The science of savagery in Iraq
He says that while many in the West see the brutality and fanaticism as unique to Islamic fundamentalism, the right cocktail of factors can make anyone an extremist. He sees five that could be at play with IS, the first simply that savagery begets savagery.
Callousness, aggression and lack of empathy are common responses by people who have been harshly treated themselves.
Robertson, Professor of Psychology at Trinity College Dublin and the founding director of Trinity College Institute of Neuroscience, believes that a group mentality also comes into play.
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.
And that can lead to seeing any other group as a dehumanised enemy.
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.
He believes that a culture of revenge can play a part, as can a blind faith in a leader.
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.
All that's well and good, I suppose, and some of Robertson's suggested motivations might make sense in the case of an Iraqi Sunni who has been victimised by members of the Shi'ite majority, and clings to a new group for survival in a lawless state as he seeks revenge for his woes.
But there doesn't seem to be much here to explain how a would-be British rap star ends up the prime suspect for cutting off another man's head, or why young Australian men would leave the comfortable southwestern Sydney suburbs to join a death cult on the other side of the world.