Soldiers in combat can kill without moral injury

Killing another person is not necessarily harmful to a soldier's mental health — in fact, the impacts on wellbeing depend more on the context than on the act itself. At least, this is the conclusion of a large-scale study of Norwegian soldiers, led by Andreas Espetvedt Nordstrand of Institute of Military Psychiatry, Norwegian Armed Forces, Norway. These findings contradict the commonly-held idea that killing is an 'unnatural' act for humans that inevitably inflicts a moral injury on the individual, the researchers write in Armed Forces and Society.

The team studied two different samples of Norwegian soldiers. The first consisted of 4,053 who had been deployed to Afghanistan between December 2001 and December 2011. These soldiers took part in NATO-led combat-oriented missions, with the aim of fighting to defeat the enemy.

The second sample was very different. These 10,605 soldiers were sent to Lebanon to act as peacekeepers, some time between 1978 and 1998. The main objective of this UN-led peacekeeping mission was not to engage or defeat an enemy, but to maintain order. In contrast to the rules of engagement for the soldiers deployed in Afghanistan, they were told that deadly force should be used only as a last resort, and only for self-defence.

The participants completed a series of questionnaires. They reported on whether they had taken a life in combat, or believed that they had killed someone in combat. They also answered questions about other types of stressful events, including whether they had witnessed someone else being killed or injured. Both groups also completed surveys that assessed various psychological symptoms.

When the team analysed the results, they found some clear differences between the two groups. Peacekeepers who had killed someone during their deployment had higher levels of PTSD symptoms, depression, anxiety, and alcohol use, as well as a lower quality of life, compared with peacekeepers who had not killed anyone.

For the group that had served in Afghanistan, however, there were no differences in scores for  soldiers who had killed someone in combat compared with those who had not.

The analyses also revealed that for the peacekeeping soldiers, feeling that their own safety had been threatened while on deployment or witnessing someone else suffering were both linked to poorer psychological wellbeing. For the soldiers who fought in Afghanistan, there were some associations between these experiences and poorer scores on the psychological measures, but the links were weaker.

The researchers' main conclusion is that when it comes to the psychological impact of taking a life, peacekeepers are more vulnerable than combat soldiers — and, because combat soldiers operate under rules of engagement that are explicitly geared towards fighting, whereas peacekeeping soldiers do not, this suggests that context really matters for the effects on wellbeing.

"There is a widespread belief in society that taking the life of another person goes against human nature, and that this will easily create what psychotraumatology refers to as 'moral injuries'," said Nordstrand in a press release. However, this is not something that is commonly reported by combat soldiers, he says — and this study suggests that it is far from the norm. "Killing another person does not in itself seem to be something that goes against human nature, and it doesn't necessarily harm the mental health of the person who does it," Nordstrand added.

The team acknowledges that this is a sensitive topic — and that the idea that a person may be unaffected by taking a life will be difficult for some to accept. However, "I think it is important to bring evidence-based perspectives on this taboo subject into the public debate," Nordstrand said.

The researchers do acknowledge some limitations to their study. One is that the gap between deployment and responding was much longer for the peacekeepers than the combat soldiers. It's possible that other variables after deployment, such as varying levels of traumatic experiences, influenced the differences in results.

However, the researchers add, if, as their work suggests, context is important, this makes it more important to understand how to work with context to minimise the potential for psychological suffering among soldiers who find themselves, in the course of their duties, in the position of taking a life. Pre-deployment briefings, mission statements and levels of psychological support could all affect this, they write.

For others, the results will raise another question — if combat soldiers are, as a rule, psychologically unharmed by killing an enemy, does this increase the risk that they may be willing to kill captives or civilians who they have come to class as 'the enemy'? Given allegations that this has happened, research is also needed to explore how specific rules of engagement, briefings and mission statements can prevent this.

Read the paper in full:
Nordstrand, A. E., Noll, L. K., Huffman, A. H., Gjerstad, C. L., Tveitstul, T., Reichelt, J. G., Bakker, L.-P., Kennair, L. E. O., Kristoffersen, R. H., Bøe, H. J., & Wickham, R. E. (2025). Killing in Combat as a Potentially Morally Injurious Event: The Diverging Psychological Impact of Killing on Peacekeepers and Combat-Oriented Troops. Armed Forces & Society. https://doi.org/10.1177/0095327x251321389

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