The science behind Isil's savagery Prisoners at Auschwitz: but some of the cruellest guards were prisoners themselves The first part of an answer may be horribly simple: savagery begets savagery. Callousness, aggression and lack of empathy are common responses by people who have been harshly treated themselves. In the Nazi concentration camps, for instance, many of the cruellest guards were themselves prisoners - the notorious "kapos." Sexually abused children - particularly males - are more likely to go on to become sexual abusers themselves as adults, although the majority do not. Victims, in other words, often respond to trauma by themselves becoming victimisers. 2 - Submersion in the Group Isis fighters capture a Syrian army base But victim becoming victimiser is not the only explanation for savagery. When the State breaks down, and with it law and order and civic society, there is only one recourse for survival - the group. Whether defined by religion, racial, political, tribal or clan - or for that matter by the brute dominance of a gang-leader - survival depends on the mutual security offered by the group. War bonds people together in their groups and this bonding assuages some of the terrific fear and distress the individual feels when the state breaks down. It also offers self-esteem to people who feel humiliated by their loss of place and status in a relatively ordered society. To the extent that this happens, then individual and group identities partially merge and the person's actions become as much a manifestation of the group as of the individual will. When this happens, people can do terrible things they would never have imagined doing otherwise: individual conscience has little place in an embattled, warring group, because the individual and group selves are one so long as the external threat continues. It is groups which are capable of savagery, much more than any individual alone. You can see it in the faces of the young male Islamic State militants as they race by on their trucks, black flags waving, broad smiles on their faces, clenched fists aloft, fresh from the slaughter of infidels who would not convert to Islam. What you can see is a biochemical high from a combination of the bonding hormone oxytocin and the dominance hormone testosterone. Much more than cocaine or alcohol, these natural drugs lift mood, induce optimism and energise aggressive action on the part of the group. And because the individual identity has been submerged largely into the group identity, the individual will be much more willing to sacrifice himself in battle - or suicide bombing, for that matter. Why? - Because if I am submerged in the group, I live on in the group even if the individual "me," dies. When people bond together, oxytocin levels rise in their blood, but a consequence of this is a greater tendency to demonise and de-humanise the out-group. That is the paradox of selfless giving to your in-group - it makes it easier for you to anaesthetise your empathy for the out-group and to see them as objects. And doing terrible things to objects is fine because they are not human. 3 - The out-group as objects The revered (Shia) Askari mosque in 2006, after (Sunni) al-Qaeda blew it up But here is one daunting fact as we contemplate the Sunni-Shia carnage in Iraq and Syria: in-group tribalism is strengthened - and loathing for the out-group correspondingly increased - where religion defines the groups. Even when aggression against the other group is self-destructive - as we can see so tragically across the Middle East - religiously-based groups advocated a degree of aggression against their opponents which was absent in non-religiously defined groups. 4 - Revenge Isis militants apparently leading away captured Iraqi soldiers in Tikrit Revenge, which is a strong value in Arab culture, may play a part in perpetuating the savagery. Of course vengeful retaliation for savagery begets more savagery in a never-ending cycle. But more, while revenge is a powerful motivator, it is also a deceiver, because the evidence is that taking revenge on someone, far from quelling the distress and anger which drives it, actually perpetuates and magnifies it. 5 - Leaders Shakir Wahiyib is a feared enforcer for the Islamic State who does not cover up his face in videos of his killings Finally, people will do savage things if their leaders tell them it is acceptable to do so, particularly if they have given their selves to the group self. The Rwandan genocide was switched on by a series of radio broadcasts by a small group of leaders to a population who, by that instruction, were turned into savage murderers of former friends and neighbours who were in the out-group. The soldiers of the Soviet army committed mass rape as they invaded Germany in 1945 because senior commanders had advocated it. Islamic State fighters are slaughtering unarmed Christians and Yazidis because their leaders have told them that this is the right thing to do. Leaders at many levels from the tribe to the country, are responsible for this savagery, and so leaders can eventually stop it - just as they chose to do in Rwanda, after international pressure. But the trouble is, as we have seen, when leaders choose to encourage savagery, not quell it, there is nothing hard-wired into human beings to stand up against it. Ian Robertson is Professor of Psychology at Trinity College Dublin and was the founding director of Trinity College Institute of Neuroscience IS: 'world's richest terror group' - in 60 seconds