The Morality of War: A Review and Research Agenda

The principle of discrimination

Just-war theory prioritizes the principle of discrimination as the paramount moral principle in war. Its priority is echoed by customary International Humanitarian Law, which codifies how wars should be fought (i.e., places legal limits on how war can be conducted). Of the 161 rules of International Humanitarian Law, 96 concern variations on the principle of discrimination in that they determine who (and what) is a legitimate target for attack during war (Henckaerts & Doswald-Beck, 2005). According to this principle, civilians (noncombatants) are not legitimate targets of attack (Orend, 2013) and should be protected as far as possible (International Committee of the Red Cross, 2005). Combatants, by contrast, are legitimate targets of attack, and part of the challenge is to define a combatant in an appropriately restrictive way (Gade, 2010; Kutz, 2005). The principle of discrimination is not a recent invention. Warrior codes of ethics and humanitarian limits on conflict appear to have included—throughout history—some version of the principle of discrimination in that some categories of people are considered more legitimate targets of attack than others (French, 2005; Traven, 2015). The prescriptive philosophical, legal, and historical literature thus suggests that the principle of discrimination will be an important part of a descriptive morality of war as well.

Two studies have investigated whether lay judgments of the conduct of war align with the principle of discrimination. Benbaji, Falk, and Feldman (2015) asked a representative sample of Israeli participants to imagine that “they were military commanders with an opportunity to gain a significant military advantage in an attack that targeted individuals” (p. 3) in a hypothetical war between two unnamed states. Overall, participants were more reluctant to order the attack when the targeted individuals were civilians rather than soldiers. Watkins and Laham (2018) likewise found that U.S. participants made harsher moral judgments of a hypothetical soldier (from a fictional country) who killed an enemy civilian compared with one who killed an enemy soldier. The findings of this latter study speak directly to the philosophical and legal principle of discrimination given that participants were positioned as uninvolved third-party observers to the conflict. Furthermore, not only did laypeople discriminate between the killing of civilians and soldiers, they also discriminated between the killing by civilians and soldiers: Soldiers were judged as more morally permitted to kill in war than were civilians. This finding is important because the principle of discrimination determines not only who is a legitimate target but also who is a legitimate moral agent in war (Gade, 2010).

The prescriptive and descriptive principles of discrimination thus appear closely aligned. But why are soldiers and civilians judged differently? In the philosophical and legal literature, the emphasis is often on posing a threat to others. On this analysis, a civilian who engages in hostilities can acquire the status of combatant and be liable to attack (Deakin, 2014), whereas a combatant who is hors de combat is afforded some of the same protections as civilians are (International Committee of the Red Cross, 1977). In the Israeli study cited above, the principle of discrimination was moderated by involvement in wartime activities: Enemy combatants who were directly involved in the fighting were seen as the most legitimate targets for attack, civilians who were uninvolved were seen as the least legitimate, and uninvolved soldiers and involved civilians fell somewhere in between (Benbaji et al., 2015). And, in the U.S. study, (Western) soldiers were perceived as more harmful than their civilian counterparts (as well as braver and colder), and perceptions of harmfulness mediated the effect of soldier status on moral judgments (Watkins & Laham, 2018). The current evidence thus suggests that the psychological principle of discrimination is indeed closely related to the perception that soldiers are generally more actively involved in causing harm. This is consistent with previous research (using animals and aliens as targets) demonstrating that harmful creatures are afforded less moral concern than harmless creatures (Piazza, Landy, & Goodwin, 2014).

The principle of discrimination is a central part of the morality of war. It is important to note that the principle is not about favoring my soldiers over enemy soldiers (although such biases certainly exist as well; Benbaji et al., 2015; Hewstone et al., 2002; Pratto & Glasford, 2008) but is instead an aspect of how people make moral judgments about the conduct of war from an uninvolved third-party perspective. The principle is well established in the philosophical and legal literature on war, and here I have discussed two studies that investigate discrimination between soldiers and civilians from a psychological, descriptive perspective. Tackling the principle of discrimination using psychological methods (i.e., vignette studies) and constructs (i.e., from the person perception literature; Piazza et al., 2014) can help illuminate not only the extent to which the principle describes laypeople’s judgments but also its possible causes.

The independence thesis and the moral equivalence of combatants

The principle of discrimination is one of the prescriptive principles of the conduct of war. Combatants on both (all) sides of a war are expected to comply with this and other principles (Boyle & Brown, 2016; Orend, 2013). That is, judgments of combatants’ conduct should be made independently of which side of the war they are on and independently of that side’s decision to initiate military action in the first place (Walzer, 2015). The second set of studies I discuss here address precisely this independence thesis drawn from traditional just-war theory: The studies aimed to investigate the relationship between moral judgments of the conduct of war and the resort to war.

The independence thesis states that the prescribed rules of conduct in war—the world of permissions and prohibitions—apply equally to all combatants regardless of the country or cause for which they are fighting (Lazar, 2017a). Consequently, as long as they do in fact follow the prescribed rules of conduct, the actions of combatants on either side of a war are morally equivalent and should be judged symmetrically (referred to as the symmetry thesis). The independence and symmetry theses are key aspects of traditional just-war theory. However, these theses have recently been subject to a great deal of debate. “Revisionist” theorists counter the “traditionalist” position by arguing that moral judgments of combatants’ conduct should take into account the cause for which they are fighting (Lazar, 2017a; McMahan, 2004). If the (just or unjust) reason the combatants’ country had for going to war in the first place ought to influence judgments of the combatants themselves, then even if they are engaging in the same conduct, combatants on either side of a war are no longer moral equals. Their conduct should be judged asymmetrically, with greater leniency afforded to the combatants fighting for a just cause.

The argument between revisionist and traditionalist just-war theorists is partly an argument about the relationship between the two aspects of just-war theory—the resort to war (the international aspect) and the conduct of war (the interpersonal aspect). The corresponding psychological research question, then, is how this relationship plays out in laypeople’s moral judgments. To what extent are third-party moral judgments of interpersonal actions (i.e., of the conduct of war) influenced by the country or cause for which the combatants are fighting (i.e., by the resort to war)? Are moral judgments of combatants on either side of a conflict symmetrical?

Watkins and Goodwin (2019) comprehensively investigated the conditions under which lay judgments of soldiers on either side of a war are symmetrical or asymmetrical. In a large set of studies, they presented participants with information about various wars in which there was a “just” and an “unjust” side (e.g., an aggressor state invaded a neighboring state, which defended itself). They then asked participants to judge a soldier from either the just or the unjust side engaged in the same action (e.g., killing enemy soldiers). Across all studies, covering many different types of wars and many different actions, laypeople judged the conduct of soldiers on the just side of a war as more morally defensible than the same conduct by soldiers on the unjust side. In other words, lay judgments were asymmetrical. The U.S. participants in these studies did not uphold the independence or symmetry theses, and thus their judgments appeared to be most in line with these aspects of revisionist just-war theory.

Despite the main effect—an asymmetry in moral judgment—aligning with the normative position of revisionist just-war theorists, there is currently no evidence that it is driven by “ordinary people’s appreciation and endorsement of the philosophical bedrock on which revisionist theorizing is based” (Watkins & Goodwin, 2020, p. 439). According to revisionist just-war theorists, one justification for asymmetrical judgments is that combatants on the unjust side of a conflict causally contribute to an unjust cause (whereas just combatants contribute to a just cause; Lazar, 2016). However, Watkins and Goodwin (2019) found that the asymmetry remained unchanged whether the soldiers’ actions causally contributed to their side’s war effort or not, which suggests—as opposed to the revisionist argument—that this factor is not contributing substantially to the asymmetry. Meanwhile, according to traditional just-war theorists, one justification for symmetrical judgments is that combatants on both sides of a conflict are operating under a “shared servitude” (Walzer, 2015). However, Watkins and Goodwin also found that the asymmetry remained unchanged whether the soldiers were acting under orders or on their own initiative, which suggests that this factor is not contributing substantially to the asymmetry either.

So what does explain the asymmetry? Which psychological processes are leading laypeople in the United States to judge the actions of combatants on the just side of a conflict as more morally defensible than the identical actions of those on the unjust side? Watkins and Goodwin (2019) found support for two processes. First, in these studies, participants tended to assume that the soldiers on either side of the conflict endorsed and supported their own country’s cause for war. When this assumption was overturned—that is, participants were told that the soldiers did not endorse their country’s cause for war but were fighting out of a sense of duty—the asymmetry was attenuated; primarily because the conduct of the soldier on the unjust side was judged more leniently under these conditions. This finding is consistent with previous research, in an everyday context, showing that an agent’s identification with a morally bad act increases condemnation by third-party judges even as the outcome remains the same (Woolfolk, Doris, & Darley, 2006). Second, in these studies, participants considered a war between two hypothetical countries. However, in making their judgments, they tended to implicitly align themselves with the just side of the war, which meant that part of the asymmetry in judgments of the combatants was driven by in-group bias (see, e.g., Hewstone et al., 2002). When this implicit alignment was overturned—by explicitly asking participants to imagine themselves either as citizens of a third, uninvolved country or as citizens of the unjust country—the asymmetry was again attenuated (Watkins & Goodwin, 2020; see also Benbaji et al., 2015). Thus, the asymmetry appears to be substantially driven by two factors: an assumption about the combatants’ endorsement of their country’s cause for war and a tendency of uninvolved third-party observers to align themselves with the just side.

The studies reported by Watkins and Goodwin (2020) uncovered a particular feature of the descriptive morality of war—asymmetry, rather than symmetry—and partially explained it by reference to psychological processes of perceived identification and group alignment. Neither of these processes is emphasized in the philosophical or legal literature on the conduct of war, and, of course, the debate between revisionists and traditionalists will not be resolved by reference to any single psychological study. However, this discrepancy between the descriptive and the prescriptive is important nonetheless.

Looking to international laws, the symmetry and independence theses (of traditional just-war theory) emerge as an attempt at impartiality. The International Criminal Court is a third-party institution that applies the laws governing the conduct of war to individual cases irrespective of the countries involved (The International Criminal Court, 2002). Ad hoc international tribunals set up after conflict, such as the Truth and Reconciliation Commission in South Africa and the International Criminal Tribunal for the former Yugoslavia, likewise aim at fairness and even-handedness when dealing with both perpetrators and victims of these complex conflicts.

Watkins and Goodwin (2019) wrote that “traditional just war theorists face an uphill battle in convincing lay individuals to judge soldiers on either side of an unjust war as moral equals” (p. 21). Certainly, these tribunals are not always successful in being or appearing impartial: They are plagued by both accusations of “victor’s justice” and recalcitrant UN Security Council members (Goldsmith, 2003; Meernik, 2003; Schabas, 2004). These problems are part of what motivates revisionists to reexamine the assumptions of traditional just-war theory: “What revisionists hope is that their work can be a source of guidance in establishing new international institutions that will eventually make it possible to reform the law of armed conflict” (McMahan, 2012, para. 31). One of these reforms concerns combatants’ taking responsibility for their endorsement (or otherwise) of their country’s cause for war: Revisionist just-war theorists believe their insistence on no independence and an absence of symmetry “can be used to instill a sense of individual responsibility and conscience in citizens and members of the armed forces” (Leveringhaus, 2012, p. 17).

Both the International Criminal Court’s impartiality and revisionists’ desire for “individual responsibility” represent worthy values and goals. I do not intend to adjudicate between them. However, in working toward these goals or trying to reconcile them and reform international institutions, philosophers and legal scholars may wish to take into account what psychology reveals about the capacities and limitations of laypeople’s morality of war or risk developing theories and guidelines that are at odds with such a morality.

Intermission and future directions

Just-war theory provides a prescriptive map of the morality of war, and the studies outlined above illuminate some psychological, descriptive features of this map. The two sets of studies differed in that one found that moral judgments of laypeople (in the United States, Australia, and Israel) were generally consistent with the principle of discrimination, whereas the other found that moral judgments of laypeople (in the United States) were inconsistent with the independence and symmetry theses. This latter discrepancy is particularly interesting in light of a recent debate within just-war theory between traditionalists who maintain that the independence thesis is the normatively correct position and the revisionists who instead argue for normative asymmetry. Despite this difference in their correspondence to (traditional) just-war theory, both sets of studies also diverge from just-war theory in a similar way: They both demonstrate that in explaining lay moral judgments, we must look beyond philosophical justifications (for the principle of discrimination and the independence thesis) and instead draw on established psychological phenomena and mechanisms such as person perception, (perceived) identification, and side taking (e.g., DeScioli, 2016; Piazza et al., 2014; Woolfolk et al., 2006). These studies thus suggest the potential to learn a great deal about the morality of war if our research questions are informed by just-war theory and our methods and investigations are informed by past psychological research.

The studies discussed in the previous section are also similar in that they focus on a dialogue between the moral philosophy of war, the international legal literature, and vignette-based experimental studies. Vignette-based studies allow researchers tight control over experimental factors (Aronson & Carlsmith, 1969) and can often be adapted directly from philosophical thought experiments (e.g., trolley problems; Cushman, 2014). But there are many other methodological avenues to explore and exploit as well. In addition to written vignettes, psychologists studying morality in general (not in war) have, for example, developed comprehensive image and video sets that capture the key features of moral judgment (Clifford, Iyengar, Cabeza, & Sinnott-Armstrong, 2015; Crone, Bode, Murawski, & Laham, 2017; McCurrie, Crone, Bigelow, & Laham, 2018); brain imaging techniques have been used to investigate the neural correlates of justified and unjustified harm (Domínguez et al., 2018) and moral as opposed to nonmoral cognition (Theriault, Waytz, Heiphetz, & Young, 2017); and beyond the lab, experience-sampling methods provide a snapshot of the moral judgments people make in everyday life (Bollich et al., 2016; Hofmann, Wisneski, Brandt, & Skitka, 2014). Furthermore, through conceptual analyses of anthropological, religious, and philosophical texts (Haidt & Graham, 2007; Haidt & Joseph, 2004), linguistic analyses of speeches by religious and political leaders (Graham, Haidt, & Nosek, 2009), and analyses of legal distinctions in systems of criminal law around the world (Mikhail, 2007), general models of moral judgment such as universal moral grammar (Mikhail, 2011) and moral-foundations theory (MFT; Haidt & Graham, 2004; Haidt & Joseph, 2004) have been developed.

MFT has been particularly influential in demonstrating that in an everyday context, judgments of right and wrong can be categorized using taxonomies of moral values, or foundations. According to MFT, five key foundations—care–harm, fairness–cheating, loyalty–betrayal, authority–subversion, and sanctity–degradation—comprehensively capture the content of moral judgments across cultures. Although debate remains about whether additional (or perhaps fewer) foundations are necessary (Iyer, Koleva, Graham, Ditto, & Haidt, 2012; Janoff-Bulman & Carnes, 2013; Schein & Gray, 2015), this taxonomy may also generate future research on the moral psychology of war. For example, can moral judgments made in war be captured by these five foundations as well, or are some foundations—perhaps authority-subversion and loyalty-betrayal—more relevant to the conduct of war than others? What is the relationship between the sanctity/degradation foundation (which captures the intuition that some actions are wrong on the grounds that they are degrading, unnatural, or impure; Haidt, 2012) and judgments of weapons that are mala in se? The mala in se principle applies to weapons that are bad in and of themselves and thus has been leveraged to develop prohibitions on, for example, landmines, blinding lasers, incendiary weapons, and biological weapons (Dige, 2012; Human Rights Watch, 2015; International Campaign to Ban Landmines, n.d.). In contemporary military affairs, the use of (autonomous) armed drones is hotly debated; opponents argue against “killer robots” in ways that appear—anecdotally—closely aligned with concerns about the sanctity of human life, the degrading effects of involving a machine in the kill-chain, and a visceral disgust and anger at the unnaturalness and unfairness of remote warfare (Asaro, 2012; Chamayou, 2015; Garcia, 2015; Human Rights Watch, 2005). Is the “impurity” of certain weapons what is at stake in these debates?

In studying these and related questions, I hope that researchers will be as inventive and pluralistic in their methodological approaches to the moral psychology of war as moral psychology has been in general by employing self-report surveys, image stimuli, video stimuli, experimental manipulations and economic games, brain imaging, big data, and linguistic analysis. As with morality in general, comparisons among these methods and the findings they yield will no doubt reveal some disagreement—among individuals, groups, nations, or historical eras—as to what the most relevant values and most inviolable rules of war are (Coker, 2008). In addition, there will be occasional disagreement among researchers about the best way to describe and explain this context-bound morality. But if we share the framework of just-war theory, we will collectively get closer to figuring out what is judged as morally right and wrong in war, how these judgments are made, and the broader implications they have.

The conflict approach: interpreting war through the morality of peace

If we ask what is morally right and wrong in an everyday context, many answers will correspond to some version of “it is morally wrong to intentionally harm people.” That is, a prohibition on intentional harm is a key moral value, recognized in most taxonomies of everyday morality (Haidt, 2012; Hofmann et al., 2014; Greene, 2014; Schein & Gray, 2017). But if one of the primary dictates of everyday morality is “do no harm,” what is going on with war?

As the world was shocked and horrified by the atrocities committed by the Nazi regime during WWII, one version of this question—“What turns ordinarily ‘moral’ and ‘peace-loving’ people into killers in war?”—provided the impetus to a great deal of research in social psychology (Leidner, Tropp, & Lickel, 2013; Reicher, Haslam, & Rath, 2008; Ross, Lepper, & Ward, 2010). This body of work provides the first alternative approach to morality and war, and I refer to it as the conflict approach. The conflict approach highlights—and responds to—a conflict between the dictates of an everyday morality, on the one hand, and actions committed during war and other kinds of intergroup conflict on the other hand: People have a

psychic need . . . to resolve two seemingly inconsistent cognitions when they decide whether or not to support war: the desire to maintain a favorable view of the self and the need to justify actions that violate socialized self-sanctions against harming others. (Aquino, Reed, Thau, & Freeman, 2007, p. 386)

How are these “inconsistent cognitions” resolved? One comprehensive answer is provided by Bandura’s moral disengagement theory (Bandura, 1999, 2002; Bandura, Barbaranelli, Caprara, & Pastorelli, 1996). Bandura proposed that we all possess a self-regulatory system that monitors our (moral) behavior, judges it as good or bad, and reacts (if necessary) to modify subsequent behavior. This system thus “keeps conduct in line with personal standards”; people behave morally because doing otherwise would “bring self-condemnations” (Bandura, 1999, p. 194). However, the self-regulatory system can be selectively disengaged (preemptively or retroactively). Moral disengagement strategies include, among others, minimizing one’s responsibility for a bad act, downplaying its negative consequences, morally justifying it, or denying the victims’ humanity (Bandura et al., 1996).

Originally conceived of as a “first-person” phenomenon (allowing the moral agents themselves to act immorally), moral disengagement has also been extended to “indirect” commission of immoral acts; that is, endorsement of immoral acts committed by others with whom one shares a group identity (e.g., fellow citizens of a particular country). Research using this theory has shown that when faced specifically with the dissonance evoked by war, people can indeed respond by morally disengaging: morally justifying the acts involved (e.g., Aquino et al., 2007; Hartmann, Krakowiak, & Tsay-Vogel, 2014), diffusing responsibility (de Graaff, Schut, Verweij, Vermetten, & Giebels, 2016), and dehumanizing both the enemy (Castano & Giner-Sorolla, 2006; Giner-Sorolla, Leidner, & Castano, 2011; Leidner, Castano, Zaiser, & Giner-Sorolla, 2010) and innocent bystanders (Rai, Valdesolo, & Graham, 2017). Moral disengagement theory is primarily discussed by Bandura as a process; however, when moral disengagement is assessed as a trait, it is also related to support for war and violence in general (Grussendorf, McAlister, Sandström, Udd, & Morrison, 2002; Jackson & Gaertner, 2010; McAlister, 2000, 2001; McAlister, Bandura, & Owen, 2006; C. Moore, 2015). Overall, then, moral disengagement theory has been highly influential and generative in investigating how individuals can commit or support acts—including killing in war—that they might “ordinarily” condemn.

My proposed approach differs from the conflict approach in a number of ways. The simplest difference relates to the distinction between the resort to war and the conduct of war (Table 1, Column 2). I believe moral disengagement theory is potentially a good match for analyzing resort to war; that is, the guidelines proposed by philosophers and legal scholars for governing a country’s decision to deploy the military and go to war. Considering this international aspect of just-war theory, the prescriptive question is, “When is a country morally justified in going to war?” The answer is—according to just-war theory—when the war has a just cause; when it has the right intention; when the correct authority declares war; when the war effort will not be disproportionate; and when it is used as a last resort (Orend, 2005). A researcher taking a conflict approach to war might analyze these prescriptive philosophical and legal requirements as examples of institutionalized moral disengagement. The just cause and right intention requirements sound like moral justifications in that they portray the war as serving valued social or moral purposes (Bandura et al., 1996). Insisting on a correct authority seems to involve diffusion or displacement of responsibility. Considerations of proportionality possibly minimizes harm to the victims. And so on.

This is no doubt an interesting project (and similar examples are used in the moral disengagement literature; Bandura, 2002), but it is not the one I am here proposing to undertake. I am instead proposing that researchers focus directly on the interpersonal conduct of war as a separate project. This means setting aside the decision to resort to war, sidestepping the contrast between “ordinarily peace-loving people” and their support for war (more on this below), and instead investigating what is judged as morally right and wrong in war. Once a war is under way, how is its interpersonal violence regulated? Which actions are proscribed or prescribed during war? This simple shift in focus represents the first difference between my proposed approach and the conflict approach.

The second, more conceptual difference relates to the relationship between morality and war (the “continuum” in Table 1, Column 4). The conflict approach starts from a baseline assumption about “everyday morality” in which ordinarily peace-loving people are socialized into avoiding and condemning intentional harm. If researchers treat the moral norms of peace—norms that include a strong prohibition against intentional harm—as the default, then the fact of killing in war indeed represents a troublesome violation of those norms. But using everyday morality as the starting point anchors the investigations at the “pacifist” end of the continuum. It forces research questions about moral judgments of the conduct of war into a comparison with the (assumed default, harm-avoiding) morality of peace. This inherent comparison in turn shapes and perhaps restricts the type of questions that are raised and answered by moral and social psychologists (Fiske & Rai, 2014). If we instead treat war as a discrete domain, with its own standards and norms, we can then ask a number of additional questions related specifically to moral judgments in this context. For example, who is considered a legitimate as opposed to illegitimate target of violence in war? What role does implied consent play in these distinctions? How are judgments about the treatment of wounded or imprisoned enemy soldiers influenced by ideals of honor (e.g., “fighting fair”; O’Dea, Martens, & Saucier, 2019)? How do expectations of fairness and bravery in war relate to judgments of drone warfare (Schulzke, 2017)? What makes urinating on corpses seem so especially morally abhorrent (Bowley & Rosenberg, 2012)?

The conflict approach focuses on deviations from an everyday morality; that is, on explaining how violence is initiated or perpetrated. For this reason, it does not facilitate the descriptive mapping of more nuanced distinctions in moral judgments within a context that is dominated by violence. But for researchers who want to explore and explain these particular judgments, it may be both more tractable and parsimonious to treat the morality of war as a separate landscape rather than to assume that it is always merely a distortion of the morality of peace (for a related argument, see Rai & Fiske, 2011,). Just-war theory provides a (prescriptive) map of this landscape and may thus prove useful to researchers interested specifically in descriptive and explanatory questions about the morality of war.

The third important difference between my approach and the conflict approach concerns who is making the judgment. Although it does not distinguish between the moralities of peace and war or between resort to war and conduct of war in the same way as just-war theory does, the conflict approach does discuss violence at many levels—interpersonal and intergroup as well as international. According to moral disengagement theory and related analyses, group-based structures (e.g., governmental and educational institutions) facilitate disengagement at the individual level and individuals collectively construct institutions as scaffolding for their disengagement in turn (Bandura, 1999; Bar-Tal, 2007). Thus, first-party, collective, and intragroup processes all actively help people resolve the potential conflict between their (or their group’s) immoral actions and their sense of themselves as morally good people.

But the questions I raised above—about the nuances in moral judgment across different types of harm—can be asked not only of first-party or in-group members but also of uninvolved third-party observers. People who are not from the same country as the moral perpetrators or victims, but rather from the broader international community, can and do make moral judgments about the conduct of war. These third-party observers do not have the same motivation to morally disengage as do the moral agents; there is no conflict between their actions and self-concept. In theory, uninvolved third-party observers are free to apply their everyday moral standards when judging war as well and to maintain, for example, that killing of civilians and soldiers is equally wrong: They are both equally human, and intentionally harming humans is wrong. Certainly, some individuals do reach this conclusion and identify as pacifists (Fiala, 2018). Yet, as discussed in the section on the principle of discrimination, on average, uninvolved observers judge the killing of (and by) soldiers more leniently than the same killing of (and by) civilians. They perceive soldiers as more dangerous, cold, and brave (Watkins & Laham, 2018). Furthermore, they make divergent judgments of the same actions by soldiers on either side of a (just vs. unjust) war (Watkins & Goodwin, 2020) and are more accepting of other people’s parochial moral actions in war (Watkins & Laham, 2019). Because moral-disengagement theory and similar lines of research focus on processes that are triggered by a (potential) conflict between actions and moral standards, they would have to explain whence this conflict is arising for these external, uninvolved observers.

This is not to say that the conflict approach has been wrong to focus on first-party antecedents of violence (whether interpersonal or intergroup). It is important to know what enables the initiation and perpetuation of violence, and exploring how people disengage from moral self-sanctions has been highly influential: Research on reducing moral disengagement, for example, has shown promise in encouraging people to behave more morally (for a review, see C. Moore, 2015). However, uninvolved third parties also make judgments about the conduct of war, and these judgments are important. Both during and in the aftermath of war, uninvolved third parties play a large role in judging—and potentially punishing—transgressions against the laws and norms of war. The legal frameworks that enable such third-party judgments in turn have their roots in philosophical theories of the morality of war (Orend, 2013). And, through informal, public, channels, third-party observers can also have an influence on the political and military elite (Holsti, 1996; Keck & Sikkink, 1999). Thus, which actions are judged as morally right or wrong by external observers once a war is under way represents an important alternative question, complementing the question of how individuals justify their own act of killing or justify their group’s going to war in the first place. Again, for researchers interested specifically in the morality of conduct in war, just-war theory provides a fruitful starting point for investigating this question.

The continuity approach: no distinction between morality of peace and war

I called the above approach the conflict approach because it starts from the observation that the core activity of war (killing) is in conflict with the dictates of everyday morality. The second approach to war in moral psychology involves no such conflict. In fact, the second approach does not directly involve war at all. I call this alternative the continuity approach.

In [the Korean War], a company of Marines was way outnumbered and was retreating before the enemy. The company had crossed a bridge over a river, but the enemy were mostly still on the other side. If someone went back to the bridge and blew it up, with the head start the rest of the men in the company would have, they could probably then escape. But the man who stayed back to blow up the bridge would not be able to escape alive. The captain himself is the man who knows best how to lead the retreat. He asks for volunteers, but no one will volunteer. If he goes himself, the men will probably not get back safely as he is the only one who knows how to lead the retreat. (Colby & Kohlberg, 1987, p. 611)

Should the captain order one of his men to stay back? Moral psychologists have a long tradition of creating moral dilemmas and scenarios (or drawing them from moral philosophy) for their research endeavors (Cushman, 2014). People’s reasoning about or reactions to these vignettes are then used to uncover general features of moral psychology, for example, the domains of moral concern (e.g., Clifford et al., 2015; Crone et al., 2017; McCurrie, Crone, Bigelow, & Laham, 2018), or the emotional (e.g., Russell & Giner-Sorolla, 2013) and cognitive (Cushman, Young, & Hauser, 2006; Feltz & May, 2017) processes underpinning moral judgment and decision-making. Kohlberg’s scenario about Heinz, who steals medicine for his sick wife (Colby & Kohlberg, 1987), is perhaps the most famous of these vignettes. More recently, trolley problems have become the lingua franca of moral psychology (Cushman, 2013). Trolley problems, as a broad class of scenario or dilemma, involve the possibility of sacrificing the life of one person to save the lives of several people. There are many inventive variations on these (and other) vignettes (A. B. Moore, Clark, & Kane, 2008; Petrinovich, O’Neill, & Jorgensen, 1993). What they have in common is that they are employed in an attempt to learn something general about moral psychology. Researchers frequently focus on broad commonalities or emotional and cognitive processes assumed to operate across domains and are therefore less interested in differences between the domains in which these scenarios are set.

Because of this emphasis on drawing general conclusions, the vignettes are often set in a wide variety of contexts. For example, the scenario about the marines and the captain, above, is included in Kohlberg’s moral judgment interview alongside the scenario about Heinz. Kohlberg and his colleagues sought to understand the development of moral reasoning; in particular, reasoning about justice. The differences between the specific scenarios—Heinz as opposed to the captain—were less important than understanding the abstract reasoning processes that underpinned people’s answers to both. A popular narrative about moral psychology begins with Kohlberg and his focus on reasoning about justice and then emphasizes how the scope of moral psychology has since broadened to include the study of intuitive and emotional processes as well as concerns such as authority, loyalty, and purity (Haidt, 2012; Monin, Pizarro, & Beer, 2007; for alternative narratives, see Skitka, 2009; Skitka, Bauman & Mullen, 2008). Yet part of Kohlberg’s legacy has not been shaken off. Scenarios incidentally set in a war context are still used alongside scenarios set in an everyday context, for example to investigate motivated moral reasoning (Ditto, Pizarro, & Tannenbaum, 2009; Uhlmann, Pizarro, Tannenbaum, & Ditto, 2009), the authority dependence or independence of judgments about harm (Piazza, Sousa, & Holbrook, 2013; Sousa, Holbrook, & Piazza, 2009), the relationship between intentions and harmful outcomes (Knobe, 2003), the dissociation between deontology and utilitarianism (Conway & Gawronski, 2013), authority as a moral foundation (Graham et al., 2013), and asymmetries in force attributions for morally bad as opposed to neutral behavior (Phillips & Young, 2011). This use of war-based scenarios to investigate general processes and principles implies that these operate in the same way across both everyday and war contexts and that war is simply another, relatively undifferentiated context within which all our regular moral principles are present and active.

Although the researchers I cited above do not explicitly state that the moral principles they have uncovered will apply in all contexts, for all people, their practice in choosing scenarios implies a certain continuity between peace and war. I am not the first to point out potential problems with this relatively decontextualized nature of moral psychological research and raise subsequent questions around morality’s potential context sensitivity (Bauman, McGraw, Bartels, & Warren, 2014; Bloom, 2011; Schein, 2020). Moral judgments may, for example, depend on the relationships between the people involved in the scenario (e.g., family vs. strangers, Shaw, DeScioli, Barakzai, & Kurzban, 2017; Simpson, Laham, & Fiske, 2016) or the social context (e.g., intimates vs. task groups, Carnes, Lickel, & Janoff-Bulman, 2015). Watkins and Laham (2019) specifically compared moral judgments in war and peace contexts (using trolley problem scenarios) and found that people were overall more likely to accept a sacrifice of one life for many in war compared with in peace. Thus, although the use of a wide range of scenarios in the interest of generalizability might be justifiable or even desirable for some research programs, this approach will tend to obscure potential differences between war and everyday contexts.

It is an open question whether some moral principles or processes do, in fact, generalize across multiple contexts (including war and peace), while others do not. For example, although the symmetry thesis of just-war theory has no parallel in everyday morality, the processes that appear to drive the asymmetry in lay judgments—perceived identification and implicit alignment—have been established in an everyday context as well (e.g., Woolfolk et al., 2006). Thus, these processes might be candidates for context-invariant principles. On the other hand, Watkins and Laham (2019) used both footbridge and switch versions of the trolley problems in their studies and found that the usually robust difference in judgments between these two versions was significantly attenuated in a war context compared with in a peace context. Crucially, we cannot know which principles and processes will vary and which will not until we have a better grasp of the morality of war as distinct from the morality of peace.

My proposed approach thus partly provides a simple methodological critique of the continuity approach in that I suggest some caution when using scenarios set in war and peace contexts interchangeably. But my critique is also broader in that it touches on an important theoretical debate about the distinctiveness of the war and peace contexts. To what extent can and should war be considered a unique sphere of activity, separate from peace?

On the one hand, it seems likely—commonsense and intuitive—that discontinuities between the two contexts exist. Traditional just-war theory is explicit about the morality of war being different from the morality of peace (Shue, 2008; Walzer, 2006); the international community has (with some success) developed a separate legal framework for judging armed conflict that is independent from domestic law (Blum, 2014); and outside of academia, writers (James, 1910/1968), soldiers (Harrison, 2002; Hedges, 2002), journalists (Junger, 2016), and filmmakers (Mechanic & Gibson, 2016) have long observed how starkly different the war context is to the peace context, including the moral aspects of this context.

Despite these traditional, doctrinal, and pop-cultural precedents, insisting on a distinction between the morality of war and the morality of peace is not entirely uncontroversial, even within just-war theory. As outlined earlier, traditionalist and revisionist just-war theorists debate precisely how the relationship between the conduct of war and the resort to war should be understood (i.e., the debate about the independence thesis). This complex debate has far-reaching implications, and it hinges on the extent to which (moral judgments of) violence in war are or should be understood as continuous with (moral judgments of) violence in peace (Garner, 2016). Traditionalists insist that the two contexts are discontinuous, which is why they can (logically, morally) justify the independence thesis, for which no parallel exists in everyday morality (Walzer, 2006). Revisionists, however, believe the independence thesis is not (logically nor morally) justified, partly because they insist that the morality of war and the morality of peace must be continuous and consistent (McMahan, 2006). In a sense, revisionists thus provide a prescriptive version of the continuity approach to war in moral psychology in that they assume that principles of morality are invariant across the two contexts. They also share with the conflict approach a concern about the many ways in which everyday morality is violated in war. Although the revisionist perspective is, coincidentally, well represented by existing approaches to war in psychology (see Table 1), traditionalist just-war theorists have no counterpart within psychology to draw on. One way to understand my argument, then, is to say that psychologists should take a leaf out of the traditionalists’ book and study the morality of war as if it were discontinuous with the morality of peace.

Summarizing the proposed context approach

Both lines of research reviewed above have advanced our knowledge of war and morality. The conflict approach has taught us a lot about how people maneuver around moral transgressions—committed by themselves or their group members—to maintain a moral self-image. The continuity approach has uncovered what appear to be broad generalities of morality. I propose a third approach, which I refer to as the context approach. This approach differs from the alternatives in a number of ways (summarized in Table 1). First, psychologists should study the morality of war separately from the morality of everyday life to empirically uncover continuities and discontinuities between the two contexts. Second, and similarly, although the conflict approach tells us how people manage to not apply certain moral standards in war, it tells us less about which moral principles and processes are applied in war. Studying the morality of war in its own right will add nuance to our understanding of moral judgment and conduct in war. Third, these war-specific prescriptions and prohibitions on conduct in war can (and according to traditional just-war theory, should) be studied separately from broader questions about moral justifications for going to war in the first place.

Finally, in-group bias appears to be an inevitable part of human psychology, but third-party uninvolved observers to a (interpersonal or intergroup) conflict also play an important role in condemning and condoning behavior during the conflict (DeScioli, 2016). Regulating the conduct of war through moral and legal judgments is one effective way of reducing its scope and violence, and increasing our understanding of the psychological factors influencing these judgments is thus a worthwhile project. When it comes to understanding all the complexities and nuances of the morality of war, most likely all three approaches have merit for tackling different terrains in this moral landscape. I hope the findings yielded by these three distinct approaches will eventually be integrated with findings by other researchers studying morality in war as well—be they ethicists training military leaders (French, 2005; Matthews, 2014), clinical psychologists treating veterans for moral injury (Brock & Lettini, 2012), or political scientists investigating the military-civilian gap (Taylor, 2011). The context approach promises to uncover one unique piece of the puzzle, and I next explore the broader implications this approach may have for the practical, psychological, and philosophical aspects of judging the conduct of war.

Implications for the psychological study of morality and war

First, and most simply, having concrete evidence about how moral principles are applied and processed in a war context will enable researchers to make more informed decisions about which scenarios to use in their investigation of general principles or processes. It is certainly possible that future research reveals that the morality of war and the morality of peace overlap to such an extent that scenarios drawn from the two contexts can be used interchangeably (see, e.g., Watkins & Brandt, 2019). Such a finding would be a challenge to the present proposal but consistent with the continuity approach and with revisionist just-war theorists’ treating everyday and war morality as continuous.

Second, the studies reviewed above open up a range of further research questions at the intersection of the international laws of armed conflict, the moral philosophy of war, and social and moral psychology. For example, underpinning the discussion about the principle of discrimination above was the assumption that most people would default to thinking about a male soldier; most soldiers in the United States and other Western nations are indeed men (Office of the Deputy Assistant Secretary of Defense, 2013). How might the stereotype of a soldier interact with other identities? How do we think about female soldiers, child soldiers, or drone operators? Are they perceived as equally brave and harmful as male soldiers, and are they dehumanized to the same degree? As dehumanization (in other contexts) has consequences for how a social group is treated (Haslam & Loughnan, 2014), dehumanization of soldiers may also have important downstream consequences for the support they receive both during and after war (e.g., Bain & Hirst, 2011). In this way, a psychological study of the morality of war may speak to concerns of military and clinical psychologists and veterans’ organizations about combatants’ return to civilian life (Sherman, 2015).

Third, the present proposal has broader conceptual and theoretical implications for how (moral) psychologists study war. Investigating the morality of war using the context approach involves taking the perspective of an uninvolved third-party observer, in line with International Humanitarian Law and just-war theory, rather than the perspective of the (vicarious) perpetrator or victim. Although previous research on, for example, moral disengagement has been extremely influential in our understanding of (the perpetration of) violence and intergroup conflict, the uninvolved third-party perspective is also important because most people in the world today are not (direct or indirect) perpetrators of war (Hayes, 2012), and wars do not tend to break out within the territories of modern democracies (Wimmer, 2014). Further, war (and the judgment thereof) is being increasingly individualized in that the laws of war now consider combatants as “individual men and women, capable of and responsible for independent decision-making” to a greater degree than in the past (when they were more likely to be considered as “mere instruments or agents of an all-powerful sovereign,” Blum, 2014, p. 62). In line with this individualizing movement, the recent edited volume Just and Unjust Warriors (Rodin & Shue, 2008) complements Walzer’s original Just and Unjust Wars (1977) with a focus specifically on the individuals fighting in war and how we judge their conduct. Some of these judgments are not easily captured by current approaches to war (and morality) in psychology. The present proposal complements these approaches, to provide a fuller picture of the morality of war.

Implications for the philosophy and laws of war and beyond

The above section outlines considerations for psychological researchers to take into account when exploring the morality of war. I hope that psychological research systematically investigating third-party judgments of the conduct of war can also more easily comment on (and potentially critique) just-war theory. This ability will be important for developing an ongoing dialogue between philosophy and psychology (see e.g., Cushman, 2014; Cushman & Young, 2009) and between psychology and international institutions such as International Humanitarian Law and the International Criminal Court. One of the broad goals of the present article is to initiate a conversation between scholars of just-war theory (both traditional and revisionist forms) and moral and social psychologists in a way that serves the goals of both fields.

For example, international norms of war are currently aligned with traditionalist just-war theory, which takes the morality of war to be discontinuous with the morality of peace. However, as discussed above, it appears lay judgments—at least in the United States—are more closely aligned with some aspects of revisionist just-war theory. Just-war theorists may of course choose not to take into account information about what people do believe given that their enterprise primarily concerns what people should believe (Abend, 2013; Birnbacher, 1999; Cushman & Young, 2009; Kahane, 2013; Kumar & Campbell, 2012). However, it seems likely that just-war theorists hope to have a concrete impact on the world: Again, “the point of just war theory is to regulate warfare, to limit its occasions, and to regulate its conduct and legitimate scope” (Margalit & Walzer, 2009, p. 2). One way it achieves this aim is through international law and other institutions (McMahan, 2010; Orend, 2013; Rodin & Shue, 2008) that design and enforce a legal framework around war. As I have demonstrated, tools and theories from psychological research can be constructively applied to phenomena drawn from just-war theory and reveal a complex picture of the morality of war. These studies thus begin to establish the current capacities and limitations of laypeople’s intuitive moral judgments in war.

Despite most people in the Western world not being directly exposed to war, modern democratic states (the United Kingdom, United States, and western European nations) have been involved in more interstate wars in the past 200 years than less developed nations, even including China and Russia (Wimmer, 2014). Citizens of these democracies influence wars by voting, protesting, campaigning, or lobbying (Burstein & Freudenburg, 1978; Foyle, 2004). In so doing, they are clearly considering their political leaders’ reasons for resorting to war—the international aspect of just-war theory. But the interpersonal aspect of war—the details of its conduct—is also becoming increasingly important given that, for example, terrorists blur the lines between combatants and noncombatants (Clark & Raustiala, 2007), the sovereignty of states is being challenged (Brunstetter & Holeindre, 2018), and drones and other technological developments change the manner in which wars are fought (Schulzke, 2017). Gaining an understanding of what people judge as morally right or wrong in war means—in turn, eventually—knowing how best to hold individuals responsible for their actions in war.