According to the synopsis of a report by Mr Atran and two colleagues delivered to staff in the White House National Security Council in March: “Small group dynamics – rather than personality, ideology, education or income – is the prime factor in deciding which few, among millions of potential jihadis will actually go on to commit violence.”
Mr Atran and Marc Sageman, a former Central Intelligence Agency officer, are building a database of terror groups based on court records and interviews. Their preliminary conclusions sometimes run counter to conventional wisdom. Terrorists, for example, are usually turned within groups and there is not much evidence of top-down recruitment or brainwashing of plotters.
About 70 per cent of terrorists enlist in groups through friendship and about 20 per cent through kinship. The preferred cell size is eight members and consists of friends made between the ages of 15 and 30.
Neither is social deprivation a factor. A 2004 survey by Mr Sageman showed more than 70 per cent of jihadis were from middle or upper class backgrounds. More than 40 per cent were, like the group allegedly behind last week’s attacks in the UK, in the professions: teachers, lawyers and doctors.
Often within groups, there is a leader or hands-on figure, often not the most devout, who converts radical words to actions. In the Madrid group, this appeared to have been Jamal Ahmidan, who married a Christian and was a notorious drug smuggler.
Once in the group, what leads them to resort to terrorism and suicide? Christopher Heffelfinger, a senior analyst at the West Point military academy, has identified four steps on the road: introduction to the group; immersion in extremist doctrine; an initial effort to effect peaceful change; and, lastly, the step from nonviolence to violence.
Mr Sageman has described three generations of jihadis: the first the foreign mujahideen in Afghanistan and the second a younger generation of educated youths, such as those that hatched the September 11 plot in Hamburg. The third wave consists mostly of semi-skilled or marginalised people, such as those behind the Madrid bombings or the London attacks of July 7 2005.
On this nomenclature, the “medical cell” allegedly behind the attacks in London and Glasgow look like a late example of the second wave.