Do Moral Judgments of War Support the Principle of Combatant Equality?: What Empirical Studies Tells Us

A Moral Psychology Study of Just War Theory

Imagine a U.S. soldier and an Iraqi soldier, shooting at each other during the First Gulf War. Are these two soldiers moral equals?

According to traditional just war theory, the answer is yes. And under international humanitarian law, combatants on all sides of a conflict should be held responsible only for their conduct during the war, not for the justness of its cause. This independence of jus in bello (law governing the conduct of war) and jus ad bellum (law governing the resort to war) is a fundamental principle underlying international legal systems and ethical doctrines of war. More colloquially, a similar idea is captured by the bumper sticker, “Hate the war, love the warrior.” This also implies that combatants on all sides of a conflict should be judged symmetrically – known as the principle of combatant equality. If they engage in identical actions, they should be judged identically: The actions of the U.S. soldier and the Iraqi soldier are therefore morally equivalent.

But do regular people’s moral judgments in fact correspond to this prescription from  philosophy and law? The symmetry idea described above – combatant equality – might be a worthy ideal, but does it accurately represent judgments made by the general public? We investigated this question in a large set of experimental studies (recently published in the Journal of Experimental Psychology: General).

First, when we explained the principle to the regular Joe or Jane – U.S. residents recruited online using Amazon MTurk – more than half were happy to endorse it. Yes, you should judge a soldier only by what he or she does, not by which side he or she is on. No, your beliefs about the justness of going to war should not influence your judgments of the soldiers fighting in the war. Judgments of soldiers on the just or the unjust side of the war should by implication be symmetrical. Sure, that sounds good.

But even though many people endorsed the principle in the abstract, this did not mean the principle was applied consistently when judging more concrete scenarios. In fact, we were surprised by how far from symmetric participants’ judgments were in all our subsequent studies. In these studies we asked participants to make moral judgments about hypothetical scenarios: For example, imagine that a soldier sees an enemy soldier running away from a firefight, and he shoots and kills him. How morally defensible do you think this action is? The exact same act was judged as much more morally defensible if it was performed by a soldier on the just side of the war, than by a soldier on the unjust side.

We asked participants in our studies to consider many different actions – for example killing enemy soldiers, risking civilian lives during a bombing campaign, torturing a captured soldier, killing enemy politicians, or bombing government buildings. The asymmetry emerged for the vast majority of them.

But, we also found some exceptions. Abhorrent acts, like defiling enemy corpses, or failing to whistleblow on egregious war crimes, were judged as equally indefensible no matter which side of the conflict the soldier was from. On the flip side, exceptionally good acts, like protecting enemy civilians during a firefight, or sacrificing one’s life for fellow soldiers, were seen as equally laudable for soldiers on both sides of the war. (Note however, that at the extremes of moral judgment the limitations of our measurement tools – self-reported judgments on 7-point response scales – might mean that we were unable to detect to pick up smaller asymmetries.)

We also asked about soldiers’ conduct in many different, hypothetical, war scenarios. One was a war of aggression versus self-defense. In another, a country’s government was oppressing an ethnic minority, and – after all attempts at a diplomatic solution failed – a nearby country undertook a military humanitarian intervention. In a third type of war, a small country was unjustly invaded by its neighbor, and a third, larger, country provided military support in repelling the invasion. Finally, we used a war scenario in which there was initially no clear just or unjust side – until one country unjustly breached a ceasefire by bombing a hospital. Regardless of what international law might suggest about these types of war, for all scenarios, participants in our studies believed that one country had a just cause for war, while the other did not. And despite the majority endorsing the principle of combatant equality (and the symmetry thesis) in the abstract, on the whole people’s judgments of resort to war consistently influenced their judgments of the conduct of war. A soldier on the just side of a conflict – irrespective of why that side was seen as “just” – was judged more leniently than a soldier on the unjust side of a conflict – even when the soldiers were engaged in the exact same actions.

This asymmetric pattern of judgments, which runs counter to the principle of combatant equality, seems concerning for any individual or institution invested in the impartial application of jus in bello. Traditional just war theorists, for example, suggest that the principle of combatant equality (and related ideas) contribute to limiting the scope and violence of war. So we might also want to ask – whywere moral judgments in our studies so asymmetric?

The father of contemporary just war theory, Michael Walzer, argues for symmetry partly by appealing to the “shared servitude” soldiers on all sides of a war operate under – that is, to the idea that the actions of soldiers on both sides of a conflict are equally bound to follow orders and obey their superior officers. Perhaps participants in our studies were instead assuming that the soldiers were acting freely, and could therefore be held responsible for their actions? If this was the case, we should have found that when we equally highlighted the “shared servitude” of soldierson each side of a given conflict, people’s judgments should be closer to symmetric. But our data did not bear this out. First, we found that participants in our studies saw soldiers as highly constrained in general, but that the degree of constraint did not influence the degree of asymmetry. Second, in one study we specified that the soldier was ordered to do X, and he obeyed the order. We did not find any difference in moral judgments when comparing these obedient soldiers to soldiers acting on their own initiative.

However, there is a different factor that did help explain the asymmetry. Absent any additional information, participants in our studies thought that a soldier on the just side of a war endorsed his country’s cause for war (be it self-defense or a humanitarian intervention), and they assumed that a soldier on the unjust side endorsed his country’s cause (be it territorial expansion or ethnic cleansing). Crucially, we were able to disrupt this default assumption by specifying in some cases that the soldiers (on both sides) did not endorse their country’s cause for war. In fact, they had made the independent decision that to resort to war in this case would be morally wrong, and they protested the war with their friends. Nonetheless, they were conscripted and trained in the military, and fought competently, upholding their professional duty as soldiers. In such cases, the conduct of soldiers on each side of the war was judged more equally. The actions of the soldier on the unjust side were still seen as less defensible than those of the soldier on the just side – but the asymmetry was much smaller, and thus more in line with the principle of combatant equality.

What are we to make of all of this? As research psychologists, we are interested in mapping out and explaining how people actually make moral judgments of soldiers. The principle of combatant equality is only one feature of this map; we have a long way to go yet. Meanwhile, international legal scholars, philosophers, and readers of Just Security may typically theorize about the morality of war independent of the intuitions of lay people – ordinary Americans with no training in international law or just war theory. But gaps between prescriptive doctrines and lay intuitions may matter, for example if they threaten the legitimacy of institutions built on such doctrines: The principle of combatant equality and the independence thesis are at stake in debates about how U.S. soldiers should be held responsible for alleged war crimes; in how veterans of unpopular (maybe unjust) wars are treated; and in the success or failure of  the International Criminal Court. In understanding these phenomena, information about public intuitions may be crucial. Experimental moral psychology provides the tools for quantifying them, and for understanding the factors that drive them.


IMAGE: US soldiers aim their guns during a ceremony on the outskirts of Kabul on March 15, 2012. (Photo by MASSOUD HOSSAINI/AFP/Getty Images)


About the Author(s)

Hanne Watkins

Postdoctoral Research Associate with the Psychology of Peace and Violence Program at the University of Massachusetts Amherst - Follow her on Twitter ().

Geoffrey Goodwin

Geoff Goodwin is an Associate Professor of Psychology at the University of Pennsylvania.