leonardsson, 2004). Although this type of certain death does not deter some insects, such as worker honeybees or African termite soldiers (Wilson, 1978; Ridley & Dawkins, 1981), it appears to be a cost that natural selection has rendered parental mammals unwilling to pay.
On the other hand, when that power differential is not as large, the parent can attempt to fight back to protect its young and simultaneously maintain a realistic chance for its own survival (Magalhães, Janssen, Montserrat & Sabelis, 2005). When intentional self-sacrifice would not be required by the situation, basic protective instincts suffice. This may be one reason why we see adult buffalo charging lions and adult chimpanzees chasing leopards to protect their kin (Boesch, 1991; Kluger, 2007), but not mice charging cats or rabbits charging wolves. Additional reasons will be discussed later in the present review.
Conspecific Infanticide
Conspecific (same-species) infanticide is a particularly good test for the possible existence of a tendency to commit intentional self-sacrifice, because unlike many aggressive encounters between predators and prey it allows us to distinguish deadly threats to the offspring from deadly threats to the parent. In the vast majority of cases, infanticidal conspecifics only pose a mortal threat to the offspring (Bertram, 1975; Hrdy, 1977).
What Behaviors Would Suggest a Tendency for Intentional Self-sacrifice?
It should be relatively easy to observe if mother mammals actually have an instinct to do everything in their power to prevent infanticide. If so, they should never let a male intruder get even close to their offspring without immediately launching a relentless and unceasing attack, regardless of the potential risks to themselves. In this scenario, even if the mother had only minimal hope of actually winning the fight, she should fight to the death in an attempt to seriously wound the male intruder and force his retreat. In fact, by being willing to fight to the death, mother animals would remove the primary motive for male attackers to commit infanticide in the first place: future breeding (Bertram, 1975; Hrdy, 1977). It would be maladaptive for males to be infanticidal in the presence of self-sacrificial mothers, because from the male perpetrator's perspective, the goal of infanticide is to induce the female's ovulation so she can bear his young. If mothers had a behavioral tendency to commit intentional self-sacrifice, conspecific infanticide for sexual-selection purposes might cease to exist altogether.
What Does the Evidence Show?
This does not seem to be what actually occurs in nature. As summarized in Table 1, research on infanticide across a wide range of contexts suggests that during any skirmishes between infanticidal males and mothers, the mother rarely dies. In fact—far from death being the norm—the present review could not find a single case where one of these infanticidal encounters was observed to have resulted in the mother's death (Sugiyama, 1965; McCarthy & Vom Saal, 1986; Agoramoorthy & Mohnot, 1988; Parmigiani, 1989; Packer & Pusey, 1994; Watts, 1989; Wilson, Wallauer & Wrangham, 2004; Yamagiwa, Kahekwa, & Basabose, 2009). Ultimately, when their infants are threatened, mothers of mice, lions, monkeys, chimpanzees, mountain gorillas, and other mammals certainly appear willing to risk some injury for protective purposes, but are not willing to intentionally sacrifice their own lives (Sommer, 1987; Parmigiani, 1989; Watts, 1989; Packer & Pusey, 1994; Arcadi & Wrangham, 1999; Watts & Mitani, 2000).
Maternal Responses to Conspecific Infanticide Among Non-human Mammals
Notably, in many cases, infanticide occurs after the mother essentially makes the behavioral decision to preserve her own life, rather than do everything she could to prevent it. A close study of house mice found that infanticide often involves (1) an initial intrusion from a non parental male, (2) an aggressive attack by the lactating female against the male intruder, (3) a counter-attack by the male against the lactating female, (4) submission from the female, and (5) the killing and cannibalizing of the pups by the male (Parmigiani, 1989). Evidence on infanticide committed by male lions often reveals a similar pattern of events where the females either submit, are chased away from their cubs, or try to avoid conflict and do not react until it is too late (Bertram, 1975; Packer & Pusey, 1994). Similarly, studies of both hanuman langur monkeys and chimpanzees reveal that infants are often snatched from the arms of their mothers and killed in front of them (Agoramoorthy & Mohnot, 1988; Wilson, et al., 2004). Watts and Mitani (2000) specifically observed a case where an adult male chimpanzee grabbed a mother's infant “from her and ran off with it… The female started to follow… but then turned and ran in the opposite direction as the other Ngogo males arrived” (p. 359). The infant was subsequently killed.
Cannibalism and Matriphagy
What Behaviors Would Suggest a Tendency for Intentional Self-sacrifice?
If mammal mothers actually had a behavioral tendency to sacrifice anything for their offspring's welfare, we would expect some of them to give their lives and bodies to feed their young, particularly in extreme circumstances where starvation seems likely. After all, malnutrition and starvation are very common threats to the health of offspring and can have many damaging effects: stunted growth and poor physical development; the loss of body mass and organ mass; permanent physical problems; and, of course, death (McCue, 2012). The flesh of the mother could provide that fuel—even for relatively young offspring and for herbivores, both of which are often capable of digesting meat, despite popular misconceptions to the contrary (Landers, 2012). By making these sacrifices, mammal mothers could potentially increase their offspring's health, size, strength, and chances of survival.
What Does the Evidence Show?
However, despite these potential benefits, mammal parents do not appear to intentionally give up their body parts or lives for cannibalization by offspring. The heavy price of this sacrifice—the disability or early death of the parent—is something that natural selection has seemingly rendered them unwilling to pay. This is most likely because of the fundamental differences between mammals and the specific arachnids for whom matriphagy is quite common, such as the Australian social spider, black lace-weaver spider, and foliage spider (Kim, Roland & Horel, 2000; Toyama, 2001).
Intentional Self-sacrifice as a Maladaptive Strategy
Ultimately, a tendency to sacrifice one's life would be valuable only if one could recognize, with some precision, the exact moment when the sacrifice would be absolutely necessary. In the extreme, it is easy to imagine the downfall of a species with an innate tendency for self-sacrifice at the smallest sign of danger: many members would intentionally die prematurely.
Furthermore, there is often great uncertainty about whether intentional self-sacrifice would achieve its desired outcome. Consider a human mother from 50, 000 yr. ago with a hardwired instinct to sacrifice herself when confronted by a fast approaching threat, and therefore save her children. If the odds that her sacrifice would successfully save her children are 100%, perhaps she need only save more than two for the behavior to be beneficial for their shared genes, as Hamilton (1964) outlined. But absolute certainty of success rarely exists in nature; even if she sacrifices her life, the attacker could end up killing both her and the children. Or the children could survive the attacker but still end up dying a short time later, because they would lack the care of their now deceased mother. If the mother's odds of a successful sacrifice are 10%, then, on average, she needs to save more than twenty of her children for the strategy to be cost-effective. That would be far more children than the average human mother typically produces, rendering the behavioral tendency maladaptive.
Perhaps the most significant uncertainty is about whether the benefits of the sacrifice would outweigh all future contributions to one's kin. This fits with Trivers's (1972) notion of parental investment. The key is that, by definition, intentional self-sacrifice becomes the final investment: it closes the account once and for all. In general, it seems unlikely that many animals could accomplish more by dying tomorrow than they could by continuing to invest in kin for the rest of their lives. Mice, lions, monkeys, gorillas, and other mammals who sacrifice their lives would be foregoing all future investments, including all future reproductive opportunities (Hamermesh & Soss, 1974; de Catanzaro, 1991). This is particularly relevant for mammals, because in the vast majority of cases they have evolved as iteroparous breeders who gave birth a number of times over the lifecycle. Intentional self-sacrifice would seem to render this reproductive strategy worthless: the ability to “breed like rabbits” would be completely wasted by the rabbit who gives up its life to save four offspring, but loses out on producing 80 more.
Given the importance of parental investment in future offspring, it should not be surprising that mammals might be unwilling to forfeit their lives for just a gambler's chance of saving their current offspring from a deadly threat. In fact, parents will sometimes decide to stop investing in young that are ill or of poor quality, because they determine that it is instead more beneficial to save their investment for other healthier or future offspring (Trivers, 1974; Dawkins, 1976; Daly & Wilson, 1988). This is essentially the same calculation: there may not be a qualitative difference between the survival prospects of offspring that are ill or of poor quality and the survival prospects of offspring that are facing mortal threats from predation, infanticide, or starvation. If parental investment in sick offspring is sometimes deemed too costly, because it requires excessive time and effort (Trivers, 1974; Dawkins, 1976; Daly & Wilson, 1988), surely the same could be said of intentional self-sacrifice, for which the cost is much greater.
Overall, these major limitations regarding the timing, probability of success, and opportunity cost of intentional self-sacrifice suggest that it would not constitute what Dawkins (1976) and other scholars have referred to as an “evolutionarily stable strategy” (p. 69)—at least not for mammals.
For Which Organisms Would Intentional Self-sacrifice Increase Inclusive Fitness?
Ultimately, because of these major uncertainties we would expect that a behavioral tendency to commit intentional self-sacrifice would be naturally selected only in organisms for whom the potential benefits are far greater than the minimum threshold cited by Hamilton (1964), or for whom the costs are relatively low. Organisms with very high numbers of closely related genetic kin would have the most to gain from selfsacrifice. And organisms with short life spans would have the least to lose by intentionally dying, because their future reproductive potential and investment in kin would be minimal anyway. Based on these principles, we would expect an innate tendency to commit intentional self-sacrifice to be far more likely to appear in insects and arachnids than it would in mammals. In fact, among mammals human beings would seem like one of the least likely species to have an innate tendency for intentional self-sacrifice, given the many uncertainties involved, the limits on their reproductive ability to spawn large numbers of closely-related kin, and their relatively long life span and high potential for future investment.
Sure enough, kin selection does appear to accurately explain intentional self-sacrifice among certain insects. For example, sterile honeybee workers essentially commit suicide attacks when they sting an intruder to protect their genetic kin. The venom injection process that causes the worker bee to die also makes its sting more effective, and thus increases its ability to defend the hive (Ridley & Dawkins, 1981). Similarly, sterile African termite soldiers attack threats to their kin by dispersing a glandular spray that often ends both their own lives and the lives of the intruders. In many cases, these termites seem to explode with bursts of the fatal spray, almost like suicide bombers (Wilson, 1978). We do not need to read these insects' minds to assert that they commit intentional self-sacrifice, because their behavior is the product of natural selection, not individual pathology or malfunction.
Similarly, in arachnid species that practice matriphagy—such as the Australian social spider, black lace-weaver spider, and foliage spider—mothers often give their bodies to feed 100 spiderlings from a single brood (Kim, et al, 2000; Toyama, 2001). Controlled experiments have found precisely what we would expect: the net benefits for the mother's genes (via those passed to her offspring) outweigh the costs of her death, because spiderlings who eat their mother grow significantly larger and have significantly higher survival rates than those who do not (Kim, et al., 2000; Toyama, 2001). In turn, the honeybee workers and termite soldiers that sacrifice their lives in defensive suicide attacks can potentially save tens of thousands of genetic kin (Wilson, 1978; Ridley & Dawkins, 1981). They share a common mother (the queen), and have particularly high degrees of relatedness to large numbers of kin far beyond what is typical for mammals (Hughes, Oldroyd, Beekman, & Ratnieks, 2008).
Furthermore, the organisms that commit intentional self-sacrifice have very small or dwindling investment accounts, so not much is lost because of their deaths. Spiders who commit matriphagy are very short-lived, and in many cases are semelparous (breed once in their lifespan). In other cases, they may be foregoing one more birth cycle, but by dying to feed their young they significantly increase their overall reproductive success (Kim, et al., 2000; Toyama, 2001). And because honeybees and African termites are both eusocial insects, the specific workers and soldiers who commit suicide attacks are not part of the reproductive process anyway. As Dawkins (1976) explains, “The death of a single sterile worker bee is no more serious to its genes than is the shedding of a leaf in autumn to the genes of a tree” (p. 172). With the possible exception of sterile members of naked mole rat colonies, there seem to be few relevant corollaries among mammals.
Conclusions
In the decades since kin selection theory has become widely accepted, some scholars have begun to discount the primacy of survival instincts and self-preservation to the evolutionary process, and instead assert that what really matters is only the successful propagation of one's genes (de Catanzaro, 1991; Kirkpatrick & Navarrete, 2006).
As a summary of mammalian psychology, this may be a significant oversimplification. It is true that overall the future prospects of one's genes must indeed be a higher priority than one's own survival, because otherwise organisms would not take such significant risks or bear such costs to procreate and care for their young (Trivers, 1972; Dawkins, 1976). However, in a more narrow sense, having survival as a top short-term priority seems essential to accomplishing other long-term priorities, such as reproduction and replication. This is why, in The Selfish Gene; Dawkins (1976) repeatedly refers to organisms as “survival machines” (p. 20). It appears that there may be a hard limit to “selfless” or “altruistic” behavior among non-human mammals, which have the naturally selected tendency to save themselves rather than deliberately give up their lives to protect offspring from predation, infanticide, or starvation.
Much of evolutionary psychology depends on accurate understandings of these basic mammalian drives for self-preservation and survival. Future research in this area—both in the form of field observations and controlled experiments—could be of tremendous value. Scholars may be able to better identify the precise set of biological and environmental factors that can lead mammals to abandon their offspring and save themselves.
Furthermore, a more accurate grasp of these evolved tendencies should help us better understand human behavior. For thousands of years, people have celebrated the notion that heroic individuals embrace certain death because they are so passionately committed to their family, group, or cause. The glorification of intentional self-sacrifice shows up in everything from sacred religious texts and cherished works of fiction to chart topping pop songs and blockbuster Hollywood movies. However, both the present review and prior research suggests that this behavior does not appear to be attributable to any evolved sacrificial tendencies (Wrangham & Glowacki, 2012).
Overall, the explanation for why mammals seem to lack a fatally self-sacrificial tendency may be similar to the explanation for why most terrorists do not volunteer to intentionally die for the cause by committing a suicide bombing. The expected value of the potential sacrifice is undermined by too many unknowns about timing, probability of success, and opportunity cost. No matter how passionately committed you may be to the cause, given the many uncertainties involved it is generally not advantageous to intentionally give up your life tomorrow, rather than invest your life in that same cause for many future decades (Lankford, 2014).
For example, past Islamic terrorists who have refused to volunteer for suicide attacks have explained that they could better serve the cause by remaining alive and continuing to fight (Merari, 2010; Lankford, 2013, 2014). And the statistics support this logic. For the most part, suicide bombings are characterized by uncertainty and relatively small tactical benefits. In Afghanistan, 40–50% of suicide bombers end up killing only themselves, and in Israel nearly 75% of Palestinian suicide bombers have failed to damage their targets (Robinson, 2007; Atran, 2010; Byman & Fair, 2010; Lankford, 2013; 2014). More broadly, despite the actions of suicide bombers, the empirical evidence strongly suggests that terrorist violence usually fails to achieve political goals (Abrahms, 2006, 2012). Ultimately, much like other mammals, humans who want to contribute to their kin, group, or cause may be programmed with the evolutionarily stable strategy that they can typically accomplish more in any scenario by trying to remain alive.