Image caption Congolese rebels have been accused of cannibalism by the UN

The recent case of a Syrian rebel appearing to take a bite from an opponent's heart seems utterly shocking and disturbing. But is this incident more inhuman than all the other countless atrocities that have already taken place in this war?

We are used to reports of mass graves, torture, killings and mutilations of civilians, and the eradication of complete villages.

But this particular barbarous act has attracted special attention. Cannibalism seems to contradict common moral and ethical beliefs on what is acceptable in war scenarios and what is not. So has the violence really reached a new dimension and what motivation underlies these acts?

Our research unit at the department of psychology, University of Konstanz, is dedicated to understanding the mechanisms motivating cruelty.

In interviewing more than 2,500 former combatants in Uganda, Rwanda, Colombia and the Democratic Republic of Congo, we found evidence that the numerous outrages that are committed in these regions - beyond the glare of Western publicity - are no less brutish or less inhuman than this recent incident from Syria.

About the author Roland Weierstall is an assistant professor of clinical psychology at the University of Konstanz The psychobiology of human aggression is his specialism He has conducted extensive studies with former combatants of Uganda, Rwanda, Colombia, the Democratic Republic of Congo, Burundi and World War II

On the contrary, when moral barriers fall and violence is legitimised in the context of war, the inhibition towards intra-species killing breaks down. Seeing the victim suffer can be a sufficient reward for violence, irrespective of secondary rewards like honour, status or material rewards.

There are two basic motivations that foster such violent behaviour.

Firstly, there is violence that results from negative emotions like rage and hatred and that appears as a response towards a threat. And secondly, there are "positive" emotions like excitement or hedonistic pleasure.

The former is easier for civilians to understand. No-one knows the story behind the Syrian video and consequently no-one knows the motives that underlie the rebel's behaviour. It cannot be excused, but I will suggest it can be understood, bearing in mind what might have happened prior to the incident.

Out of utter rage and vengeance, humans can behave in a way that might appear to be inhumane.

The executions of Saddam Hussein or Bin Laden remind us that under certain circumstances even civilians can overcome their moral beliefs and values, which include refraining from killing. Instead of appalled reactions, the video of an execution and the picture of the blood of a man in his bedroom became seen as trophies after a great hunt.

This gruesome episode is also a powerful reminder of the bitterness and increasingly sectarian nature of the fighting in Syria Jonathan Marcus, BBC diplomatic correspondent Syria video pinpoints West's dilemma

Consequently, we should not be amazed by the brutality of violent acts of war. Can emotions and consequently also violent acts be any more intense than in an enduring and irreconcilable conflict like the one in Syria?

The second form of violent behaviour, called appetitive aggression - committed for "pleasure" - is less familiar to most of us. About one third of all former combatants in our studies said that to some extent the violence and the struggling of the victim could be fascinating, emotionally arousing and even linked to excitement. In these cases, blood must be shed as the victim is killed.

Systematic torture to make the opponent suffer, the mutilation of civilians, like cutting off ears, lips or genitals and the desecration of dead bodies are surprisingly common by-products of war and escalating violence, and can be found across different cultural and historical backgrounds.

Hence, the Syrian case should not surprise anyone. We should rather be surprised that the extent of human right violations we should expect to happen in Syria is kept secret.

Image caption An estimated two million children in Syria face malnutrition, disease and trauma

Besides appetitive aggression as a core motivation for cruel behaviour, cannibalism in particular can have a considerable social and ritual significance. There are rebel groups that resort to superstitious cannibalism as part of a traditional culture.

Almost 10% of former rebels in the Democratic Republic of Congo reported in one of our studies that they had eaten human flesh and one out of four reported the witnessing of cannibalism among their comrades.

The Mai-Mai fighters of eastern Congo drink blood of the enemy or ingest their heart or genitals to gain the power of the enemy warrior. However, as there is no evidence for a historical tradition of cannibalism in Syria, speculation in this direction appears to be far-fetched and exaggerated.

It's likely that the behaviour in this video will become more frequent and persistent in the Syrian conflict.

Violence against civilians and unspeakably cruel acts against the enemy are powerful and effective coercive strategies to systematically intimidate and demoralise the enemy, even if they risk revenge.

The media attention given to this incident and the intense indignation might demonstrate the rebels' resolve to resort to violence and increase their bargaining position within the conflict - or it could further fuel the cycle of violence.

The motivation underlying human cruel behaviour is complex. An adequate appraisal of this incident in Syria depends on sufficient information which - especially for the Syrian conflict in general - is seldom available.